December 14, 2017

Archives for January 2006

CD DRM: Attacks on the Player

Alex and I are working on an academic paper, “Lessons from the Sony CD DRM Episode”, which will analyze several not-yet-discussed aspects of the XCP and MediaMax CD copy protection technologies, and will try to put the Sony CD episode in context and draw lessons for the future. We’ll post the complete paper here later in the week. Until then, we’ll post drafts of a few sections here. We have two reasons for this: we hope the postings will be interesting in themselves, and we hope your comments will help us improve the paper.

Today’s excerpt is the last one we will post. By now, you have seen drafts of the all sections of the paper except the introduction, conclusion, and discussion of related work. The deadline for submission is (late) tomorrow night.

Please note that this is a draft and should not be formally quoted or cited. The final version of our entire paper will be posted here when it is ready.

Attacks on the Player

Increasingly, personal computers—and portable playback devices that attach to them—are users’ primary means of organizing, transporting, and enjoying their music collections. Sony and its DRM vendors recognized their trend when they designed their copy protection technologies. Rather than inhibit all use with PCs, as some earlier anti-copying schemes did~cite{}, XCP and MediaMax allow certain limited uses subject to restrictions imposed by DRM software.

XCP and MediaMax facilitate use on PCs using their own proprietary media players that are shipped on each protected CD. The schemes use active (and, with XCP, passive) protection to block other applications from accessing the CD audio, but a back door allows the scheme’s own player to bypass the protections.

The XCP and MediaMax players launch automatically using autorun when a protected disc is inserted into a PC. Both players have similar feature sets. They provide a rudimentary player interface, allowing users to listen to protected albums, and they allow access to “bonus content,” such as album art, liner notes, song lyrics, and links to artist web sites. [Footnote: Curiously, this bonus content is seldom copy protected, perhaps because it is of little value.]

XCP and MediaMax both permit users to burn copies of the entire album a limited number of times (typically 3). These copies are create using a proprietary burning application integrated into the player. They include the player applications and active (and passive, for XCP) protection as the original album, but they do not allow any subsequent generations of copying.

Another feature of the player applications allows users to rip the tracks from the CD to their hard disks, but only in DRM-protected audio formats. Both schemes support the Windows Media Audio format by using a Microsoft product, the Windows Media Data Session Toolkit [citation[, to deliver DRM licenses that are bound to the PC where the files were ripped. The licenses allow the music to be transferred to portable devices that support Windows Media DRM scheme or burned onto CDs, but the Windows Media files will not be usable if they are copied to another PC.

Attacks on Player DRM

The XCP and MediaMax players were designed to enforce usage restrictions specified by content providers. In practice, they provide minimal security, because there are a number of ways that users can bypass the limitations.

Perhaps the most interesting class of attacks targets the limited number of burned copies permitted by the players. Both players are designed to enforce this limit without communicating with any networked server; therefore, the player must keep track of how many allowed copies remain by storing state on the local machine.

It is well known that DRM systems like this are vulnerable to rollback attacks. In a rollback attack, the state of the machine is backed up before performing the limited operation (in this case, burning the copy). When the operation is complete, the old system state is restored, and the DRM software is not able to determine that the operation has occurred. This kind of attack is easy to perform with virtual machine software like VMWare, which allows the entire state of the system to be saved or restored in a few clicks. The XCP and MediaMax both fail under this attack, which allows unlimited copies to be burned with their players.

A refined variation of this attack targets only the specific pieces of state that the DRM system uses to remember the number of copies remaining. The XCP player uses a single file, %windir%system32$sys$filesystem$sys$parking, to record how many copies remain for every XCP album that has been used on the system. [Footnote: We did not determine how the MediaMax player stores the number of copies remaining.] This file is hidden and protected by the XCP rootkit. With the rootkit disabled, a user can back up the file, copy the album, and then restore the backup to set the remaining copy counter back to its original value.

A more advanced attacker can go further and modify the $sys$parking file to set the counter to an arbitrary value. The file consists of a 16 byte header followed by a series of 177 byte records. For each XCP disc used on the machine, the file contains a whole-disc record and an individual record for each track. Each disc record stores the number of permitted copies remaining for the disc as a 32-bit integer beginning 100 bytes from the start of the record.

The file is protected by primitive encryption. Each record is XORed with a repeating 256-bit pad. The pad—a single one is used for all records—is randomly chosen when XCP is first installed and stored in the system registry in the key HKLMSOFTWARE$sys$referenceClassID. Note that this key, which is hidden by the rootkit, is intentionally misnamed “ClassID” to confuse investigators. Instead of a ClassID it contains the 32 bytes of pad data.

Hiding the pad actually doesn’t increase the security of the design. An attacker who knows only the format of the $sys$parking file and the current number of copies remaining can change the counter to an arbitrary value without. Say the counter indicates that there are x copies remaining and the attacker wants to set it to y copies remaining. Without decrypting the record, she can XOR the padded bytes where the counter is stored with the value (x XOR y). If the original value was padded with p, the new value is (x XOR p) XOR (x XOR y) = (y XOR p), which is just y padded with p.

iPod Compatibility

Ironically, Sony itself furnishes directions for carrying out another kind of attack on the player DRM.

Conspicuously absent from the XCP and MediaMax players is support for the Apple iPod—by far the most popular portable music player with more than 80% of the market [citation]. A Sony FAQ blames Apple for this shortcoming and urges users to direct complaints to them:”Unfortunately, in order to directly and smoothly rip content into iTunes it requires the assistance of Apple. To date, Apple has not been willing to cooperate with our protection vendors to make ripping to iTunes and to the iPod a simple experience.” [citation]. Strictly speaking, it is untrue that Sony requires Apple’s cooperation to work with the iPod. They ship thousands of albums that work “smoothly” with iTunes every day: unprotected CDs. What Sony has difficulty doing is moving music to the iPod while keeping it wrapped in copy protection. This is because Apple has so far refused to license its proprietary DRM, a system called FairPlay.

Yet so great is consumer demand for iPod compatibility that Sony gives out—to any customer who fills out a form on its web site [citation] —instructions for working around its own copy protection and transforming the music into a DRM-free format that will work with iTunes. The procedure is simple but cumbersome: users are directed to use the player software to rip the songs into Windows Media DRM files; use Windows Media Player to burn the files to a blank CD, which will be free of copy protection; and then use iTunes to rip the songs once more and transfer them to the iPod.

XCP’s Hidden iPod Support

A further irony came to light in the weeks following the public disclosure of the XCP rootkit when it was discovered that XCP itself apparently infringes on the copyrights to several open source software projects. In one case, Sam Hocevar found strong evidence that part of XCP’s code was copied from a program called DRMS, which he co-authored with Jon Lech Johansen and released under the terms of the GPL open source license. This was particularly curious, because the purpose of DRMS is to break Apple’s FairPlay DRM. Its presence is interesting enough to warrant a brief diversion from our discussion of player-related attacks.

We discovered that XCP utilizes the DRMS code not to remove Apple DRM but to add it, as part of a hidden XCP feature that provides iTunes and iPod compatibility. This functionality shipped on nearly every XCP CD, but it was never enabled or made visible in the XCP user interface. Despite being inactive, the code appears to be fully functional and was compatible with the current version of iTunes when the first XCP CDs were released. [Footnote: XCP’s FairPlay-compatibility code works with iTunes up to iTunes version 4.8. iTunes 4.9, released June 28, 2005, included changes unrelated to FairPlay that cause the XCP code to fail. XCP CDs released after this date do not appear to contain an updated version of the code.] This strongly suggests that the apparently infringing DRMS code was deliberately copied by XCP’s creator, First4Internet, rather than accidentally included as part of a more general purpose media library used for other functions in the copy protection system.

This isn’t the first time another vendor has tried to make its DRM compatible with FairPlay. FairPlay is the only DRM compatible with iPod, and Apple has declined to license it to rival music distributors, effectively locking rivals out from the iPod platform (at least as long as the rivals insist on using DRM). In 2004, RealNetworks attempted to work around Apple and reverse engineered FairPlay so that Real Player could create FairPlay files for use with the iPod [citation]. Apple responded by making vague legal threats and updating iTunes to break this compatibility. The evidence suggests that First4Internet wanted to create their own iPod compatibility system, but rather than take the time to reverse engineer FairPlay themselves, they copied critical pieces of code from DRMS in violation of the GPL license.

Understanding how XCP uses code from DRMS requires a some background information about FairPlay. When a customer purchases a song from the iTunes Music Store, she receives a FairPlay encrypted audio file that can only be played with knowledge of a secret key assigned to her by Apple. iTunes retrieves this key from an Apple server and stores it on the hard drive in an encrypted key database (a file called SC Info.sidb). When the user plays the song again, or if she copies it to an iPod, iTunes reads her key from the database instead of reconnecting to the server.

FairPlay’s security depends on the encrypted key database being difficult anyone but Apple to decipher, so it is protected using a proprietary encryption method and a system-dependent secret key. [Footnote: As security experts predicted, this protection was quickly broken. Today DRMS is able to defeat FairPlay because Jon Lech Johansen reverse engineered the database decryption code in iTunes.] iTunes encrypts the key database using a two step process. First it pads the plaintext database by XORing it with the output of a proprietary pseudorandom number generator (PRNG) using a system-dependent seed; then it applies AES encryption in ECB mode with a system-dependent key. As a consequence of this design, the code for the PRNG is exactly the same whether the file is being encrypted or decrypted. To decrypt, iTunes applies AES decryption, then XORs the same PRNG output again. This explains why parts of the DRMS code—in particular, a function called DoShuffle, which computes the PRNG’s output—are useful for encryption as well as their original purpose, decryption.

The proprietary PRNG must have been especially difficult to reverse engineer. Rather than expend this effort themselves, XCP’s authors appear to have lifted the DoShuffle code verbatim from DRMS. XCP uses this code to manipulate the iTunes key database in the process of adding FairPlay protection. Starting with an unencrypted audio file, such as a track from a protected CD, XCP applies AAC compression in memory, then encrypts using the same algorithm as FairPlay. Instead of using an Apple-assigned user key, XCP creates a new random user key and, with the help of the DRMS code, adds it to the iTunes key database. This ensures that the song file can only be used on the computer where it was created.

The XCP FairPlay compatibility code is contained in a file named ECDPlayerControl.ocx that is installed the first time an XCP CD is played. The code can be tested by jumping to a function at debugger offset 0x10010380 (apparently the start of a thread for transferring music to iTunes). This function takes one parameter, a wide character string of the form <m P 3>< "C:test.mp3">. This syntax causes the function to convert an MP3 file to a FairPlay-protected AAC file. Variations can be used to specify other audio sources: WAV files, raw audio files, standard unprotected audio CDs, and XCP copy-protected CDs. Before calling the function, you must initialize a Windows CriticalSection object and set the ECX register to the object’s address minus 0x6C.

The parent function calls a subroutine (offset 0x10027D20) that converts an audio file into a FairPlay-protected AAC file. A second subroutine (offset 0x1008A470) reads the iTunes key database, decrypts it, and, if necessary, adds the XCP user key to the database and re-saves it in encrypted form. The iTunes database encryption function
(0x1008A0C0) and decryption function (0x1008A300) both made use of the DoShuffle routine (0x10089E00) taken from DRMS.

MediaMax Player Security Risks

Besides suffering from several kinds of attacks that expose the music content to copying, the MediaMax player make the user’s system more vulnerable to attack. When a MediaMax CD is inserted into a computer, Windows autorun launched an installer from the disc. Even before the installer displays a license agreement, it copies almost 12 megabytes of files and data related to the MediaMax player to the hard disk and stores them in a folder named
%programfiles%Common FilesSunnComm Shared. Jesse Burns and Alex Stamos of iSec partners first discovered that the MediaMax installer sets insecure permissions on this directory and the files and programs it contains [citation]. The installer grants “Everyone” (all users) the “Full Control” privilege. Normally, application files shared by all users on a Windows system can only be modified by members of the “Administrators” and “Power Users” groups.

As Burns and Stamos realized, this misconfiguration could lead to a dangerous privilege escalation attack. The incorrect permissions allow a non-privileged user to replace the executable code in the MediaMax player files. A user might plant malicious code deliberately in order to attack the system, or accidentally as the result of an email virus. The next time a user plays a MediaMax-protected CD, the attack code will be executed with that user’s security privileges. The MediaMax player requires Power User or Administrator privileges to run, so it’s likely that the attacker’s code will run with almost complete control of the system.

Normally, this problem could be fixed by manually correcting the errant permissions. However, MediaMax aggressively updates the installed player code each time the software on a protected disc autoruns or is launched manually. As part of this update, the permissions on the installation directory are reset to the insecure state.

We discovered a variation of the attack suggested by Burns and Stamos that allows the attack code to be installed and triggered even more easily—simply by inserting MediaMax CDs without ever consenting to the software’s installation. In the original attack, the user needed to accept the MediaMax license agreement before attack code could be inserted or executed, because the code was placed in a file called MMX.EXE that was not copied to the system until after the agreement was accepted. In our version, the attacker modifies a different file, MediaMax.dll, which MediaMax installs even before displaying a license agreement, and places attack code in the file’s DllMain() procedure. The next time a MediaMax CD is inserted, the installer autoruns and immediately attempts to check the version of the installed MediaMax.dll file. The installer calls the Windows LoadLibrary function on the DLL file, which causes the file’s DllMain() procedure to execute, together with any attack code inserted there.

This problem was exacerbated because part of the MediaMax software are installed automatically and without consent. Users who declined the license agreement would likely assume that MediaMax was not installed, and so most were unaware that they were vulnerable. The same installer code performs the dangerous version check as soon as the CD is inserted. A CD that prompted the user to accept a license before installing would give the user a chance to head off the attack.

Fixing the problem permanently without losing the use of protected discs requires applying a patch from MediaMax. Unfortunately, we discovered, the initial patch released by Sony in response to the iSec report was capable of triggering precisely the kind of attack it attempted to forestall. In the process of updating MediaMax, the patch checks the version of MediaMax.dll just like the MediaMax installer does. If this file has already been booby trapped by an attacker, the process of applying the security patch could execute the attack code. Prior versions of the MediaMax uninstaller had the same vulnerability, though both the uninstaller and the patch have since been replaced with versions that do not suffer from this problem.

CD DRM: Attacks on Installation

Alex and I are working on an academic paper, “Lessons from the Sony CD DRM Episode”, which will analyze several not-yet-discussed aspects of the XCP and MediaMax CD copy protection technologies, and will try to put the Sony CD episode in context and draw lessons for the future. We’ll post the complete paper here later in the week. Until then, we’ll post drafts of a few sections here. We have two reasons for this: we hope the postings will be interesting in themselves, and we hope your comments will help us improve the paper.

Today’s section is part of the technical core of the paper.

Please note that this is a draft and should not be formally quoted or cited. The final version of our entire paper will be posted here when it is ready

Attacks on Installation

Active protection measures cannot begin to operate until the DRM software is installed on the user’s system. In this section we consider attacks that either prevent installation of the DRM software, or try to capture music files from the disc in the interval after the disc has been inserted but before the DRM software is installed on the computer.

Autorun

Both XCP and MediaMax relies on the autorun feature of Windows. Whenever removable media, such as a floppy disc or CD, is inserted into a Windows PC (and autorun is enabled), Windows looks on the disc for a file called autorun.ini; if a file with that name is found, Windows executes commands found in it. Autorun allows a disc to pop up a splash screen or simple menu, for example to offer to install software found on the disc. However, the autorun mechanism will run any program that the disc specifies.

Other popular operating systems, including MacOS and Linux, do not have an autorun feature, so this mechanism does not work on these other systems. XCP ships only Windows code and so has no effect on other operating systems. MediaMax ships with both Windows and MacOS code on the CD, but only the Windows code can autorun. The MacOS code relies on the user to double-click an installer on the CD, which few users will do.

Current versions of Windows ship with autorun enabled by default, but the user can choose to disable it. Many security experts advise users to disable autorun, to protect against disc-borne malware. If autorun is disabled, the XCP or MediaMax active protection software will not load or run.

Even if autorun is enabled, the user can block autorun for a particular disc by holding down the Shift key while inserting the disc. This will prevent the active protection software from running.

Even without disabling autorun, a user can prevent the active protection software from loading by covering up the portion of the disc on which it is stored. Both XCP and MediaMax discs contain two sessions, with the first session containing the music files and the second session containing DRM content, including the active protection
software and the autorun command file. The first session begins at the center of the disc and extends outward; the second session is near the outer edge of the disc.

By covering the outer edge of the disc, the user can cover up the second session’s files, effectively converting the disc back to an ordinary single-session disc. The edge of the disc can be covered with nontransparent material such as masking tape, or by writing over it with a felt-tip marker. Exactly how much of the disc to cover can be determined by iteratively covering more and more until the disc’s behavior changes, or by visually inspecting the disc to look for a difference in appearance of the disc’s surface which is often visible at the boundary between the two sessions.

Temporary Protection

Even if the copy protection software is allowed to autorun, there is a period of time, between when a protected disc is inserted and when the active protection software is installed, when the music is vulnerable to copying. It would be possible to have the discs immediately and automatically install the active protection software, minimizing this window of vulnerability, but legal and ethical requirements should preclude this option. Installing software without first obtaining the user’s consent appears to be illegal in the U.S. under the Computer Fraud and Abuse Act (CFAA) as well as various state anti-spyware laws [citation].

Software vendors conventionally obtain the user’s consent to installation of their software by displaying an End User License Agreement (EULA) and asking the user to agree to it. Only after the user agrees to the EULA is the software installed. The EULA informs the user, in theory at least, of the general scope and purpose of the
software being installed, and the user has the option to withhold consent by declining the EULA, in which case no software is installed. As we will see below, the DRM vendors do not always follow this procedure.

If the discs didn’t use any other protection measures, the music would be vulnerable to copying while the installer waited for the user to accept or reject the EULA. Users could just ignore the installer’s EULA window and switch tasks to a CD ripping or copying application. Both XCP and MediaMax employ temporary protection mechanisms to
protect the music during this time.

XCP Temporary Protection

The first time an XCP-protected disc is inserted into a Windows machine, the Windows autorun feature launches the XCP installer, the file go.exe located in the contents folder on the CD. The installer displays a license agreement and prompts the user to accept or decline it. If the user accepts the agreement, the installer installs the XCP active protection software onto the machine; if the user declines, the installer ejects the CD and exits.

While the EULA is being displayed, the XCP installer continuously monitors the list of processes running on the system. It compares the image name of each process to a blacklist of nearly 200 ripping and copying applications hard coded into the go.exe program. If one or more blacklisted applications are running, the installer replaces the EULA display with a warning (shown at right [in the paper version, but not here]) indicating that the applications need to be closed in order for the installation to continue. It also initiates a 30-second countdown timer; if the any of the applications are still running when the countdown reaches zero, the installer ejects the CD and quits. [Footnote: Similar application blacklisting techniques have been used in other security contexts. The client software for World of Warcraft, a massively multiplayer online role playing game, checks running applications against a regularly updated blacklist of programs used to cheat. [citation]]

This technique might prevent some unsophisticated users from copying the disc while the installer is running, but it can be bypassed with a number of widely known techniques. For instance, users might kill the installer process (using the Windows Task Manager) before it could eject the CD, or they might use a ripping or copying application that locks the CD tray, preventing the installer from ejecting the disc.

The greatest limitation of the XCP temporary protection system is the blacklist. Users might find ripping or copying applications that are not on the list, or they might use a blacklisted application but rename its executable file to prevent the installer from recognizing it. Since there is no mechanism for updating the blacklist on existing CDs, they will gradually become easier to rip and copy as new applications not on the blacklist come into widespread use. Application developers may also adapt their software to the blacklisting technique by randomizing their process image names or taking other measures to avoid detection. [Footnote: An extreme extension of this would be to adopt rootkit-like techniques to conceal the copying application’s presence, just as XCP hides its active protection software.]

MediaMax Temporary Protection

The MediaMax system employs a different—and highly controversial, if not illegal—temporary protection measure. It defends the music while the installer is running by installing, and at least temporarily activating, the active protection software before displaying the EULA. The software is installed without obtaining consent, and it remains installed (and in some cases, permanently active) even if the user explicitly denies consent by declining the license agreement. This practice is uncomfortably close to the behavior of spyware and
may be illegal.

Prior to license acceptance, both MediaMax version 3 and version 5 discs install the active protection driver. (At this writing, version 5 is the current version. To our knowledge, there was no version 4.) The driver file sbcphid.sys is copied to the Windows drivers directory, configured as a service in the registry, and launched. Initially, the driver’s startup type is set to “Manual,” so it will not re-launch the next time the computer boots; however, it remains running until the computer is shut down and remains installed permanently. Albums that use MediaMax version 5 additionally install components of the MediaMax player software before displaying a license agreement—almost 12 megabytes of programs and data that are stored in %programfiles%Common FilesSunnComm Shared. These files are not removed if the EULA is declined.

Even more troublingly, under some common circumstances the MediaMax installer will permanently activate the active protection software (by setting its startup type to “Auto,” which causes it to be launched every time the computer boots). This behavior is related to a mechanism in the installer apparently intended to upgrade the active protection software if an older version is already installed. Under the following scenarios, it is triggered even if the user previously declined the EULA:

  • The user inserted a CD-3 (older version of MediaMax) album, then sometime later inserts an MM-5 (current version of MediaMax at this writing) album.
  • The user inserted an MM-5 album, then sometime later inserts a CD-3 album.
  • The user inserted an MM-5 album, reboots, then sometime later inserts the same album or another MM-5 album.

These steps do not have to take place in a single session. They can happen over a period of weeks or months, as users purchase new albums.

We can think of two possible explanations for this behavior. Perhaps the vendor, SunnComm, did not test these scenarios to determine what their software did, and so did not realize that they were activating the software without consent. Or perhaps they did know what would happen in these cases and deliberately chose these behaviors. Either possibility is troubling, indicating either a badly deficient design and testing procedure or a deliberate decision to install software after the user denied permission to do so.

Even if poor testing is the explanation for activating the software without consent, it is clear that SunnComm deliberately chose to install the MediaMax software code on the user’s system even if the user did not consent. These decisions are difficult to reconcile with the ethical and legal requirements on software companies. But they are easy to reconcile with the vendor’s platform building strategy, which rewards the vendor for placing its software on as many computers as possible.

Even the activation of temporary protection software before the user consents to anything raises troubling ethical questions. It is hard to argue that the user has consented to loading and running software merely by the act of inserting the disc. Most users do not expect the insertion of a compact disc to load software, and although many (but not all) of the affected discs did contain a statement about protection software being on the discs, the statements generally were confusingly worded, were written in tiny print, and did not say explicitly that software would install or run immediately upon insertion of the disc. Some in the record industry argue that the industry’s need to block potential infringement justifies the short-term execution of the temporary protection software on every user’s computer. We think this issue deserves more ethical and legal debate.

Passive Protection

Another way to prevent copying before active protection software is installed is to use passive protection measures. Passive protection exploits subtle differences between the way computers read CDs and the way ordinary CD players do. By changing the layout of data on the CD, it is sometimes possible to confuse computers without affecting ordinary players. In practice, the distinction between computers and CD players is less precise. Older generations of CD copy protection, which relied entirely on passive protection, proved easy to copy in some computers and impossible to play on some CD players [citation]. Furthermore, computer hardware and software has tended to get better at reading the passive protected CDs over time as it became more robust to all manner of damaged or poorly formatted discs. For these reasons, more recent CD DRM schemes rely mainly on active protection.

XCP uses a mild variety of passive protection as an added layer of security against ripping and copying. This form of passive protection exploits a quirk in the way Windows handle multisession CDs. When CD burners came to market in the early 1990s, the multisession CD format was introduced to allow data to be appended to partially recorded discs. (This was especially desirable at a time when recordable CD media cost tens of dollars per disc.) Each time data is added to the disc, it is written as an independent series of tracks called a session. Multi-session compatible CD drives see all the sessions, but ordinary CD players, which generally do not support the multisession format, recognize only the first session.

Some commercial discs use a variant of the multisession format to combine CD audio and computer accessible on a single CD. These discs adhere to the Blue Book [citation] or “stamped multisession” format. According to the Blue Book specification, stamped multisession discs must contain two sessions: a first session with 1–99 CD audio tracks, and a second session with one data track. The Windows CD audio driver contains special support for Blue Book discs. It presents the CD to playing and ripping applications as if it was a normal audio CD. Windows treats other multisession discs as data-only CDs.

XCP discs deviate from the Blue Book format by adding a second data track in the second session. This causes Windows to treat the disc as a regular multisession data CD, so the primary data track is mounted as a file system, but the audio tracks are invisible to player and ripper applications that use the Windows audio CD driver. This includes Windows Media Player, iTunes, and most other widely used applications.

Using a specialized procedure, it is possible to create discs with this flavor of passive protection with standard CD burning hardware and software [citation].

Limitations

This variety of passive protection provides only limited resistance to ripping and copying. There are a number of well-known methods for defeating it. Advanced ripping and copying applications avoid the Windows CD audio driver altogether and issue MMC commands [citation] directly to the drive. This allows programs such as Nero [citation] and Exact Audio Copy [citation] to recognize and read all the audio tracks. Non-Windows platforms, including Mac and Linux systems, read multisession CD more robustly and don’t suffer from the limitation that causes ripping problems on Windows. The felt-tip marker trick can also defeat this kind of passive protection, as noted above.

CD DRM: Unauthorized Deactivation Attacks

Alex and I are working on an academic paper, “Lessons from the Sony CD DRM Episode”, which will analyze several not-yet-discussed aspects of the XCP and MediaMax CD copy protection technologies, and will try to put the Sony CD episode in context and draw lessons for the future. We’ll post the complete paper here next week. Until then, we’ll post drafts of a few sections here. We have two reasons for this: we hope the postings will be interesting in themselves, and we hope your comments will help us improve the paper.

Today’s section is part of the technical core of the paper.

Please note that this is a draft and should not be formally quoted or cited. The final version of our entire paper will be posted here when it is ready.

Unauthorized Deactivation Attacks

As described previously, active protection methods rely on installing and running software components that interfere when ordinary software tries to access the disc. If an adversary can remove or deactivate the active protection software, then the DRM scheme will fail to prevent arbitrary use or ripping of the music on the disc. In this section we discuss such deactivation attacks.

One attack strategy is to manually deactivate or uninstall the active protection software. This can be done by using standard system administration tools, which are designed to find, characterize, and control the programs installed on a machine. This attack is very difficult to stop if the user has system administrator privileges on the machine.

Deactivating MediaMax

The MediaMax active protection software is simple to deactivate since it is comprised of a single device driver with a consistent service name, sbcphid. The driver can be removed by using the Windows command sc delete sbcphid to stop the driver, and then removing the sbcphid.sys file containing the driver code. Once the driver is deactivated, MediaMax-protected albums can be accessed as if they were unprotected.

Defenses Against Deactivation

To counter these deactivation attacks, a vendor might try to use technical tricks to evade detection and frustrate removal of the active protection software. The best example of this kind of defense is the rootkit-like behavior of XCP, famously discovered by Mark Russinovich [citation], as described earlier.

When XCP installs its active protection software, it also installs a second program—the rootkit—that hides the software’s presence. Specifically, it conceals any file, process, or registry key with a name that begins with the prefix $sys$. The result is that XCP’s main installation directory, and most of its registry keys, and almost all of its individual files and processes are made invisible to normal programs and administration tools.

The rootkit is installed as a kernel-level driver named $sys$aries and set to automatically load early in the boot process. When the rootkit starts, it hooks several Windows system calls by modifying the system service dispatch table, the kernel’s KeServiceDescriptorTable structure. This structure is an array of pointers to the kernel functions that implement basic system calls. The rootkit changes five of these addresses so that they point to functions within the rootkit. When an application invokes one of these patched system calls, it is handled by the rootkit instead of the original function in the kernel. The rootkit calls the real kernel function with the same parameters and filters the results before returning them to the application.

The system calls intercepted by the rootkit are:

  • NtQueryDirectoryFile – This function is used to list the contents of a directory; the rootkit version filters out directory entries that begin with $sys$, rendering such files and directories invisible to applications.
  • NtCreateFile – This call is used for creating and opening files. The rootkit version returns an invalid filename error when programs attempt to open existing files with names starting with $sys$, protecting XCP’s files from reading or writing by other programs.
  • NtQuerySystemInformation – One use of this function is to obtain a list of running processes. The rootkit filters out any processes with names prefixed by $sys$, making them invisible to other applications.
  • NtEnumerateKey – This function returns a list of the subkeys of a registry key. The rootkit filters the results to remove subkeys with names starting with $sys$. Note that it does not conceal individual fields within the registry (“values” in Windows parlance) with names starting with $sys$.
  • NtOpenKey – This function opens a registry key for reading or modifying. The rootkit intercepts this function call but does not alter its behavior. Its authors may have intended to restrict access to hidden registry keys in the same way that the hooked NtQueryDirectoryFile call restricts access to hidden files, but for some reason they did not ship a working implementation of this behavior.

The rootkit begins each intercepted function by checking the name of the calling process. If the process’s name begins with $sys$, the rootkit returns the results of the real kernel function without alteration. This allows XCP’s own processes to bypass the rootkit’s filters for a complete view of the system.

The XCP rootkit increases users’ vulnerability to many kinds of attacks because it can be used to hide arbitrary software, not just XCP. The rootkit is indiscriminate about what it conceals—any files, registry keys, or processes with names beginning in $sys$ will be hidden. Spyware and malware authors can leverage this functionality on systems where the rootkit is installed. This saves attackers the trouble of installing their own rootkits, but more importantly, it gives them access to a rootkit in situations where they would not be able to install one themselves because of the system’s security policies.

Only kernel-level processes are allowed to patch the Windows system service dispatch table, and only privileged users—normally, members of the Administrators or Power Users groups—are allowed to install such processes. (XCP iteslf requires these privileges to install.) Malicious code executed by an unprivileged user wouldn’t normally be allowed to install a rootkit that intercepted system calls in the kernel. However, if the XCP rootkit is installed, its cloaking behavior applies to all users regardless of their security privileges. Unprivileged malware can adopt the $sys$ prefix to become invisible to both privileged and unprivileged users. This privilege escalation attack has already been exploited by at least two Trojan horses discovered in the wild [citations].

Another privilege escalation attack facilitated by the XCP rootkit allows an unprivileged application to crash the system. Russinovich demonstrated this problem using an automated testing program he created called NTCRASH2 [citation]. This utility makes repeated system calls with randomly generated invalid parameters. The original Windows kernel functions handle invalid inputs correctly and the system remains stable, but with the XCP rootkit installed, certain invalid inputs result in a system crash.

We investigated the specific circumstances when these crashes occur. The rootkit’s implementation of NtCreateFile can cause a crash if it is passed an invalid pointer as its ObjectAttribute argument, or if it is passed a valid ObjectAttributes structure that points to a ObjectName structure with an invalid Buffer pointer. We do not believe that an attacker could exploit these flaws to execute code; however, they do allow an unprivileged user to bring the system to a halt. As Russinovich and other have pointed out, these problem illustrates the security danger of installing software in secret. Users experiencing system instability due to these rootkit bugs would have great difficulty diganosing the problem, since they likely would be unaware of the rootkit’s presence.

Deactivating XCP

Deactivating XCP’s active protection software is more complicated because it is comprised of a number of processes that are more deeply entagled in the system configuration, and because these files are hidden by the XCP rootkit. Deactivation can be accomplished by a three-step procedure.

The first step is to deactivate and remove the rootkit. This is the same procedure used to deactivate MediaMax. The only change is that the driver’s name is aries.sys. Disabling the rootkit and then rebooting the system exposes the previously hidden files, registry entries, and processes (ones with names prefixed with $sys).

The second step is to edit the system registry to remove references to XCP’s filter drivers and CoDeviceInstallers. XCP uses the Windows filter driver facility to intercept commands to the CD drives and IDE bus. If these filter drivers are not removed, the CD and IDE device drivers will fail to initialize after the program files for the filter drivers are deleted. This can cause the CD drives to malfunction, or, worse, cause the system to fail to boot because the IDE device driver is disabled. XCP’s filter drivers can be neutralized by editing the
Windows Registry to remove any reference to a driver named $sys$cor from any registry entries named UpperDrivers or LowerDrivers. The CoDeviceInstallers can be neutralized by removing any lines containing $sys$caj from any list of CoDeviceInstallers.

The third step is to delete the XCP services and remove the XCP program files. Services named $sys$lim, $sys$oct, cd_proxy, $sys$drmserver, and $sys$cor can be deactivated using the sc delete command, and then files named crater.sys, lim.sys, oct.sys, $sys$cor.sys, $sys$caj.dll, and $sys$upgtool.exe can be deleted. After the system is rebooted, the two remaining files, named CDProxyServ.exe and $sys$DRMServer.exe can be removed.

After performing these steps, XCP will now deactivated, and only the passive protection on XCP CDs will continue to be in force. Of course, these steps could easily be automated, creating a point-and-click tool for removing XCP.

CD DRM as Spyware

Tactics like the rootkit function, and the engineering of programs so that removal attempts can system instabilty, iare often used by spyware programs. That active DRM systems would be drawn to the same tactics as spyware should come as no surprise, as the two have the same goal: to prevent a user from removing unwanted software. In both cases, the user wants to remove the software (if he can find it) because the software provides no value to the user and can only harm him.

These tactics harm users, primarily by undermining users’ ability to manage their computers. If users lose track of which programs are running on their computers, they lose the opportunity to remove or patch programs that are malfunction and to remove unneeded programs. Maintaining a secure configuration is difficult already, and spyware tactics make it even more difficult. Though it is not surprising that spyware tactics would have attraction for DRM designers, it was a bit surprising that mass-market DRM vendors chose to use those tactics despite the risk of harming users. If only one vendor had chosen to use such tactics, we could write it off as an aberration. But two vendors made that choice, which is probably not a coincidence.

We suspect that the explanation may lie in the DRM vendors’ platform building strategy, which relies on keeping the software installed on as many computers as possible, coupled with the risk tolerance of DRM startup companies. The vendors may not have realized the extent of damage they could be causing, but they must have known that they were doing some harm. Our hypothesis is that the vendors allowed the lure of platform building to override the risk to users.

Authorized Uninstallers

Once users began to complain about the spyware-like behavior of the XCP and MediaMax software, the vendors offered access to uninstallers that would remove their software from users’ systems. Uninstallers had been available previously, but they were very difficult to obtain. For example, to get the original XCP uninstaller, a user had to fill out an online form involving some personal information, then wait a few days for a reply email, then fill out another online form and install some software, then wait a few days for yet another email, and then finally click a URL in the last email. We can think of no explanation for the complexity of this procedure, other than a desire to deter users from uninstalling the software.

The uninstallers, when a user did succeed in getting one of them, did not behave like ordinary software uninstallers. Normally an uninstaller is a standalone program that the user runs, either by double-clicking it or by using a system-provided user interface to designate the program to be removed. One advantage of ordinary uninstallers is that they can be acquired and used by any user who has the software.

The first XCP uninstaller did not work this way. Instead, the uninstaller was customized for each user, so that it would work only for a limited time and only on the computer on which the user had filled out the second form. This meant, for example, that if a user uninstalled the XCP software but it got reinstalled later—as might happen if the user inserted an XCP-bearing CD—the user could not use the same uninstaller again but would have to go through the entire process again to request a new one.

Customizing the uninstaller in this way is more difficult for the vendor and increases customer support costs, compared to a more traditional uninstaller, so a rational vendor would not do it unless there was some benefit. Most likely, the benefit is to the vendor’s platform building strategy, which takes a step backward every time a user uninstalls the vendor’s software. Customizing the uninstaller allows the vendor to contol who receives the uninstaller and to change the terms under which it is delivered in the future.

As user complaints mounted, Sony-BMG announced that unrestricted uninstallers for both XCP and MediaMax would be made available to all users from the vendors’ web sites. Both vendors chose to make these uninstallers available as ActiveX controls accessed via a web site.

By an unfortunate coincidence, both uninstallers turned out to open the same serious vulnerability on any computer where they were used.

MediaMax Uninstaller Vulnerability

The MediaMax uninstaller employed a proprietary ActiveX control called AxWebRemove.ocx created and signed by MediaMax author SunnComm. When users visited the MediaMax uninstaller web page, Internet Explorer prompted them to install the control. Then the web page invoked one of the control’s methods to uninstall MediaMax. This method, Remove, took two parameters: key, and validate_url. The key parameter was a single-use code provided by MediaMax technical support, and the validate_url parameter specified a web page that would validate the key and deliver executable code to perform the actual uninstallation.

When Remove is called from the web page, is issues an HTTP GET request to the provided url to validate the key. If it is valid, the server responds with the message true, {uninstall_url, where uninstall_url is the URL of a DLL file containing code to uninstall MediaMax. The control retries this DLL file from the Internet and saves it to a temporary location, then calls a function in the DLL named ECF7() to perform the uninstallation. If the function returns sucess, the control issues a second HTTP GET request to validate_url to report that the uninstall was sucessful and that the single-use key should be retired.

This design is vulnerable because the control accepts an arbitrary validate_url parameter and does not check that the DLL specified by the key validation server is authentic. The ActiveX control is not itself removed during the uninstallation process, so its methods can be invoked later by any web page without further browser security warnings. A attacker can create a web page that invokes the Remove method and provides a validate_url pointing to a page under the attacker’s control. This page can return an uninstall_url pointing to a DLL created by the attacker. When the MediaMax control executes the uninstall function in this file, arbitrary attacker code will execute on the user’s machine.

XCP Uninstaller Vulnerability

The XCP uninstaller contains the same design flaw and is only slightly more difficult to exploit. XCP’s ActiveX-based uninstaller invokes a proprietary ActiveX control named CodeSupport.ocx. (Early versions of XCP’s rootkit removal patch utilized the same control.) Usually this control is installed when users perform the second step in the three-step XCP uninstall process. In this step, the user is prompted to explain why they are requesting to uninstall XCP. The user’s response is sent to an XCP server along with a pseudorandom code generated by the ActiveX control. The same code is written to the system registry. Eventually the user receives an email with a link to another web page that uses the ActiveX control to remove XCP, but only after verifying that the code sent with the request matches the code in the local system registry. This check ensures that the uninstaller is only used on the machine from which the uninstallation request was made. As a consequence of this design, the control may be present on a user’s system even if she never performed the step in the uninstallation process where XCP is removed.

Matti Nikki first noted that the XCP ActiveX control contains some suspiciously-named methods, including InstallUpdate(url), Uninstall(url), and RebootMachine() [citation]. He demonstrated that the control remained installed after the XCP uninstallation was complete, and that its methods (including one that restarted the computer) were scriptable from any web page without further browser security warnings.

We found that the InstallUpdate and Uninstall methods have an even more serious flaw. Each takes as an argument a URL pointing to a specially formatted archive that contains updater or uninstaller code and data files. When these methods are invoked, the archive is retrieved from the provided URL and stored in a temporary location. For the InstallUpdate() method, the ActiveX control extract from the archive a file named InstallLite.dll and calls a function in this DLL named InstallXCP().

Like the MediaMax ActiveX control, the XCP control does not validate the download URL or the downloaded archive. The only barrier to using the control to execute arbitrary code is the proprietary format of the archive file. We determined the format by disassembling the control. The archive file consists of several blocks of gzip-compressed data, each storing a seperate file and preceded with a short header. At the end of the archive, a catalog structure lists metadata for each of the blocks, including a 32-bit CRC. The control verifies this CRC before executing code from the DLL.

With knowledge of this file format, we constructed an archive containing sample (benign) exploit code. The most difficult detail was the CRC, which is computed with an apparently proprietary algorithm that proved tedious to reverse engineer. We saved the trouble by having the ActiveX control compute the CRC for us. The control checks the CRC by computing a CRC for the file data in the archive and verifying that it matches the CRC specified in the archive catalog. We inserted a break point where the comparison occurs and ran the control on an archive containing code we prepared. We then took the CRC computed by the control and placed it in the archive catalog. Thus modified, the archive passed the CRC check and the ActiveX control executed our code. (This illustrated why digital signatures, rather than CRCs, must be used to validate code from untrusted sources.)

This procedure would allow a malicious web site to execute arbitrary code on the user’s machine. Like the MediaMax uninstaller flaw, it is especially dangerous because users who have completed the uninstallation may not be aware that they are still vulnerable.

Obviously, these vulnerabilities could have been prevented by careful design and programming. But they would not have been possible at all if not for the decision to deliver the uninstallers via this ActiveX method rather than using an ordinary download. We conjecture that the vendors chose to use ActiveX in this way because they wanted to retain the ability to rewrite, modify, or cancel the uninstaller later, and that this desire was driven at least in part by the vendors’ platform building strategy.

Summary of Deactivation Attacks

When all is said and done, there is little a CD DRM vendor can do to stop users from deactivating active protection software. A user can do this via ordinary security and system administration tools; attempts by the vendor to interfere with these tools are harmful and will trigger a strong backlash from users. In practice, vendors will probably have to provide some kind of uninstaller – users will insist on it, and some users will need it to deal with the bugs and incompatibilities that crop up occasionally in any complex software. Once an uninstaller is released, users will be able use it to remove the DRM software. Ultimately, determined users will be able to keep CD DRM software off their machines.