This post is by , , and J. Alex Halderman.
Absolute Manage is a remote administration program that allows sysadmins to supervise and maintain client computers over the Internet. It has been in the news since early February, when Lower Merion School District in Pennsylvania was alleged to be using it to spy on students at home via their laptop webcams. The story took a new twist last Thursday, when Threat Level reported that researchers at Leviathan Security Group had discovered serious vulnerabilities in the program. These problems let attackers carry out a number of exploits, including installing malware or running other arbitrary code on the students’ laptops. The major limitation in the reported attacks is that the bad guy needs to be on the same local network as the victim, and the program’s developers, Absolute Software, says it’s a largely theoretical threat.
Unfortunately, the security problems are worse than has been reported so far, and are far from theoretical. In fact, any machine with a public IP address running Absolute Manage can be taken over by attackers anywhere on the Internet. Such an attacker can command the machine to run arbitrary code, steal data, or take photographs using the computer’s camera.
We have been investigating Absolute Manage for several months, hoping to gain a better understanding of the security measures it employs to protect users. We are disclosing this information now because, following the Threat Level post, we believe it’s only a matter of time until real attackers discover it. Users need to be aware of the vulnerabilities and take proper measures to protect themselves.
The security issues revolve around the way Absolute Manage encrypts commands sent to the clients. The software has two parts: the Absolute Manage Agent, which runs on client machines, and the Absolute Manage Server, which tracks clients under its supervision and sends commands to perform remote operations like installing new software. The clients and server exchange messages using a TCP-based protocol. Clients continuously listen for messages containing instructions from the server, and they also periodically send “heartbeat” messages to the server to deliver status updates and pull down any queued commands.
The programs compress and encrypt each message before transmitting it. They use an encryption algorithm called Blowfish, which is a credible, if outmoded, choice. The problem is how the keys are managed. Strong encryption protocols like SSL negotiate a new secret key for each communication session, but Absolute Manage uses the same hard coded key every time, in every client. Blowfish can use variable-sized keys from 1-56 bytes, and the programmers decided to use secret textual phrases for the keys. We know what the keys are. We won’t publish them here, but they were easy to figure out by inspecting the program files.
Using hard coded keys neutralizes the benefits of cryptography. Since the same, easy-to-discover key is used in every client, it’s straightforward for an attacker to unwrap the encrypted messages, modify them, or encrypt messages of his own. This problem is very similar to the broken cryptography in Diebold voting machines that Ari, Alex, and Ed discovered years ago. Diebold also used a simple fixed key, which allowed an attacker with access to one machine to learn the key and attack all the other machines.
The broken cryptography in Absolute Manage enables a variety of attacks. For instance, an attacker can eavesdrop on a command sent from the server and rewrite it to issue a new command that installs and executes malicious code. Or, he can act as a man-in-the-middle between the client and server and insert evil commands in response to a client heartbeat message. An attacker could easily do these by sniffing wireless LAN traffic, as in school and office environments where Absolute Manage is often deployed. These seem to be the attacks referred to in the Threat Level post.
Exploitable from Anywhere
The limitation of these attacks is that the bad guy usually needs to be on the same physical network as the victim. However, we discovered that potentially more dangerous attacks are possible. In these attacks, a bad guy anywhere on the Internet can exploit any Absolute Manage client with a publicly reachable IP address.
Here’s an example of a message from the server to the client, with the encryption and compression removed. The client tries to authenticate the command using a parameter called the SeedValue. This value is provided by the server when a client initially attempts to contact it after booting. After that, the client requires the SeedValue to be the same in subsequent commands. The client basically ignores all of the other parameters that look like they would be hard to guess, so the SeedValue is the only thing that makes it difficult for the attacker to generate his own command messages from whole cloth.
In our example message, the SeedValue is:
It turns out that it is encrypted using a second hard-coded textual phrase. Decrypting it yields the following bytes:
00 00 00 00 e0 03 10 03 40 03 00 00 31 00 34 00 37 00 35 00 00 00 00 00 00
The length is misleading; the value is actually just a 16-bit unicode encoding of a 7-digit number, 1401475. This is the server’s “serial number,” which was provided by Absolute Software along with the product activation key when we purchased our license.
Thus, an attacker who wants to send arbitrary commands to Absolute Manage clients just needs to figure out the server’s serial number. One way he can do that is to guess it. The attacker could try contacting the client with different values until one of them turns out to be correct. If all serial numbers are 7 digits (like ours) or less, and there is no pattern to there assignment, then an attacker can guess among 10 million possibilities. If there is a pattern (the likely case) then the attacker’s job may be much easier. Our tests show that we can make more than 330 guesses/second over a fast network link, so even assuming no pattern an attacker could expect to succeed after about four hours of guessing. Each server uses the same serial number for all its clients, so after the attacker guesses it for one client, he can compromise all the server’s other clients without any additional guesswork.
Brute forcing the server’s serial number is one method attackers can use, but there is a much more efficient attack for targeting a large set of clients: the server will tell the correct SeedValue to any client that asks. If the attacker knows the IP address of the server a client is trying to contact, he can just impersonate a freshly-booted client and ask the server to send him the correct SeedValue. The server will respond with all the information the attacker needs to impersonate the server.
A bad guy could extend this method to target all Absolute Manage clients in one attack. He could scan the entire Internet address space to discover all hosts running Absolute Manage Server and build a list of active SeedValues. (Servers generally run on public IP addresses so that they can receive status updates from clients that are away from the local network.) Such a scan would take only a few days. The attacker could then do a second Internet-wide scan to discover Absolute Manage Clients. For each of them, he would need only a few seconds to try all the active SeedValues from his list and determine the correct one. This attack could be exploited to quickly install and run malicious code on all computers running the Absolute Manage client on publicly accessible IP addresses.
Defenses and Lessons
In the short term, users can protect themselves by uninstalling the Absolute Manage client. This might be difficult on machines with privileges locked down, so system administrators will need to help.
(Attempts to work around the problem, such as firewalling the server so it can’t be found by an Internet scan, may backfire. If the server is unreachable from outside the firewall, clients that are rebooted away from the local network will be unable to obtain a SeedValue. In this situation, the clients insecurely default to accepting arbitrary commands without even the protection of a SeedValue.)
In the long term, the solution is for Absolute Manage to adopt serious cryptographic authentication. Absolute Software says they will do this in the next version later this summer–let’s hope they get it right next time.
Remote administration products like Absolute Manage carry large risks because they intentionally create a mechanism for a remote third party to take control of the machine. This can be powerful in the right hands but devastating if exploited by attackers. There will always be a risk of abuse by authorized parties, as alleged in the students’ lawsuit against Lower Merion School District, but correctly designed technology should at least prevent unauthorized third-party attacks by making sure only authorized parties can issue commands. This requires getting authentication right–exactly what Absolute Manage failed to do.
Because of these dangers, remote administration software should be designed defensively, minimizing the risk even if the authentication fails. For example, it could only allow installation of signed binaries, or it could give users prominent notification before actions are taken so that attacks can be more easily detected.
The blatant vulnerabilities in Absolute Manage suggest that this kind of remote administration software requires greater security scrutiny. We will further discuss the problems and the lessons they carry in a forthcoming paper.