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Side-Channel Leaks in Web Applications

March 23, 2010 by

Popular online applications may leak your private data to a network eavesdropper, even if you’re using secure web connections, according to a new paper by Shuo Chen, Rui Wang, XiaoFeng Wang, and Kehuan Zhang. (Chen is at Microsoft Research; the others are at Indiana.) It’s a sobering result — yet another illustration of how much information can be leaked by ordinary web technologies. It’s also really clever.

Here’s the background: Secure web connections encrypt traffic so that only your browser and the web server you’re visiting can see the contents of your communication. Although a network eavesdropper can’t understand the requests your browser sends, nor the replies from the server, it has long been known that an eavesdropper can see the size of the request and reply messages, and that these sizes sometimes leak information about which page you’re viewing, if the request size (i.e., the size of the URL) or the reply size (i.e., the size of the HTML page you’re viewing) is distinctive.

The new paper shows that this inference-from-size problem gets much, much worse when pages are using the now-standard AJAX programming methods, in which a web “page” is really a computer program that makes frequent requests to the server for information. With more requests to the server, there are many more opportunities for an eavesdropper to make inferences about what you’re doing — to the point that common applications leak a great deal of private information.

Consider a search engine that autocompletes search queries: when you start to type a query, the search engine gives you a list of suggested queries that start with whatever characters you have typed so far. When you type the first letter of your search query, the search engine page will send that character to the server, and the server will send back a list of suggested completions. Unfortunately, the size of that suggested completion list will depend on which character you typed, so an eavesdropper can use the size of the encrypted response to deduce which letter you typed. When you type the second letter of your query, another request will go to the server, and another encrypted reply will come back, which will again have a distinctive size, allowing the eavesdropper (who already knows the first character you typed) to deduce the second character; and so on. In the end the eavesdropper will know exactly which search query you typed. This attack worked against the Google, Yahoo, and Microsoft Bing search engines.

Many web apps that handle sensitive information seem to be susceptible to similar attacks. The researchers studied a major online tax preparation site (which they don’t name) and found that it leaks a fairly accurate estimate of your Adjusted Gross Income (AGI). This happens because the exact set of questions you have to answer, and the exact data tables used in tax preparation, will vary based on your AGI. To give one example, there is a particular interaction relating to a possible student loan interest calculation, that only happens if your AGI is between $115,000 and $145,000 — so that the presence or absence of the distinctively-sized message exchange relating to that calculation tells an eavesdropper whether your AGI is between $115,000 and $145,000. By assembling a set of clues like this, an eavesdropper can get a good fix on your AGI, plus information about your family status, and so on.

For similar reasons, a major online health site leaks information about which medications you are taking, and a major investment site leaks information about your investments.

The paper goes on to consider possible mitigations. The most obvious mitigation is to add padding to messages so that their sizes are not so distinctive — for example, every message might be padded to make its size a multiple of 256 bytes. This turns out to be less effective than you might expect — significant information can still leak even if messages are generously padded — and the padded messages are slower and more expensive to transmit.

We don’t know which sites the researchers studied, but it seems like a safe bet that most, if not all, of the sites in these product categories have similar problems. It’s important to keep these attacks in perspective — bear in mind that they can only be carried out by someone who can eavesdrop on the network between you and the site you’re visiting.

It’s becoming increasingly clear that securing web-based applications is very difficult, and that the basic tools for developing web apps don’t do much to help. The industry, and researchers, will be struggling with web app security issues for years to come.

Filed Under: Privacy & Security Tagged With: ,

Netflix Cancels the Netflix Prize 2

March 12, 2010 by

Today, Netflix announced it is canceling its plans for a second Netflix Prize contest, one that reportedly would have involved the release of more information than the first. As I argued earlier, I feared that the new contest would have put the supposedly private movie viewing and rating habits of Netflix customers at great risk, and I applaud Netflix for making a very responsible decision. No doubt, pressure from the private lawsuit and FTC investigation helped Netflix make up its mind, and both are reportedly going away as a result of today’s action.

Filed Under: Privacy & Security Tagged With:

A Free Internet, If We Can Keep It

January 28, 2010 by

“We stand for a single internet where all of humanity has equal access to knowledge and ideas. And we recognize that the world’s information infrastructure will become what we and others make of it. ”

These two sentences, from Secretary of State Clinton’s groundbreaking speech on Internet freedom, sum up beautifully the challenge facing our Internet policy. An open Internet can advance our values and support our interests; but we will only get there if we make some difficult choices now.

One of these choices relates to anonymity. Will it be easy to speak anonymously on the Internet, or not? This was the subject of the first question in the post-speech Q&A:

QUESTION: You talked about anonymity on line and how we have to prevent that. But you also talk about censorship by governments. And I’m struck by – having a veil of anonymity in certain situations is actually quite beneficial. So are you looking to strike a balance between that and this emphasis on censorship?

SECRETARY CLINTON: Absolutely. I mean, this is one of the challenges we face. On the one hand, anonymity protects the exploitation of children. And on the other hand, anonymity protects the free expression of opposition to repressive governments. Anonymity allows the theft of intellectual property, but anonymity also permits people to come together in settings that gives them some basis for free expression without identifying themselves.

None of this will be easy. I think that’s a fair statement. I think, as I said, we all have varying needs and rights and responsibilities. But I think these overriding principles should be our guiding light. We should err on the side of openness and do everything possible to create that, recognizing, as with any rule or any statement of principle, there are going to be exceptions.

So how we go after this, I think, is now what we’re requesting many of you who are experts in this area to lend your help to us in doing. We need the guidance of technology experts. In my experience, most of them are younger than 40, but not all are younger than 40. And we need the companies that do this, and we need the dissident voices who have actually lived on the front lines so that we can try to work through the best way to make that balance you referred to.

Secretary Clinton’s answer is trying to balance competing interests, which is what good politicians do. If we want A, and we want B, and A is in tension with B, can we have some A and some B together? Is there some way to give up a little A in exchange for a lot of B? That’s a useful way to start the discussion.

But sometimes you have to choose — sometimes A and B are profoundly incompatible. That seems to be the case here. Consider the position of a repressive government that wants to spy on a citizen’s political speech, as compared to the position of the U.S. government when it wants to eavesdrop on a suspect’s conversations under a valid search warrant. The two positions are very different morally, but they are pretty much the same technologically. Which means that either both governments can eavesdrop, or neither can. We have to choose.

Secretary Clinton saw this tension, and, being a lawyer, she saw that law could not resolve it. So she expressed the hope that technology, the aspect she understood least, would offer a solution. This is a common pattern: Given a difficult technology policy problem, lawyers will tend to seek technology solutions and technologists will tend to seek legal solutions. (Paul Ohm calls this “Felten’s Third Law”.) It’s easy to reject non-solutions in your own area because you have the knowledge to recognize why they will fail; but there must be a solution lurking somewhere in the unexplored wilderness of the other area.

If we’re forced to choose — and we will be — what kind of Internet will we have? In Secretary Clinton’s words, “the world’s information infrastructure will become what we and others make of it.” We’ll have a free Internet, if we can keep it.

Filed Under: Censorship & Filtering, Privacy & Security Tagged With: , , , ,

No Warrant Necessary to Seize Your Laptop

January 27, 2010 by

The U.S. Customs may search your laptop and copy your hard drive when you cross the border, according to their policy. They may do this even if they have no particularized suspicion of wrongdoing on your part. They claim that the Fourth Amendment protection against warrantless search and seizure does not apply. The Customs justifies this policy on the grounds that “examinations of documents and electronic devices are a crucial tool for detecting information concerning” all sorts of bad things, including terrorism, drug smuggling, contraband, and so on.

Historically the job of Customs was to control the flow of physical goods into the country, and their authority to search you for physical goods is well established. I am certainly not a constitutional lawyer, but to me a Customs exemption from Fourth Amendment restrictions is more clearly justified for physical contraband than for generalized searches of information.

The American Civil Liberties Union is gathering data about how this Customs enforcement policy works in practice, and they request your help. If you’ve had your laptop searched, or if you have altered your own practices to protect your data when crossing the border, staff attorney Catherine Crump would be interested in hearing about it.

Meanwhile, the ACLU has released a stack of documents they got by FOIA request.
The documents are here, and their spreadsheets analyzing the data are here. They would be quite interested to know what F-to-T readers make of these documents.

ACLU Queries for F-to-T readers:
If the answer to any of the questions below is yes, please briefly describe your experience and e-mail your response to laptopsearch at aclu.org. The ACLU promises confidentiality to anyone responding to this request.
(1) When entering or leaving the United States, has a U.S. official ever examined or browsed the contents of your laptop, PDA, cell phone, or other electronic device?

(2) When entering or leaving the United States, has a U.S. official ever detained your laptop, PDA, cell phone, or other electronic device?

(3) In light of the U.S. government’s policy of conducting suspicionless searches of laptops and other electronic devices, have you taken extra steps to safeguard your electronic information when traveling internationally, such as using encryption software or shipping a hard drive ahead to your destination?

(4) Has the U.S. government’s policy of conducting suspicionless searches of laptops and other electronic devices affected the frequency with which you travel internationally or your willingness to travel with information stored on electronic devices?

Filed Under: National Security & Surveillance, Privacy & Security Tagged With: ,

Another Privacy Misstep from Facebook

December 14, 2009 by

Facebook is once again clashing with its users over privacy. As a user myself, I was pretty unhappy about the recently changed privacy control. I felt that Facebook was trying to trick me into loosening controls on my information. Though the initial letter from Facebook founder Mark Zuckerberg painted the changes as pro-privacy — which led more than 48,000 users to click the “I like this” button — the actual effect of the company’s suggested new policy was to allow more public access to information. Though the company has backtracked on some of the changes, problems remain.

Some of you may be wondering why Facebook users are complaining about privacy, given that the site’s main use is to publish private information about yourself. But Facebook is not really about making your life an open book. It’s about telling the story of your life. And like any autobiography, your Facebook-story will include a certain amount of spin. It will leave out some facts and will likely offer more and different levels of detail depending on the audience. Some people might not get to hear your story at all. For Facebook users, privacy means not the prevention of all information flow, but control over the content of their story and who gets to read it.

So when Facebook tries to monetize users’ information by passing that information along to third parties, such as advertisers, users get angry. That’s what happened two years ago with Facebook’s ill-considered Beacon initiative: Facebook started telling advertisers what you had done — telling your story to strangers. But perhaps even worse, Facebook sometimes added items to your wall about what you had purchased — editing your story, without your permission. Users revolted, and Facebook shuttered Beacon.

Viewed through this lens, Facebook’s business dilemma is clear. The company is sitting on an ever-growing treasure trove of information about users. Methods for monetizing this information are many and obvious, but virtually all of them require either telling users’ stories to third parties, or modifying users’ stories — steps that would break users’ mental model of Facebook, triggering more outrage.

What Facebook has, in other words, is a governance problem. Users see Facebook as a community in which they are members. Though Facebook (presumably) has no legal obligation to get users’ permission before instituting changes, it makes business sense to consult the user community before making significant changes in the privacy model. Announcing a new initiative, only to backpedal in the face of user outrage, can’t be the best way to maximize long-term profits.

The challenge is finding a structure that allows the company to explore new business opportunities, while at the same time securing truly informed consent from the user community. Some kind of customer advisory board seems like an obvious approach. But how would the members be chosen? And how much information and power would they get? This isn’t easy to do. But the current approach isn’t working either. If your business is based on user buy-in to an online community, then you have to give that community some kind of voice — you have to make it a community that users want to inhabit.

Filed Under: Privacy & Security Tagged With: ,