Tuesday, September 30th, 2008

Report and Case Study on CSRF

Category: Security

<>p>Bill Zeller and Ed Felten have published a report on Cross-Site Request Forgery attacks on popular Web sites:

We found four major vulnerabilities on four different sites. These vulnerabilities include what we believe is the first CSRF vulnerability that allows the transfer of funds from a financial institution. We contacted all the sites involved and gave them ample time to correct these issues. Three of these sites have fixed the vulnerabilities listed below, one has not.

CSRF vulnerabilities occur when a website allows an authenticated user to perform a sensitive action but does not verify that the user herself is invoking that action. The key to understanding CSRF attacks is to recognize that websites typically don’t verify that a request came from an authorized user. Instead they verify only that the request came from the browser of an authorized user. Because browsers run code sent by multiple sites, there is a danger that one site will (unbeknownst to the user) send a request to a second site, and the second site will mistakenly think that the user authorized the request.

The chaps share deatils on the following attacks:

1. ING Direct (ingdirect.com)

Status: Fixed

We found a vulnerability on ING’s website that allowed additional accounts to be created on behalf of an arbitrary user. We were also able to transfer funds out of users’ bank accounts. We believe this is the first CSRF vulnerability to allow the transfer of funds from a financial institution. Specific details are described in our paper.

2. YouTube (youtube.com)

Status: Fixed

We discovered CSRF vulnerabilities in nearly every action a user could perform on YouTube. An attacker could have added videos to a user’s “Favorites,” added himself to a user’s “Friend” or “Family” list, sent arbitrary messages on the user’s behalf, flagged videos as inappropriate, automatically shared a video with a user’s contacts, subscribed a user to a “channel” (a set of videos published by one person or group) and added videos to a user’s “QuickList” (a list of videos a user intends to watch at a later point). Specific details are described in our paper.

3. MetaFilter (metafilter.com)

Status: Fixed

A vulnerability existed on Metafilter that allowed an attacker to take control of a user’s account. A forged request could be used to set a user’s email address to the attacker’s address. A second forged request could then be used to activate the “Forgot Password” action, which would send the user’s password to the attacker’s email address. Specific details are described in our paper.

(MetaFilter fixed this vulnerability in less than two days. We appreciate the fact that MetaFilter contacted us to let us know the problem had been fixed.)

4. The New York Times (nytimes.com)

Status: Not Fixed. We contacted the New York Times in September, 2007. As of September 24, 2008, this vulnerability still exists.

A vulnerability in the New York Time’s website allows an attacker to find out the email address of an arbitrary user. This takes advantage of the NYTimes’s “Email This” feature, which allows a user to send an email about a story to an arbitrary user. This emails contains the logged-in user’s email address. An attacker can forge a request to active the “Email This” feature while setting his email address as the recipient. When a user visit’s the attacker’s page, an email will be sent to the attacker’s email address containing the user’s email address. This attack can be used for identification (e.g., finding the email addresses of all users who visit an attacker’s site) or for spam. This attack is particularly dangerous because of the large number of users who have NYTimes’ accounts and because the NYTimes keeps users logged in for over a year.

Also, TimesPeople, a social networking site launched by the New York Times on September 23, 2008, is also vulnerable to CSRF attacks. We hope the New York Times will decide to fix these vulnerabilities now that they have been made public.

And, what about mitigation?

Our paper provides recommendations for preventing these attacks. We provide a server-side plugin for the PHP MVC framework Code Igniter that can completely prevent CSRF. We also provide a client-side Firefox extension that can protect users from certain types of CSRF attacks (non-GET request attacks).

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Posted by Dion Almaer at 7:41 am
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I wonder how the provided solution will work. All I can see is that the attack is delayed a bit, the attacker needs the client to pick up the token first, before issueing the attack. That is, create two CSRF to inflict one.
This provideds no security at all. (Or am I missing something?)

The only solution I can think of (also proposed in the article about clickjacking) is to allow the pickup of the tokens only after the user proves he’s not a machine. Captcha’s and the like on every action. Although that will be to intrusive.

Comment by stim — October 1, 2008

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