Sep 29 2010

IE8 CSS-based forced tweeting

A few weeks back, I published a demo that uses a serious Internet Explorer cross-origin violation to permit a malicious web page to force the visitor to make unwarranted tweets:

http://seclists.org/fulldisclosure/2010/Sep/64

The post was light on technical details of how the attack works, so they will be filled in below. In addition, I'll quickly take care of the FAQ:

Q) Does this attack affect earlier versions of Internet Explorer, such as IE6?
A) Yes, the attack works verbatim on IE6 in my testing.

Q) When will a patch be available?
A) As far as I'm aware, Microsoft have not stated when users of IE6, IE7 and IE8
will be afforded protection. I'm not privy to any other details.

Q) Why drop this 0-day?
A) The rampant misuse of the term "0-day" is somewhat disappointing. You can look
it up on Wikipedia, but an "0-day" is a term reserved for a disclosure where the
vendor was previously unaware of it. In this instance, the bug is better described
as a "500+ day" if you look at when the bug was first publicly disclosed.

So how does the Twitter attack work?
When exploiting this bug in general, you are looking to meet various conditions:

  • Finding a text injection point in the target origin to start a CSS property.

  • Using a text injection context that does not escape vital CSS characters.

  • Arranging for some sensitive information (such as an anti-XSRF token) to appear after the text injection point.

  • Arranging that no CSS property terminator appears before the sensitive information.

In the Twitter attack, the injection point can clearly be seen; it's simply the text of a tweet in my "scarybeaststest" account:

http://twitter.com/scarybeaststest

If you look at the raw source HTML of that page on twitter.com, you'll see the following piece of HTML:

<span class="entry-content">{}body{font-family:"</span>

That's the start of a CSS descriptor injected into the Twitter page. Note that none of the characters {, }, : or " need to be escaped in the HTML tag value context -- there is no Twitter bug here.

The need for the " character in this injection context is subtle: it derives from how the IE CSS parser determines end-of-property. Essentially, IE is just looking for a ; character to denote the end of the value for the "font-family" property. Unluckily for us, the ; character appears in the Twitter HTML prior to the high-value anti-XSRF secret we are trying to steal. This would tend to terminate the injected CSS property early and spoil our day. Fortunately, ; is perfectly acceptable within a quoted section, so the use of " will ensure the stray ; characters appear between a pair of quotes and do not terminate the parse of our CSS property.

With our injection, the Twitter page ends up looking a little bit like:

<html>bla..bla..{}body{font-family:"..more HTML prop="value" some ; SECRET_XSRF_TOKEN blah EOF

The IE CSS parser keeps track of opening and closing quote pairs, and only terminates the CSS property if a ; is encountered outside of quoting. Our usage of a " character to start the property ensures that any ; characters appear within the quoted context. Eventually, the CSS parser includes the secret anti-XSRF token in the CSS property value and then hits EOF with an unterminated quote. IE happily recovers from this situation by constructing a CSS property value with all the text parsed so far!

So the attacker simply includes the above CSS-property-injected Twitter page as CSS and recovers the CSS value for the "font-family" property, which includes the secret token (as well as a bunch of quoted semi-colons and other HTML detritus which is less interesting to us).

There is quite a pile of IE CSS parsing bugs involved to pull this off. Many are CSS specification violations:

  1. Newlines permitted within quoted strings.
  2. Whitespace permitted outside quoted strings.
  3. Multiple quoted strings permitted in single property.
  4. End of file within quoted string does not invalidate descriptor.
  5. Bad Content-Type and non-CSS content ignored for cross-origin CSS.
As be seen, the interaction of CSS property terminator vs. quoting context is quite subtle. There might well be a simpler twitter.com injection to use, but this more complicated example is simply the first working one I found.
Sep 27 2010

Google Continues To Index Pages From SEO Poisoning Campaign

Google continues to include in its search index, pages from a major SEO poisoning campaign. SEO poisoning involves getting web pages listed in search engines that when accessed attempt to infect the computer with malware. We have repeatedly reported a listing of pages used by this campaign, that is available at http://www.getalllinks.info/links/0.txt, to Google using their page for reporting a malware page over a period of several months.  Google has continued to list these pages in its index leading to an unknown, but possibly large number of computers to become infected with malware . These page have also not been flagged as being malicious by Google’s Safe Browsing system during the period when they are most likely to be infect users computers.

Our recent experience has shown that public releasing the information get Google to respond, while there reporting mechanisms get ignored. We recently posted about Google providing hosting for files used in attempted hackings, after having reporting using their mechanism multiple times without any action being taken Google disabled the account the day after our posting.

Other companies have allowed this SEO poisoning campaign to continue, including The Planet who provides hosting for a critical component of the campaign.

Sep 20 2010

TA10-263A: Adobe Flash Vulnerabilities

Original release date: September 20, 2010
Last revised: --
Source: US-CERT

Systems Affected

  • Adobe Flash Player 10.1.82.76 and earlier versions
  • Adobe Flash Player 10.1.92.10 for Android
  • Adobe Reader 9.3.4 and earlier 9.x versions

Other Adobe products that support Flash may also be vulnerable.


Overview

According to Adobe Security Bulletin APSB10-22 there are vulnerabilities in Adobe Flash. These vulnerabilities affect Flash Player, Reader, and possibly other products that support Flash. A remote attacker could exploit these vulnerabilities to execute arbitrary code.


I. Description

Adobe Security Bulletin APSB10-22 describes vulnerabilities in Adobe Flash that affect Flash Player. These vulnerabilities may also affect other products that independently support Flash, such as Adobe Reader, Acrobat, Photoshop, Photoshop Lightroom, Freehand MX, and Fireworks.

An attacker could exploit these vulnerabilities by convincing a user to open specially crafted Flash content. Flash content is commonly hosted on a web page, but it can also be embedded in a PDF and other documents or provided as a stand-alone file.


II. Impact

If a user opens specially crafted Flash content, a remote attacker may be able to execute arbitrary code.


III. Solution

Update Flash

Adobe Security Bulletin APSB10-22 recommends updating to Flash Player 10.1.85.3 for Windows, Mac OS, Linux, and Solaris and Flash Player 10.1.95.1 for Android.  However, products like Adobe Reader with embedded Flash will require their own security updates.

To reduce your exposure to these and other Flash vulnerabilities, consider the following mitigation technique.

Disable Flash in your web browser

Uninstall Flash or restrict which sites are allowed to run Flash. To the extent possible, only run trusted Flash content on trusted domains. For more information, see Securing Your Web Browser.

Additional workarounds are available in US-CERT Vulnerability Note VU#275289.


IV. References



Feedback can be directed to US-CERT.


Produced 2010 by US-CERT, a government organization. Terms of use


Revision History

September 20, 2010: Initial release

Sep 17 2010

How to Hide Apache Banner

You should hide apache banner information from being displayed so the attackers are not aware of what version of Apache version you are running and thus making it more difficult for them to exploit any system holes and thus making vulnerability scanners work harder and in some cases impossible without knowing banner information.

Example:

Apache/2.2.0 (Ubuntu) PHP/5.2.6-2ubuntu4.6 with Suhosin-Patch mod_perl/2.0.4 Perl/v5.10.0 Server at www.webaddress.com Port 80

How To:

Modify /etc/httpd/conf/httpd.conf
Change the ServerSignature line to: ServerSignature Off
Change the ServerTokens line to: ServerTokens Prod
Restart Apache: /sbin/service httpd restart

For ubuntu you will find these setting in /etc/apache2/conf.d/security
Restart Apache: service apache2 restart