ISC Diary

WMF FAQ

Published: 2006-01-07,
Last Updated: 2006-01-07 17:16:23 UTC
by Swa Frantzen (Version: 5)

This version has been updated to reflect the release of the official patch by Microsoft. Take care with the translations as they might very well not be up to date.

[a few users offered translations of this FAQ into various languages. Obviously, we can not check the translation for accuracy, nor can we update them. Most of these translations are hosted on servers operated by the translation authors. So use at your own risk: Deutsch and Deutsch (pdf), Catalan , Espa�ol , Italiana and Italiana, Polski, Suomenkielinen, Danish, Japanese, Slovenian, Chinese, Norwegian and Nederlands ]

To assist with internal presentations about this issue, we made a slide set available:
PDF, Power Point , OpenOffice 2.0

Why is this issue so important?

The WMF vulnerability uses images (WMF images) to execute arbitrary code. It will execute just by viewing the image. In most cases, you don't have click anything. Even images stored on your system may cause the exploit to be triggered if it is indexed by some indexing software. Viewing a directory in Explorer with 'Icon size' images will cause the exploit to be triggered as well. Microsoft initially announced that an official patch would not be available before January 10th 2006 (next regular update cycle). But they did release a patch out of cycle earlier.

Is it better to use Firefox or Internet Explorer?

Internet Explorer will view the image and trigger the exploit without warning. New versions of Firefox will prompt you before opening the image. However, in most environments this offers little protection given that these are images and are thus considered 'safe'.

What versions of Windows are affected?

Windows XP, (SP1 and SP2), Windows 2003 are affected by the currently circulating exploits. Other versions may be affected to some extent. Mac OS-X, Unix or BSD is not affected.

Note: If you're still running on Win98/ME, this is a watershed moment: we believe (untested) that your system is vulnerable and there will be no patch from MS. Your mitigation options are very limited. You really need to upgrade.

What can I do to protect myself?

We did, due to the expectional conditions, endorse an unofficial patch for a while, but are now recommending you install the official Microsoft supported patch ASAP.

The unofficial patch was made available by Ilfak Guilfanov. Our own Tom Liston reviewed the patch and we tested it. The reviewed and tested version is not available form us any longer (last version was 1.4, MD5: 15f0a36ea33f39c1bcf5a98e51d4f4f6), PGP signature (signed with ISC key) here. THANKS to Ilfak Guilfanov for providing the patch!!

We also advised the suggested mitigation action by Microsoft to unregister another DLL, due to the availability of an official patch there is no more need for that.

How do I re-register the DLL and remove the patch?

Important: You need to REBOOT the system before re-enabling the DLL. Malicious WMF images may still be sitting in memory waiting to be displayed. Our testing has shown, that they will be executed as soon as the DLL is re-enabled.

To re-register the DLL, click State, click Run, type
regsvr32 %windir%\system32\shimgvw.dll
This is the same command as you used to unregister, with the -u part).

To remove the patch, open the control pannel, open the "Add/Remove Programs" icon, find the patch in the list and uninstall.

To uninstall the patch from the command line (vs. using the Control Panel), enter this command:
msiexec.exe /X{E1CDC5B0-7AFB-11DA-8CD6-0800200C9A66} /qn

How does the unofficial patch work?

The wmfhotfix.dll is injected into any process loading user32.dll. The DLL then patches (in memory) gdi32.dll's Escape() function so that it ignores any call using the SETABORTPROC (ie. 0x09) parameter. This should allow Windows programs to display WMF files normally while still blocking the exploit. The version of the patch we distributed was carefully checked against the source code provided as well as tested against all known versions of the exploit. It would work on WinXP (SP1 and SP2) and Win2K. The documentation provided by Microsoft show the official patch does remove exactly the same functionality from gdi32.dll than the unofficial patch prevent from begin used.

Are there other patches?

ESET, the maker of the antivirus product NOD32, published another patch. We haven't gotten around to check it as thoroughly as Ilfan's patch and the official patch release pre-empted possible efforts to do much with it. The ESET patch claimed to work for older versions of Windows (e.g. ME). The ESET patch does not require a reboot. Please refer to the ESET site for any additional questions. Microsoft's official (preliminary) patch was leaked and posted to some web sites. Microsoft recommends against installing that version.

Is there a test to see if I am vulnerable?

We created an image that will start the windows calculator. See http:// sipr.net / test.wmf. This image should be detected by all up to date antivirus scanners and uses an easy to detect version of the exploit. Your antivirus scanner should detect this image. However, it detecting the image is not sufficient to prove that it will detect other versions of the exploit.

Would unregistering the DLL (without using the official or unofficial patch) protect me?

It might help. But it is not foolproof. We want to be very clear on this: we have some very stong indications that simply unregistering the shimgvw.dll isn't always successful. The .dll can be re-registered by malicious processes or other installations, and there may be issues where re-registering the .dll on a running system that has had an exploit run against it allowing the exploit to succeed. In addition it might be possible for there to be other avenues of attack against the Escape() function in gdi32.dll. We strong recommend to install the official patch.

Should I just delete the DLL?

It might not be a bad idea, but Windows File Protection will probably replace it. You'll need to turn off Windows File Protection first. Also, once an official patch is available you'll need to replace the DLL. (renaming, rather than deleting is probably better so it will still be handy).

Should I just block all .WMF images?

This may help, but it is not sufficient. WMF files are recognized by a special header and the extension is not needed. The files could arrive using any extension, or embeded in Word or other documents.

What is DEP (Data Execution Protection) and how does it help me?

With Windows XP SP2, Microsoft introduced DEP. It protects against a wide range of exploits, by preventing the execution of 'data segements'. However, to work well, it requires hardware support. Some CPUs, like AMD's 64 Bit CPUs, will provide full DEP protection and will prevent the exploit. You can learn more about DEP, how to enable it and check that it is running, here.

How good are Anti Virus products to prevent the exploit?

At this point, we are aware of versions of the exploit that will not be detected by all the antivirus engines. We hope they will catch up soon. But it will be a hard battle to catch all versions of the exploit. Up to date AV systems are necessary but likely not sufficient.

How could a malicious WMF file enter my system?

There are too many methods to mention them all. E-mail attachments, web sites, instant messaging are probably the most likely sources. Don't forget P2P file sharing and other sources.

Is it sufficient to tell my users not to visit untrusted web sites?

No. It helps, but its likely not sufficient. We had at least one widely trusted web site (knoppix-std.org) which was compromissed. As part of the compromise, a frame was added to the site redirecting users to a corrupt WMF file. "Tursted" sites have been used like this in the past.

What is the actual problem with WMF images here?

WMF images are a bit different then most other images. Instead of just containing simple 'this pixel has that color' information, WMF images can call external procedures. One of these procedure calls can be used to execute the code.

Should I use something like "dropmyrights" to lower the impact of an exploit.

By all means yes. Also, do not run as an administrator level users for every day work. However, this will only limit the impact of the exploit, and not prevent it. Also: Web browsing is only one way to trigger the exploit. If the image is left behind on your system, and later viewed by an administrator, you may get 'hit'.

Are my servers vulnerable?

Maybe... do you allow the uploading of images? email? Are these images indexed? Do you sometimes use a web browser on the server? In short: If someone can get a image to your server, and if the vulnerable DLL may look at it, your server may very well be vulnerable.

What can I do at my perimeter / firewall to protect my network?

Not much. A proxy server that strips all images from web sites? Probably wont go over well with your users. At least block .WMF images (see above about extensions...). If your proxy has some kind of virus checker, it may catch it. Same for mail servers. The less you allow your users to initiate outbound connections, the better. Close monitoring of user workstations may provide a hint if a work station is infected.

Can I use an IDS to detect the exploit?

Most IDS vendors are working on signatures. Contact your vendor for details. Bleedingsnort.org is providing some continuosly improving signatures for snort users. Recent releases of this exploit take advantage of http compression and randomization of the exploit to evade IDS signatures.

If I get hit by the exploit, what can I do?

Not much :-(. It very much depends on the exact exploit you are hit with. Most of them will download additional components. It can be very hard, or even impossible, to find all the pieces. Microsoft offers free support for issues like that at 866-727-2389 (866 PC SAFETY).