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 GnuPG: remotely controllable function pointer [CVE-2006-6235]
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                          2006-12-04

Summary

Tavis Ormandy of the Gentoo security team identified a severe and exploitable bug in the processing of encrypted packets in GnuPG.

[ Please do not send private mail in response to this message. The mailing list gnupg-devel is the best place to discuss this problem (please subscribe first so you don't need moderator approval [1]). ]

Impact

Using malformed OpenPGP packets an attacker is able to modify and dereference a function pointer in GnuPG. This is a remotely exploitable bug and affects any use of GnuPG where an attacker can control the data processed by GnuPG. It is not necessary limited to encrypted data, also signed data may be affected.

Affected versions: All versions of GnuPG < 1.4.6 All versions of GnuPG-2 < 2.0.2 All beta versions of GnuPG-2 (1.9.0 .. 1.9.95) Affected tools: gpg, gpgv, gpg2 and gpgv2. Affected platforms: All.

gpg-agent, gpgsm as well as other tools are not affected.

A workaround is not known.

Solution

If you are using a vendor supplied version of GnuPG:

  • Wait for an update from your vendor. Vendors have been informed on Saturday December 2, less than a day after this bug has been reported.

If you are using GnuPG 1.4:

  • Update as soon as possible to GnuPG 1.4.6. It has been uploaded to the usual location: ftp://ftp.gnupg.org/gcrypt/gnupg/. This version was due to be released anyway this week. See http://www.gnupg.org/download/ for details.

  • Or: As another and less intrusive option, apply the attached patch to GnuPG 1.4.5. This is the smallest possible fix.

If you are using GnuPG 2.0:

  • Apply the attached patch against GnuPG 2.0.1.

  • Or: Stop using gpg2 and gpgv2, install GnuPG 1.4.6 and use gpg and gpgv instead.

If you are using a binary Windows version of GnuPG:

  • A binary version of GnuPG 1.4.6 for Windows is available as usual.

  • Gpg4win 1.0.8, including GnuPG 1.4.6, is available. Please go to http://www.gpg4win.org .

Background

GnuPG uses data structures called filters to process OpenPGP messages. These filters ware used in a similar way as a pipelines in the shell. For communication between these filters context structures are used. These are usually allocated on the stack and passed to the filter functions. At most places the OpenPGP data stream fed into these filters is closed before the context structure gets deallocated. While decrypting encrypted packets, this may not happen in all cases and the filter may use a void contest structure filled with garbage. An attacker may control this garbage. The filter context includes another context used by the low-level decryption to access the decryption algorithm. This is done using a function pointer. By carefully crafting an OpenPGP message, an attacker may control this function pointer and call an arbitrary function of the process. Obviously an exploit needs to prepared for a specific version, compiler, libc, etc to be successful - but it is definitely doable.

Fixing this is obvious: We need to allocate the context on the heap and use a reference count to keep it valid as long as either the controlling code or the filter code needs it.

We have checked all other usages of such a stack based filter contexts but fortunately found no other vulnerable places. This allows to release a relatively small patch. However, for reasons of code cleanness and easier audits we will soon start to change all these stack based filter contexts to heap based ones.

Support

g10 Code GmbH, a Duesseldorf based company owned and headed by GnuPG's principal author, is currently funding GnuPG development. As evident by the two vulnerabilities found within a week, a review of the entire code base should be undertaken as soon as possible. As maintainers we try to do our best and are working slowly through the code. The long standing plan is to scrutinize the 2.0 code base, write more test cases and to backport new fixes and cleanups to 1.4. However, as a small company our resources are limited and we need to prioritize other projects which get us actual revenues. Support contracts or other financial backing would greatly help us to improve the quality of GnuPG.

Thanks

Tavis Ormandy found this vulnerability.

[1] See http://lists.gnupg.org/mailman/listinfo/gnupg-devel .