5.4 KiB
Multiple Messages Problem in GnuPG and GPGME
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2007-03-05 [CVE-2007-1263]
Summary
Gerardo Richarte from Core Security Technologies identified a problem when using GnuPG in streaming mode.
The problem is actually a variant of a well known problem in the way signed material is presented in a MUA. It is possible to insert additional text before or after a signed (or signed and encrypted) OpenPGP message and make the user believe that this additional text is also covered by the signature. The Core Security advisory describes several variants of the attack; they all boil down to the fact that it might not be possible to identify which part of a message is actually signed if gpg is not used correctly.
[ 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
All applications using GnuPG without properly using the status interface to verify signed or signed and encrypted messages.
All GPGME versions up to and including 1.1.3.
Starting with version 1.4.7 and 2.0.3, GnuPG implements an additional and sufficient protection against this common usage problem.
Detached signatures are in no way affected by this problem.
Description
When using gpg (or gpg2) in a pipeline or with redirected input and output additional data may be inserted into a message. This allows to forge a signed message by prefixing it with arbitrary material. A way to create such a message is:
echo "This is my sneaky plaintext message" > foobar.txt gpg -z0 --output prefix.gpg --store foobar.txt cat prefix.gpg original-signed-message.gpg > forged.gpg
Using gpg naively this results in:
$ gpg <forged.gpg This is my sneaky plaintext message Either I'm dead or my watch has stopped. -- Groucho Marx's last words gpg: Signature made Mon Feb 26 09:57:04 2007 CET using DSA key ID 68697734 gpg: Good signature from "Alfa Test (demo key) alfa@example.net" [...]
and thus gives the impression that the sneaky message is part of the signed Groucho quote. The correct way to use gpg with redirection is by taking care of the status interface:
$ gpg --status-fd 1 <forged.gpg [GNUPG:] PLAINTEXT 62 1172479053 foobar.txt [GNUPG:] PLAINTEXT_LENGTH 36 This is my sneaky plaintext message [GNUPG:] PLAINTEXT 62 1172480224 original-signed-message [GNUPG:] PLAINTEXT_LENGTH 86 Either I'm dead or my watch has stopped. -- Groucho Marx's last words gpg: Signature made Mon Feb 26 09:57:04 2007 CET using DSA key ID 68697734 [GNUPG:] SIG_ID UncMPBJYgbG/uszJVNKoCAz+hvY 2007-02-26 1172480224 [GNUPG:] GOODSIG 2D727CC768697734 Alfa Test (demo key) alfa@example.net gpg: Good signature from "Alfa Test (demo key) alfa@example.net" [...]
Here the PLAINTEXT status lines clearly identify the start of a new message.
Note, that using gpg on the command line is in almost all cases not done with redirection but by letting gpg save the signed message. In this case gpg will save the message to different files or in case the file names are identical, prompt the over to overwrite the first one again.
Because the problem of identifying the actual signed content when mixing the signed data and the signature is very common, the long standing suggestion for all digital signatures is to use a detached signature. A detached signature allows to clearly identify what is signed and what is the signature. This is also the reason why PGP/MIME signed messages are in general to be preferred over the old style clear signed messages.
Solution
Given that there are many applications in use which are subject to the described problem, we have decided to change GnuPG so that such forged OpenPGP messages are detected and the signature verification will fail. GnuPG 1.4.7 has been released today and is available from the usual places [2]. If you don't want to update, a minimal patch against GnuPG 1.4.6 is available at
ftp://ftp.gnupg.org/gcrypt/gnupg/patches/gnupg-1.4.6-multiple-message.patch
Many applications are using the library GPGME which implements an easy way to process OpenPGP messages using gpg. We have updated GPGME to make it immune against this problem even if an old version of gpg is being used. GPGME 1.1.4 is available from the usual places [2]. A patch (against version 1.1.3 or 1.1.2) is available at
ftp://ftp.gnupg.org/gcrypt/gpgme/patches/gpgme-1.1.3-multiple-message.patch
Please note that - after applying one of these patches - some vulnerable applications (mainly MUAs) may fail to handle certain messages which are composed of several OpenPGP messages. To continue the support of such messages fixing the application is required as there is no way for GnuPG to do it.
Support
g10 Code GmbH [3], a Duesseldorf based company owned and headed by GnuPG's principal author, is currently funding GnuPG development. Support contracts or other financial backing will greatly help us to improve the quality of GnuPG.
Thanks
Gerardo Richarte found this problem. David Shaw greatly helped to analyse and describe the core of the problem.
[1] See http://lists.gnupg.org/mailman/listinfo/gnupg-devel [2] See http://www.gnupg.org/download/ [3] See http://www.gnupg.org/service.html