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diff --git a/doc/detached-sig-verification-bug.txt b/doc/detached-sig-verification-bug.txt new file mode 100644 index 000000000..a938cd7ee --- /dev/null +++ b/doc/detached-sig-verification-bug.txt @@ -0,0 +1,231 @@ + False positive signature verification in GnuPG + ============================================== + (released 2006-02-15) + + +Summary +======= + +The Gentoo project identified a security related bug in GnuPG. When +using any current version of GnuPG for unattended signature +verification (e.g. by scripts and mail programs), false positive +signature verification of detached signatures may occur. + +This problem affects the tool *gpgv*, as well as using "gpg --verify" +to imitate gpgv, if only the exit code of the process is used to +decide whether a detached signature is valid. This is a plausible +mode of operation for gpgv. + +If, as suggested, the --status-fd generated output is used to decide +whether a signature is valid, no problem exists. In particular +applications making use of the GPGME library[2] are not affected. + +To solve this problem an update of the current stable version has been +released (see below). + +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: +======= + +Signature verification of detached signatures does not work, thus +modified versions of signature protected files may not be detected. + +All versions of gnupg prior to 1.4.2.1 are affected if they are used +in certain unattended operation modes. + +There is no problem using GnuPG in an interactive way because GnuPG +won't print any signature status at all; i.e. no "Good signature". + +Scripts and applications using gpg or gpgv with the --status-fd option +and properly parsing this output are not affected. + +Applications using the GPGME library[2] are not affected. + +The GnuPG versions 1.9 are not affected unless the currently +deprecated gpg part has been enabled. + + +Solution: +========= + +Update GnuPG as soon as possible to version 1.4.2.1. There are no +fixes for older versions available, although the fix described below +may be adjusted for them. + +To update please follow the instructions found at +http://www.gnupg.org/download/ or read on: + +GnuPG 1.4.2.1 may be downloaded from one of the GnuPG mirror sites or +direct from ftp://ftp.gnupg.org/gcrypt/ . The list of mirrors can be +found at http://www.gnupg.org/mirrors.html . Note, that GnuPG is not +available at ftp.gnu.org. + +On the mirrors you should find the following files in the *gnupg* +directory: + + gnupg-1.4.2.1.tar.bz2 (2.8M) + gnupg-1.4.2.1.tar.bz2.sig + + GnuPG source compressed using BZIP2 and OpenPGP signature. + + gnupg-1.4.2.1.tar.gz (4.0M) + gnupg-1.4.2.1.tar.gz.sig + + GnuPG source compressed using GZIP and OpenPGP signature. + + gnupg-1.4.2-1.4.2.1.diff.bz2 (39k) + + A patch file to upgrade a 1.4.2 GnuPG source. + +Select one of them. To shorten the download time, you probably want to +get the BZIP2 compressed file. Please try another mirror if +exceptional your mirror is not yet up to date. + +In the *binary* directory, you should find these files: + + gnupg-w32cli-1.4.2.1.exe (1.4M) + gnupg-w32cli-1.4.2.1.exe.sig + + GnuPG compiled for Microsoft Windows and OpenPGP signature. + Note that this is a command line version and now comes with a + graphical installer tool. The source files are the same as + given above. Note, that a new version of the Gpg4Win + package[4], including an updated version of GnuPG will be + available later the day. + + +In order to check that the version of GnuPG which you are going to +install is an original and unmodified one, you can do it in one of +the following ways: + + * If you already have a trusted version of GnuPG installed, you + can simply check the supplied signature. For example to check the + signature of the file gnupg-1.4.2.1.tar.bz2 you would use this command: + + gpg --verify gnupg-1.4.2.1.tar.bz2.sig + + This checks whether the signature file matches the source file. + You should see a message indicating that the signature is good and + made by that signing key. Make sure that you have the right key, + either by checking the fingerprint of that key with other sources + or by checking that the key has been signed by a trustworthy other + key. Note, that you can retrieve the signing key using "finger wk + 'at' g10code.com" or "finger dd9jn 'at' gnu.org" or using the + keyservers. From time to time I prolong the expiration date; thus + you might need a fresh copy of that key. + + Never use a GnuPG version you just downloaded to check the + integrity of the source - use an existing GnuPG installation! + Watch out for a "Good signature" messages. + + * If you are not able to use an old version of GnuPG, you have to + verify the SHA-1 checksum. Assuming you downloaded the file + gnupg-1.4.2.1.tar.bz2, you would run the sha1sum command like this: + + sha1sum gnupg-1.4.2.1.tar.bz2 + + and check that the output matches the first line from the + following list: + +1c0306ade25154743d6f6f9ac05bee74c55c6eda gnupg-1.4.2.1.tar.bz2 +cefc74560f21bde74eed298d86460612cd7e12ee gnupg-1.4.2.1.tar.gz +98d597b1a9871b4aadc820d8641b36ce09125612 gnupg-1.4.2-1.4.2.1.diff.bz2 +a4db35a72d72df8e76751adc6f013b4c96112fd4 gnupg-w32cli-1.4.2.1.exe + + +Background: +=========== + +If a file with arbitrary data, for example 64 times the character +0xCA, is used as the detached signature, any data file will lead to +gpg exiting with 0 (success). There won't be any messages indicating +that the signature is valid or false: + + $ fortune >x.txt + $ perl -e 'print "\xca"x"64"' >x.txt.sig + $ gpgv x.txt.sig x.txt + $ echo $? + 0 + +Cleary this should not return success. + +The same problem appears when using "gpg --verify" in place of gpgv. +However in this case any application should to do further checks to +make sure that the key verifying the signature is actually the desired +one, thus using "gpg --verify" without processing the --status-fd +generated output is in general the wrong approach. + +The fixed version makes sure that gpgv and "gpg --verify" won't return +success if no signature has been seen. A minimal but sufficient fix +against 1.4.2 and possible older versions is: + +====8<============ +--- g10/mainproc.c (revision 4001) ++++ g10/mainproc.c (working copy) +@@ -77,6 +77,7 @@ + int op; + int stop_now; + } pipemode; ++ int any_sig_seen; /* Set to true if a signature packet has been seen. */ + }; + + +@@ -217,6 +218,7 @@ + { + KBNODE node; + ++ c->any_sig_seen = 1; + if( pkt->pkttype == PKT_SIGNATURE && !c->list ) { + /* This is the first signature for the following datafile. + * GPG does not write such packets; instead it always uses +@@ -1137,6 +1139,18 @@ + c->signed_data = signedfiles; + c->sigfilename = sigfilename; + rc = do_proc_packets( c, a ); ++ ++ /* If we have not encountered any signature we print an error ++ messages, send a NODATA status back and return an error code. ++ Using log_error is required because verify_files does not check ++ error codes for each file but we want to terminate the process ++ with an error. */ ++ if (!rc && !c->any_sig_seen) ++ { ++ write_status_text (STATUS_NODATA, "4"); ++ log_error (_("no signature found\n")); ++ rc = -1; ++ } + m_free( c ); + return rc; + } +====>8============ + +Note that the released version also includes a test case for this bug +and prints an additional diagnostic. With the patch above the output +using the same test data as above should be: + + $ gpgv x.txt.sig x.txt + gpgv: no signature found + gpgv: verify signatures failed: eof + $ echo $? + 2 + + + +Thanks +====== + +taviso from the Gentoo project found this vulnerability and informed +me on Monday evening. Unfortunately I had already switched off my +monitor at that time. The update has been released yesterday evening +(CET). + + + + +[1] http://lists.gnupg.org/mailman/listinfo/gnupg-devel +[2] http://www.gnupg.org/related_software/gpgme +[3] http://www.gpg4win.org |