Date: Fri, 13 Sep 2013 13:15:22 -0600 From: Kurt Seifried <kseifried@...hat.com> To: oss-security@...ts.openwall.com CC: Daniel Kahn Gillmor <dkg@...thhorseman.net> Subject: Re: GnuPG treats no-usage-permitted keys as all-usages-permitted -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 09/13/2013 12:32 AM, Daniel Kahn Gillmor wrote: > RFC 4880 permits OpenPGP keyholders to mark their primary keys and > subkeys with a "key flags" packet that indicates the capabilities > of the key . These are represented as a set of binary flags, > including things like "This key may be used to encrypt > communications." > > If a key or subkey has this "key flags" subpacket attached with all > bits cleared (off), GnuPG currently treats the key as having all > bits set (on). While keys with this sort of marker are very rare > in the wild, GnuPG's misinterpretation of this subpacket could lead > to a breach of confidentiality or a mistaken identity > verification. > > Potential Confidentiality Breach -------------------------------- > > For example, if Alice has a subkey X whose "key flags" subpacket > has all bits cleared (because she is using it for something not > documented in the spec, perhaps something experimental or risky), > and Bob sends Alice an e-mail encrypted using GnuPG, Bob may > accidentally encrypt the message to key X, depsite Alice having > clearly stated that the key is not to be used for encrypted > communications. If Alice's intended use of X turns out to > compromise the key itself somehow, then the attacker can read Bob's > otherwise confidential communication to Alice. > > Potential Mistaken Identity Verification > ---------------------------------------- > > Consider the scenario above, but where Bob is in general willing to > rely on OpenPGP certifications made by Alice. The legitimate form > of these certifications are usually made by Alice's primary key, > which is marked as "certification-capable". Because Bob's GnuPG > misinterprets the usage flags on subkey X, Bob may be able to be > tricked into believing that Alice has certified someone else's > OpenPGP identity if an attacker manages to coax Alice into using > subkey X in a way that is replayable as an OpenPGP certification. > > > > These risks are unlikely today (there are very few certifications > in the wild with an all-zero key flags subpacket), and they are > not particularly dangerous (for a compromise to happen, there needs > to also be a cross-context abuse of the mis-classified key, which i > do not have a concrete example of). But the keyholder's stated > intent of separating out keys by context of use is being ignored, > so there is a window of vulnerability that should not be open. > > There is also a (maybe non-security) functionality issue here, in > that GnuPG may mis-use the user's own keys if they are marked as > described above (e.g. signing messages or certifying identities > with a subkey that is explicitly marked as not being for that > purpose). > > > This problem was first reported to the GnuPG team back in March > . Patches are available, but appear to only be applied on the > development branch (2.1.x). So stable branches 1.4.x and 2.0.x > remain vulnerable at the moment. > > Could a CVE be issued for this? > > Regards, > > --dkg > >  https://tools.ietf.org/html/rfc4880#section-220.127.116.11  > http://thread.gmane.org/gmane.comp.encryption.gpg.devel/17712/focus=18138 > > Please use CVE-2013-4351 for this issue. - -- Kurt Seifried Red Hat Security Response Team (SRT) PGP: 0x5E267993 A90B F995 7350 148F 66BF 7554 160D 4553 5E26 7993 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.14 (GNU/Linux) iQIcBAEBAgAGBQJSM2RKAAoJEBYNRVNeJnmTcakQAMXS9Myd4ZpJeqGCX3dZlTHj rU4u//iuOC3cfRYwYV8EycQdAxkdXKGt5Fc2ZImjhwNiY70b9Luq+5MGPlDHRM1n vojp65uKJRHmiWrAsIMZJQiKDKdYtNys/rWh5YEbptUIIjILSyPCpO7vw8CAlAWW vwi4OYQoWKrPKt7vyRsTGd2kaKK4yhs3Lt3ULC9olbIE8M4qa7CXwc6+uO44jMLu MLToG7gHT3Y0hun6O58736vemkS/3Iqh1G0QRLWUZGj2qPC6u30OxpuwFIyDaZSu ng5+5zOfUyLnZNE6tdPcx1io2Dm5VaRE0gtM4HknNEHf8t2u8LOExWhTU5g0yR5e nyq0dKrTTG84nGSw/HRINXV0v2hbSpyA15Pz4gQoZ3KMkflCSw4XOlelT6sA2PCe sJkUWXGy/lgNC8PUUV7IH1uWkP7RSqh8oam3bD4h6ZBpZlodDXzwKBvSG5V9cdL7 PKGZKfAzETueBAxKPXmYWLf9KRXZsulfud6yRctIa1n27C/wSSTw1L9VBY1XY4Ko ILT3Lg0tTk/Y/Q1YC2QgtAQu6u2jPUdXiDY1d2D5tv5EvxDTZUCipSpcw8LnGnjc pnvJ3XrbyDHxLe6nzSJwm/5NO5cKmq7uklr7SYIEhyPvuCFlC/NvD1rtfLhcsbxp iGpgXUg54W9hL1m4OA3u =Dg6j -----END PGP SIGNATURE-----
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