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GnuPG treats no-usage-permitted keys as all-usages-permitted
From: Daniel Kahn Gillmor <dkg () fifthhorseman net>
Date: Fri, 13 Sep 2013 02:32:47 -0400

RFC 4880 permits OpenPGP keyholders to mark their primary keys and
subkeys with a "key flags" packet that indicates the capabilities of the
key [0].  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 [1].
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

[0] https://tools.ietf.org/html/rfc4880#section-5.2.3.21
[1] http://thread.gmane.org/gmane.comp.encryption.gpg.devel/17712/focus=18138

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