subkey binding signature with no usage flags and/or a critical notation

[Daniel Kahn Gillmor]

On Wed 2013-09-11 17:46:41 -0400, Daniel Kahn Gillmor wrote:
> Without this patch, if a subkey with a usage flags subpacket that is
> all-zero appears, GnuPG thinks that the key is valid for all
> capabilities (usage: SCEA).
>
> This is dangerous in several ways:
>
>  * if it's your own key, GnuPG will use it to sign messages or certify
>    other OpenPGP certificates
>
>  * if it's someone else's key, GnuPG will encrypt to it
>
>  * if it's someone else's key, GnuPG may be willing to rely on OpenPGP
>    certifications made by the key, despite the owner having clearly
>    marked that it is not acceptable for that use.
>
> Depending on how such a key is actually used in practice, this may cause
> unrelated confidential data to be revealed, or it may let GnuPG follow
> spoofable paths through the WoT. 

 [...]

> I'm inclined to see this as a security vulnerability (because of the
> confidentiality and certification-following concerns); i'd like to see
> it fixed in the stable branches and assigned a CVE, so that i can push
> for getting it resolved in those distros that care about CVE coverage.

This is now CVE-2013-4351.

Regards,

        --dkg
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