Earl Hood wrote:
In the DKIM SSP draft, the following is stated:
If the Sender Signing Policy record does not exist, verifier systems
MUST assume that some messages from this entity are not signed and
the message SHOULD NOT be considered to be Suspicious.
I'm wondering if this a safe policy to assert, especially in the
context of past SSP discussions on ietf-mailsig where the SSP may
need to be examined always, and not just for invalid signatures.
It's all a matter of precedence, whether one should believe in the
signature or in the signing policy in the event of a conflict between
the two. If you believe (as I do) that any attacker who can get a valid
signature for the OA on a message can just as easily override the SSP
for that domain, then the SSP doesn't add any value when you have a
valid OA signature.
Why is it not safe? Because a malicious domain can send out messages
with forged rfc2822.From addresses where the domain portion does not
have any SSP defined. Therefore, when a DKIM verifier checks the
SSP for rfc2822.From, the message would not be considered suspicious
since no SSP record is available.
But how would they get a valid signature on behalf of the OA? Or are
you saying that one should treat the message differently from an
unsigned message because there is an invalid OA signature present?
There isn't any reason to apply a signature unless you know it will
verify correctly. Conversely, there isn't any reason to downgrade a
message simply because it has an invalid signature.
-Jim (theoretically on vacation, so responses will be slow)
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