[ietf-dkim] detecting header mutations after signing
hsantos at isdg.net
Fri Oct 8 08:06:51 PDT 2010
Wietse Venema wrote:
> With this signer-side configuration solution, the verifier can
> detect attempts to "spoof" any header that was covered by the DKIM
> signature (spoof as in "add a forged header, and hope that naive
> programs will use the forged header instead of the authentic one").
> So the solution is already available in DKIM. We just need to use
> the solution, and make it part of routine DKIM tests.
>> Having the signer put the extra junk in h= should make existing verifiers
>> do the right thing, although I doubt the bit of verification code that
>> checks for the non-existence of the N+1st header for N>0 is well tested in
>> DKIM implementations.
> To address this, make this solution part of routine DKIM test suites.
This is only part of the solution. A temporary one to allow current
operators to cover themselves using their "Required Header"
configuration, if any.
The real solution is to void double 5322.From messages. Either the
DKIM compliant MSA, MDA do it or the "better" DKIM signer/verification
engine does it to cover for legacy MSA, MDA or to make sure customers
using a 3rd party signing engine are sending the proper mail to sign.
Can someone come up with IETF amenable copy text for Dave to add to
4871bis that won't prohibit or slow it down its progress?
IMV, all would be implementers need to read is a basic idea of:
"Make sure there are no two or more 5322.From headers when signing
or verifying. These messages should be voided."
and thats it.
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