Re: [ietf-dkim] ISSUE: 4871bis - Security Loop hole with Multiple 5322.F

Hector Santos wrote:

> Julian Mehnle wrote:
>
> > I interpret RFC 4871, section 5.4 (actually, exactly the part you
> > quoted originally), such that signing a message that has a dingle
> > From field with h=From:From ensures that adding another From field
> > will break the signature.  If you're saying there is a way to add a
> > second From field a message thusly signed without breaking the
> > signature, could you please explain to me how?
> You are correct. Adding a second from: to the h= tag:
>
>          "h=from:from:.........."
>
> can address this.   But no implementation does that.
I don't think this is a matter of implementation, but one of *configura- 
tion*.  Are there any DKIM implementations that *hard-code* the value of 
h=?

In any case I think all that's warranted is adding an explicit note that 
From should be included in h= twice, be it in a hard-coded (default) 
value or in user-made configuration.

> [...]
>
> Obviously, the ease of this exploit is a concern.  Any high value
> domain mail can now be found and replayed with a phished or spooked
> 2nd or more 5322.From:
Agreed.  Assuming you're right and single-"From" h='s are indeed a 
frequent configuration, then apparently the significance of this was 
underestimated.

-Julian

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