[Mimedefang] Invalid "mimedefang.pl -structure" output and virusscanning behaviour

[Paul Murphy]

> > The *ONLY* safe way to handle these situations is to 
> > canonicalize every message that passes through your mail 
> > server by calling "action_rebuild()" in filter_end.

> Is there any harm in doing this? 

Potentially, but not as much as could be caused by not doing it.  Some messages
which are badly constructed could have sections stripped out, or the whole
message trashed, but then they were badly constructed, and by definition this is
non-standard and a potential risk.

> Does it take a lot of CPU? 

Yes, if the message is complex and/or large, but not as CPU-intensive as virus
scanning.

> Will it break digital signatures? 

Possibly - but then most digital signature systems encapsulate the whole message
as a single attachment, and the public certificate of the sender as a separate
attachment, so there shouldn't be any impact if the signature system is a decent
one.

> It sounds like a good idea, but I'm afraid of potential downsides.

By using such a system, there will undoubtedly be some messages which are
corrupted because they have an invalid structure.  If these come from a valid
source using a badly-behaved e-mail client, you will have enormous problems in
coping with it.  On the other hand, if you attempt to correct the problem at
your end, they have no incentive to fix their software....

Best Wishes,

Paul.
__________________________________________________
Paul Murphy
Head of Informatics
Ionix Pharmaceuticals Ltd
418 Science Park, Cambridge, CB4 0PA

Tel. 01223 433741
Fax. 01223 433788


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