[Exim] potential security problem with lookups

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Author: Alexey Promokhov
Date:  
To: exim-users
Subject: [Exim] potential security problem with lookups
Greetings.

I just discover the following situation.

There was the following ACL statement:

accept senders = ${if exists {/usr/local/etc/exim/whitesender+$domain} {/usr/local/etc/exim/whitesender+$domain} {:}}

It means a whitelist for users in one of virtual domains. But if sender
of processed message is <>, i.e. it's a bounce message, then lookup is
hit, even if recipient is in foreign domain. So, the above construction
gives an open relay.

{:} construction was in our config file for historic reason. Replacing it
with {} does correct the situation.

I suggest to process looking with key=<empty> in special manner, because such
lookup is mostly useless, and can give undesired results in case of typos
and carelessness.

--
Alexey Y. Promokhov, system administrator
Joint Stock Venture "GP Telecom", Moscow, Russia