joel, all,

On Wed, 6 Feb 2002, Joel Maslak wrote:
> The fix for this requires sophisticated bounce tracking software. The
> only real way to fix this problem is to send each recipient a message with
> a custom-encoded FROM envelope address, such as:
> bounce-<user-id>-<security-key>example.com
> Where the user-id is some sort of database identifyer and the security key
> is simply a random number kept in the database to prevent malicious
> activity (it could also be some sort of cryptographic code). When the
> example.com mail server receives a message to bounce-xxx-yyyexample.com,
> it checks the security key, verifies that the bounce is a permanent
> bounce, and deletes the user.

it's worth noting that this is a succinct description of VERP (variable
envelope return path), something used by ezmlm and qmail to accomplish
exactly this--make it difficult to forge a bounce and easy to determine
true per-recipient bounces. VERP makes handling large mailing lists
trivial and significantly reduces this security problem.

see http://www.lifewithqmail.org/lwq.html#verp for a good description.

-- 
todd underwood, vp & cto
oso grande technologies, inc.
toddosogrande.com
"Those who give up essential liberties for temporary safety deserve
neither liberty nor safety." - Benjamin Franklin

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