2003-09-25T09:06:58 MightyE:
> There are two methods which you can use in the writing of your
> email virus scanner; you can either decode it every known way that
> any client does so, [...] Alternatively you can accept it only if
> it is properly encoded, [...]
There's a third method, which I think is rather better than either
of those.
You can re-code everything into a canonical form. Some email client
drop some punctuation characters in filenames? Delete all such
characters from filenames. Different tools handle various i18n
encoded filenames differently? Map to US-ASCII. Enforce length
limits. Recode base64. Recode uuencoded chunks. Regularize
non-standard MIME.
Do all this canonicalization before the message hits your
attachment type policy enforcement and malware scanner, so they only
have to deal with the common forms that everybody handles the same.
> There are two methods which you can use in the writing of your
> email virus scanner; you can either decode it every known way that
> any client does so, [...] Alternatively you can accept it only if
> it is properly encoded, [...]
There's a third method, which I think is rather better than either
of those.
You can re-code everything into a canonical form. Some email client
drop some punctuation characters in filenames? Delete all such
characters from filenames. Different tools handle various i18n
encoded filenames differently? Map to US-ASCII. Enforce length
limits. Recode base64. Recode uuencoded chunks. Regularize
non-standard MIME.
Do all this canonicalization before the message hits your
attachment type policy enforcement and malware scanner, so they only
have to deal with the common forms that everybody handles the same.
-Bennett
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