[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: base64

I agree, I don't think it's unreasonable to reject improperly formatted messages. Chances are much higher that they're spam or virii, and the minority with broken clients will find their way to non-broken clients. If you are parsing the message, particularly looking for malware, and encounter an improper encoding, bounce the message with a meaningful error, this way you don't have to worry about a targeted exploit that depends on the way one email client (mis)interprets a message in a different way from your virus scanner.

The RFC does declare an = to indicate that the end of the data stream has been reached, further data should be truncated, though it seems each email client actually handles this differently. Take the low road catchall, and simply reject them as a matter of course.

-Eric Stevens
mightye a@t mightye d.o.t org

Christian Vogel wrote:

Hi,

On Tue, Sep 23, 2003 at 07:50:56PM +0300, Alexander Ogol wrote:


decision in all situations. Some mailing lists (debian-russian, for example)
add some 7bit information after letter body while re-forwarding, regardless
of was the letter base64/QP encoded or not, resulting of such malformed



Then this software is severly broken (MIME-wise), imho, and needs to be
updated/changed/dumed.



So I think that the right solution (before antivirus software would be
rewritten) is to write filters by yourself - decode base64 as that do
popular mail clients and give them to antivirus.



With this approach, you are always on the "one step behind" side of
the problem. It's only a matter of time that someone finds out that
(made up example:) you can use a UTF8-mis-encoded "=" in Microsoft's
base64-decoder... The only sane way is to check if it's in the
standard-form ("abcABC=") and reject or convert if it's not.

99.99% of all software should create the standard form, so please
let the tiny fraction of users with broken software suffer
when their mails get rejected.

(Note: this of course applies not only to Base64 but to all aspects
of header-parsing, file-format guessing etc...)

Chris