MS Doesn't completely block spam with faulty attachments

Fri Sep 2 12:42:33 IST 2011

"How can you tell if it's a good double extension file (ie what the server
THINKS NOW is non-spam/non-malware)"
You can't, that's my whole point. I think the behavior to just replace the
file with a warning message and send the mail through is the perfect
solution. My only problem is that this method should NOT be used when the
mail hits other spam traps. (Virus Scanner, Spamassassin) In that case, the
message should just be handled like any other spam message.

What happens now is that the file with double extension (or other stuff
hitting the file name rules like ridiculously long file names) gets replaced
with a warning message and directly send to the recipient. Mailscanner
doesn't care if it's a spam message or not. This places phishing or spam
messages in my users mailboxes. The harmful attachments are stripped but the
message should have been blocked because it woul've hit other spam traps IF
Mailscanner would take the trouble of processing the message further.

I could just block all E-mails completely that hit such rules but with the
amount of false positives, that's not an option. Disabling these rules
altogether is possible but not a long term solution. I'm trying to revert
the 20090730 changes in the MailScanner binary. That will probably solve it
but isn't very convenient with future updates.

On 2 September 2011 13:18, Martin Hepworth <maxsec at gmail.com> wrote:

> How can you tell if it's a good double extension file (ie what the server
> THINKS NOW is non-spam/non-malware) vs what is actually bad vs a new virus
> signature that comes after the event that would flag the file as
> problematic. The double extn trap is designed to solve a particular trick
> the bad guys started playing years ago and the AV's weren't picking them up
> fast enough.
>
>  drop the double extn check and let them through if this is causing too
> many issues (false positives) - you can even do this on a per
> recipient/domain level (
> http://wiki.mailscanner.info/doku.php?id=documentation:configuration:rulesets:overloading)
> for common recipients of this stuff, or educate the users that it's bad and
> wrong and they should cleanup the filenames before sending. or both!
>
> Either
>
> --
> Martin Hepworth
> Oxford, UK
>
>
>
> On 2 September 2011 11:20, Joolee <mailscanner at joolee.nl> wrote:
>
>> A feature that i would like to be able to disable ;)
>>
>>
>> "Why would you want to spend precious resources on a meaningless check,
>> when you already decided to stop the offending attachment?!"
>> To inform my paying user why the contract he's been waiting for was
>> blocked.
>>
>> I think I already made quite clear why it's not an option for me to
>> completely block them. I can't see why other users can't be bothered by it,
>> maybe they just accept that they can't solve it? (Not my way of handling
>> problems)
>>
>>
>> On 1 September 2011 23:07, Glenn Steen <glenn.steen at gmail.com> wrote:
>>
>>> That's not a problem, it's a feature... And a much needed one at that!
>>> Why would you want to spend precious resources on a meaningless check,
>>> when you already decided to stop the offending attachment?!
>>> Don't deliver it at all, if it bothers you;-)
>>>
>>> Cheers
>>> --
>>> -- Glenn
>>> Den 1 sep 2011 19:12 skrev "Joolee" <mailscanner at joolee.nl>:
>>>
>>> > The problem with the current spam is that they're blocked for
>>> containing exe
>>> > files, not double file extensions (Although they woul've hit that one
>>> if
>>> > exe's were not clocked.)
>>> >
>>> > Only quick temporary solution is to disable all file-name validation
>>> because
>>> > this can occur with more than just exe files and double extensions.
>>> This is
>>> > no final solution though.
>>> >
>>> > On 1 September 2011 18:40, Kevin Miller <Kevin_Miller at ci.juneau.ak.us
>>> >wrote:
>>> >
>>> >> **
>>> >> Easiest thing to do in that case is to comment out the line in
>>> >> filename.rules.conf that disallows double extensions. The message will
>>> be
>>> >> accepted as normal and go through the additional tests (is it an
>>> executable,
>>> >> is it a virus, is it spam, etc.)
>>> >>
>>> >>
>>> >> ...Kevin
>>> >> --
>>> >> Kevin Miller Registered Linux User No: 307357
>>> >> CBJ MIS Dept. Network Systems Admin., Mail Admin.
>>> >> 155 South Seward Street ph: (907) 586-0242
>>> >> Juneau, Alaska 99801 fax: (907 586-4500
>>> >>
>>> >>
>>> >> ------------------------------
>>> >> *From:* mailscanner-bounces at lists.mailscanner.info [mailto:
>>> >> mailscanner-bounces at lists.mailscanner.info] *On Behalf Of *Joolee
>>> >> *Sent:* Thursday, September 01, 2011 7:32 AM
>>> >> *To:* MailScanner discussion
>>> >> *Subject:* Re: MS Doesn't completely block spam with faulty
>>> attachments
>>> >>
>>> >> I agree that it isn't a good idea to notify the sender of a spam or
>>> virus
>>> >> message I'm not planning to do that, I know the troubles of
>>> backscatter.
>>> >>
>>> >> What I've configured is that if a user sends a completely normal
>>> >> (non-virus, non-spam) E-mail but with, for instance, a file named
>>> >> "CurriculumVitae.doc.pdf" (default output for a lot of PDF printers).
>>> The
>>> >> server sends out a warning to sender and the original message stripped
>>> of
>>> >> it's attachment to the recipient of the message. Notifying the sender
>>> is not
>>> >> strictly necessary but if this is only done for such non-virus,
>>> non-spam
>>> >> message, it isn't a problem either.
>>> >>
>>> >> The situation that bugs me is when some spam message with a file named
>>> >> "CurriculumVitae.doc.pdf" is received. The message hits the filename
>>> rule
>>> >> and* isn't processed any further to check if its a spam message*.
>>> Because
>>> >> it isn't processed any further, the warning messages are send out to
>>> both
>>> >> sender and original recipient.
>>> >>
>>> >> As I stated before, I can disable the sender notification. What I
>>> can't do
>>> >> is tell my customers (the recipients) that such wrongly named files,
>>> most
>>> >> containing important documents, are silently discarded. Sending spam
>>> to my
>>> >> customers that could have been recognized isn't an option either.
>>> >>
>>> >> The simplest solution, I think, would be to *continue processing* the
>>> >> message after a file name rule is hit, decide if the E-mail is HAM and
>>> in
>>> >> that case, send out the notifications. If the E-mail is spam, silently
>>> >> discard it.
>>> >> It would add a bit of load to the server but stopping spam is what
>>> it's all
>>> >> about, isn't it? :P
>>> >>
>>> >> On 1 September 2011 16:34, Julian Field <MailScanner at ecs.soton.ac.uk
>>> >wrote:
>>> >>
>>> >>> He's probably switched on some "Notify Senders" options. Bad idea :-(
>>> >>>
>>> >>>
>>> >>> On 01/09/2011 12:32, Martin Hepworth wrote:
>>> >>>
>>> >>>> what version of MS?
>>> >>>>
>>> >>>> I never inform the sender of junk as you end up with fake messages
>>> sent
>>> >>>> out.
>>> >>>>
>>> >>>> --
>>> >>>> Martin Hepworth
>>> >>>> Oxford, UK
>>> >>>>
>>> >>>>
>>> >>>> On 1 September 2011 08:17, Joolee <mailscanner at joolee.nl <mailto:
>>> >>>> mailscanner at joolee.nl>**> wrote:
>>> >>>>
>>> >>>> Hallo Everybody,
>>> >>>>
>>> >>>> I've experienced a small flood of virus E-mails. These E-mails
>>> >>>> (subj.: "ACH Payment *random number* Canceled") contain
>>> >>>> attachments named like: "report_082011-65.pdf.exe"
>>> >>>> They obviously get blocked by the "no executables" and "No double
>>> >>>> file extensions" rules. The problem is that after blocking them,
>>> >>>> an automated E-mail is send to the original recipient and the
>>> >>>> (faked) sender of the message, informing them of the blocked
>>> >>>> attachment.
>>> >>>>
>>> >>>> Had the E-mails been processed further, they would've probably hit
>>> >>>> the virusscanner (not tested) or spamassassin (gives a score of 27
>>> >>>> when tested) and the E-mail would've silently been discarded as a
>>> >>>> virus / spam / phishing.
>>> >>>>
>>> >>>> Is it possible to let the MailScanner continue it's processing
>>> >>>> when hitting the file name rules and / or running the filename
>>> >>>> rule at a later time?
>>> >>>> --
>>> >>>> MailScanner mailing list
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>>> mailscanner at lists.mailscanner.info>
>>> >>>> <mailto:mailscanner at lists.**mailscanner.info<
>>> mailscanner at lists.mailscanner.info>>
>>> >>>>
>>> >>>>
>>> >>>> http://lists.mailscanner.info/**mailman/listinfo/mailscanner<
>>> http://lists.mailscanner.info/mailman/listinfo/mailscanner>
>>> >>>>
>>> >>>> Before posting, read http://wiki.mailscanner.info/**posting<
>>> http://wiki.mailscanner.info/posting>
>>>
>>> >>>>
>>> >>>> Support MailScanner development - buy the book off the website!
>>> >>>>
>>> >>>>
>>> >>>>
>>> >>>>
>>> >>>>
>>> >>>> Jules
>>> >>>>
>>> >>>> --
>>> >>>> Julian Field MEng CITP CEng
>>> >>>> www.MailScanner.info
>>> >>>>
>>> >>>> Buy the MailScanner book at www.MailScanner.info/store
>>> >>>> Need help customising MailScanner? Contact me!
>>> >>>>
>>> >>>> PGP footprint: EE81 D763 3DB0 0BFD E1DC 7222 11F6 5947 1415 B654
>>> >>>> Follow me at twitter.com/JulesFM
>>> >>>>
>>> >>>> 'It's okay to live without all the answers' - Charlie Eppes, 2011
>>> >>>> 'All programs have a desire to be useful' - Tron, 1982
>>> >>>>
>>> >>>
>>> >>> --
>>> >>> This message has been scanned for viruses and
>>> >>> dangerous content by MailScanner, and is
>>> >>> believed to be clean.
>>> >>>
>>> >>> --
>>> >>> MailScanner mailing list
>>> >>> mailscanner at lists.mailscanner.**info <
>>> mailscanner at lists.mailscanner.info>
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>>> http://lists.mailscanner.info/mailman/listinfo/mailscanner>
>>> >>>
>>> >>> Before posting, read http://wiki.mailscanner.info/**posting<
>>> http://wiki.mailscanner.info/posting>
>>>
>>> >>>
>>> >>> Support MailScanner development - buy the book off the website!
>>> >>>
>>> >>
>>> >>
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>>> >>
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>>> >>
>>> >> Support MailScanner development - buy the book off the website!
>>> >>
>>> >>
>>>
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>>
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>>
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>>
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>>
>>
>
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