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Attachment Security Checks

I was going back in the history seeing how the current security scan (paranoid one) came about

Code: [Select]
(iframe|\\<\\?|\\<%|html|eval|body|script\W|[CF]WS[\x01-\x0C]) //Improved regular expression detection 
(iframe|\\<\\?php|\\<\\?[\s=]|\\<%[\s=]|html|eval|body|script\W) // Don't allow the word 'description' to trigger a false positive.
(iframe|\\<\\?php|\\<\\?[\s=]|\\<%[\s=]|html|eval|body|script) // Added protection against <?= and <%=
(iframe|\\<\\?php|\\<\\?\s|\\<%\s|html|eval|body|script) // Relax the conditions for an avatar to be refused.
(iframe|\\<\\?php|\\<\\?|\\<%|html|eval|body|script) // Prevent certain ascii data to appear in avatars

The current one looks for \< or \<\ or \<% and will fail ... seems pretty strict to me, so strict in fact that probably no one uses it since the odds of find \< are darn good.

Looking at the progression, I don't think that was the intention but wanted to get some others thoughts on that.  I'm not sure what the signature in the file would be.  Even the earlier ones of |\\<\\?php  which means \<php or \<\php don't make sense to me, I could see \\<\\\?php or even \\<\\?\?php 

Any of you at heart hackers have insight on this one?
Be safe, Be kind, Happy Programing

Re: Attachment Security Checks

Reply #1
I searched a bit and found a simple example here:
http://ha.ckers.org/blog/20070604/passing-malicious-php-through-getimagesize/
I copied in an hex-editor the image code and it works (the image I mean, I can see the phpinfo output).

Then I used that image to test Elk's paranoid checks and they work (I also used the shor-tags and they are detected).
So the pattern (at least some) are correct and detect the expected code.

Then, to verify, I put the last of these regexp in here:
http://rubular.com/
meh, as it is, it doesn't match <?php
Converting the double escapes into single escapes it is able to match things.

I finally decided to test the preg_match in isolation:
Code: [Select]
$cur_chunk = '<?';
echo preg_match('~(\\<\\?)~i', $cur_chunk);
???
Why does it give 1?
Shouldn't this regexp match a \< with or without a \ after it?

 emanuele is confused...
Better go bed and forget about things I don't understand... lol
Bugs creator.
Features destroyer.
Template killer.

Re: Attachment Security Checks

Reply #2
I see this note in the preg section
Quote
Single and double quoted PHP strings have special meaning of backslash. Thus if \ has to be matched with a regular expression \\, then "\\\\" or '\\\\' must be used in PHP code.
In some testing ~\\<~
matches < or \<

~\\\\<~
only matches \< not <

~\\?~
matches ? and \?

~\\<\\?~
will match \<? or <? but not \<\? and I would have expected it to based on the above.

So this appears to be another php thing due to its use of \ as an escape sequence in a string on top of  preg's use of \ as the same escape sequence.  Still not clear on the rules though.
Be safe, Be kind, Happy Programing

Re: Attachment Security Checks

Reply #3
Do you remember if something was changed here?
Bugs creator.
Features destroyer.
Template killer.

Re: Attachment Security Checks

Reply #4
Nope ... It was mostly my confusion over both php and preg using \ as the escape character.  Forgetting that had me looking at that regex going wtf ?
Be safe, Be kind, Happy Programing

Re: Attachment Security Checks

Reply #5
Then I move it out. :P
Bugs creator.
Features destroyer.
Template killer.