Re: Generic SQL injection false positives

[Alex Kirk <akirk () sourcefire com>]

Let me ask a quick clarifying question here. As I understand my PCRE, "$"
means "end-of-buffer" (if you've specified the "m" option, or just
"end-of-line" otherwise), and "^" outside of a character class means
"start-of-buffer" (or line, depending on the use of "m"). Thus,
"/$update^/i" would never actually match anything, since the end can't come
before the beginning.

Were you trying to work some other PCRE magic I'm unfamiliar with? Or should
I just read your verbal description and instead try, say,
"/[^\nA-Z]update[^\nA-Z]/smi"?

On Mon, Dec 28, 2009 at 4:30 PM, Paul Schmehl <pschmehl_lists () tx rr com>wrote:

> --On December 28, 2009 12:10:37 PM -0600 Matt Olney
> <molney () sourcefire com> wrote:
>
> > I see a lot of false positive for generic SQL injection rules.  For
> > example, SID 13514 shown here:
> >
> > alert tcp $EXTERNAL_NET any -> $HTTP_SERVERS $HTTP_PORTS (msg:"SQL
> > generic sql update injection attempt"; flow:established,to_server;
> > content:"update"; nocase; pcre:"/update[^\n]*set/i"; metadata:policy
> > security-ips drop, service http;
> > reference:url,www.securiteam.com/securityreviews/5DP0N1P76E.html;
> > classtype:web-application-attack; sid:13514; rev:4;)
> >
> > Alas it alerts for normal traffic like this:
> >
> > GET /get_updates_1/assessment/frameset_yellow.asp  HTTP/1.1
>
> I don't see how a sql injection attempt is going to begin with any
> character other than a space preceding it.  How would the sql engine be
> able to parse that?  ISTM that the update could simply be anchored on both
> sides; e.g pcre:"$update^/i";  For update to work, the only thing that can
> be on either side of it is a non-alpha character or a single quote, which
> the sql parser will discard.  If you want to include set (which makes
> sense), I would make it a separate detection.  A typical update statement
> would be UPDATE table SET blah='foo' where blah='bar' or blah like '%doo%';
>
> Something like this would be better, in my opinion.
>
> alert tcp $EXTERNAL_NET any -> $HTTP_SERVERS $HTTP_PORTS (msg:"SQL generic
> sql update injection attempt"; flow:established,to_server;
> content:"update"; nocase; pcre:"/$update^/i"; content:"set"; nocase;
> pcre:"/$set^/i"; metadata:policy security-ips drop, service http;
> reference:url,www.securiteam.com/securityreviews/5DP0N1P76E.html;
> classtype:web-application-attack; sid:13514; rev:5;)
>
> Mind you, I haven't tested it, but it would certainly eliminate the false
> positive given in the example.
>
> Paul Schmehl, If it isn't already
> obvious, my opinions are my own
> and not those of my employer.
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-- 
Alex Kirk
AEGIS Program Lead
Sourcefire Vulnerability Research Team
+1-410-423-1937
alex.kirk () sourcefire com

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