Free BSD is a free, Open Source unix operating system. More information
is available at http://www.freebsd.org/.
Remote exploitation of a denial of service (DoS) vulnerability in
FreeBSD's memory buffers (mbufs) could allow attackers to launch a DoS
By sending many out-of-sequence packets, a low bandwidth denial of
service attack is possible against FreeBSD. When the targeted system
runs out of memory buffers (mbufs), it is no longer able to accept or
create new connections.
Exploitation of this vulnerability requires that the targeted system has
at least one open TCP port.
The DoS will last until the port is closed, either by the attacker or
the target machine.
iDEFENSE has proof of concept exploit code demonstrating the impact of
iDEFENSE has confirmed that this vulnerability exists in FreeBSD 5.1
(default install from media). FreeBSD has stated that all versions are
"It may be possible to mitigate some denial-of-service attacks by
implementing timeouts at the application level."
"Do one of the following:
1) Upgrade your vulnerable system to 4-STABLE, or to the RELENG_5_2,
RELENG_4_9, or RELENG_4_8 security branch dated after the correction
2) Patch your present system:
The following patch has been verified to apply to FreeBSD 4.x and 5.x
a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.
[FreeBSD 4.8, 4.9] # fetch
ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-04:04/tcp47.patch # fetch
b) Apply the patch.
# cd /usr/src # patch < /path/to/patch
c) Recompile your kernel as described in
<URL:http://www.freebsd.org/handbook/kernelconfig.html> and reboot the
The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN-2004-0171 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org), which standardizes names for
01/22/2004 Exploit acquired by iDEFENSE
02/17/2004 iDEFENSE clients notified
02/18/2004 Initial vendor notification
02/18/2004 Initial vendor response
03/02/2004 Coordinated public disclosure
Alexander Cuttergo is credited with this discovery.
Copyright © 2004 Verisign, Inc.
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