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Home » Hacking News » FreeBSD-SA-02:26-accept


by Nikola Strahija on May 29th, 2002 FreeBSD features an accept_filter(9) mechanism which allows an application to request that the kernel pre-process incoming connections. For example, the accf_http(9) accept filter prevents accept(2) from returning until a full HTTP request has been buffered.

No accept filters are enabled by default. A system administrator must
either compile the FreeBSD kernel with a particular accept filter
option (such as ACCEPT_FILTER_HTTP) or load the filter using
kldload(8) in order to utilize accept filters.

II. Problem Description

In the process of adding a syncache to FreeBSD, mechanisms to remove
entries from the incomplete listen queue were removed, as only sockets
undergoing accept filtering now use the incomplete queue.

III. Impact

By simply connecting to a socket using accept filtering and holding a
few hundred sockets open (~190 with the default backlog value), one
may deny access to a service. In addition to malicious users, this
affect has also been reported to be caused by worms such as Code Red
which generate URLs that do not meet the http accept filter's

Systems are not affected by this bug unless they have enabled accept
filters in the kernel and are utilizing an application configured to
take advantage of this feature. Apache (versions 1.3.14 and later) is
the only application known to utilize accept filters by default.

IV. Workaround

Do not use accept filters. If you have enabled the ACCEPT_FILTER_DATA
or ACCEPT_FILTER_HTTP options in your kernel, remove these options and
recompile your kernel as described in
and reboot the
system. If you have loaded one of the kernel accept filters by using
kldload(8), then you must modify your startup scripts not to load
these modules and reboot your system. You may list loaded kernel
modules by using kldstat(8). If loaded, the HTTP accept filter will
be listed as `accf_http.ko', and the Data accept filter will be listed
as `accf_data.ko'.

For affected versions of Apache, accept filters may be disabled either
by adding the directive ``AcceptFilter off'' to your configuration
file, or via a compile-time option, depending upon the version.
Please see the Apache documentation for details.

V. Solution

1) Upgrade your vulnerable system to 4.5-STABLE; or to the RELENG_4_5
(4.5-RELEASE-p6) security branch dated after the respective correction

2) To patch your present system:

The following patch has been verified to apply to FreeBSD 4.5-RELEASE
and 4.5-STABLE systems.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch
# fetch

b) Execute the following commands as root:

# cd /usr/src
# patch and reboot the

VI. Correction details

The following list contains the revision numbers of each file that was
corrected in FreeBSD.

Path Revision
- -------------------------------------------------------------------------
- -------------------------------------------------------------------------

VII. References

Version: GnuPG v1.0.7 (FreeBSD)


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