-----BEGIN PGP SIGNED MESSAGE----- ============================================================================= FreeBSD-SA-02:26.accept Security Advisory The FreeBSD Project Topic: Remote denial-of-service when using accept filters Category: core Module: kernel Announced: 2002-05-29 Credits: Mike Silbersack Affects: FreeBSD 4.5-RELEASE FreeBSD 4-STABLE after 2001-11-22 and prior to the correction date Corrected: 2002-05-21 18:03:16 UTC (RELENG_4) 2002-05-28 18:27:55 UTC (RELENG_4_5) FreeBSD only: YES I. Background 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 criteria. 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 dates. 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 ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-02:26/accept.patch # fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-02:26/accept.patch.asc b) Execute the following commands as root: # cd /usr/src # patch < /path/to/patch c) Recompile your kernel as described in and reboot the system. VI. Correction details The following list contains the revision numbers of each file that was corrected in FreeBSD. Path Revision Branch - ------------------------------------------------------------------------- src/sys/kern/uipc_socket.c RELENG_4 1.68.2.21 RELENG_4_5 1.68.2.17.2.1 src/sys/kern/uipc_socket2.c RELENG_4 1.55.2.15 RELENG_4_5 1.55.2.10.2.1 src/sys/conf/newvers.sh RELENG_4_5 1.44.2.20.2.7 - ------------------------------------------------------------------------- VII. References -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.7 (FreeBSD) iQCVAwUBPPUCC1UuHi5z0oilAQFApAP6ApvgOydr72UHKHXiRZnGxiwBhpyVE+mH 5xdDP45s0GaUChA7GLbpv0hLL5syNPMavo7ygRuqD6pHFA0xpVn3hUXtLh09dhwS YTDWrC2VL9QJmFWIxMNzo0OXD1uDBrlGEk3Ew0jWT2ewe46QW1czpPYCeGg4Bx+i +FzEQ9V4D8k= =W+BP -----END PGP SIGNATURE-----