FreeBSD accept_filter(9) denial of service

freebsd-accept-filter-dos (9209) The risk level is classified as MediumMedium Risk


FreeBSD is vulnerable to a denial of service attack, caused by an error that can occur if accept filtering is enabled and a program that utilizes accept filtering is being used. If a remote attacker connects to a socket that uses accept filtering and keeps several hundred sockets open, the attacker could cause a denial of service against the affected service.


Denial of Service


Upgrade to latest version of 4.5-STABLE or to RELENG_4_5 (4.5-RELEASE-p6 security branch dated later than 28-May-2002), as listed in FreeBSD Inc., Security Advisory FreeBSD-SA-02:26.accept. See References.

— OR —

Apply the appropriate patch for your system, as listed in FreeBSD Inc., Security Advisory FreeBSD-SA-02:26.accept. See References.


  • FreeBSD Security Advisory FreeBSD-SA-02:26: Remote denial-of-service when using accept filters.
  • BID-4879: FreeBSD Accept Filter Denial of Service Vulnerability
  • CVE-2002-0794: The accept_filter mechanism in FreeBSD 4 through 4.5 does not properly remove entries from the incomplete listen queue when adding a syncache, which allows remote attackers to cause a denial of service (network service availability) via a large number of connection attempts, which fills the queue.
  • OSVDB ID: 5081: FreeBSD accept_filter Listen Queue DoS

Platforms Affected:

  • FreeBSD FreeBSD 4.0
  • FreeBSD FreeBSD 4.5


May 29, 2002

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