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FreeBSD -- Infinite loop in SACK handling


Problem description:

When insufficient memory is available to handle an incoming selective acknowledgement, the TCP/IP stack may enter an infinite loop.


By opening a TCP connection and sending a carefully crafted series of packets, an attacker may be able to cause a denial of service.


On FreeBSD 5.4, the net.inet.tcp.sack.enable sysctl can be used to disable the use of SACK:

# sysctl net.inet.tcp.sack.enable=0

No workaround is available for FreeBSD 5.3.



portaudit: FreeBSD -- Infinite loop in SACK handling

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If you have found a vulnerability in a FreeBSD port not listed in the database, please contact the FreeBSD Security Officer. Refer to "FreeBSD Security Information" for more information.

Oliver Eikemeier <>