Remote DoS in libevent DNS parsing <= 1.2a Feb 19 2007 02:00AM
Jon Oberheide (jon oberheide org)
Author: Jon Oberheide <jon (at) oberheide (dot) org [email concealed]>
Date: Sun, February 18th, 2007


Application: libevent
Affected Versions: 1.2 - 1.2a
Vendor Website: http://monkey.org/~provos/libevent/
Type of Vulnerability: Denial of Service - Remote


The libevent API provides a mechanism to execute a callback function
when a specific event occurs on a file descriptor or after a timeout
has been reached. Furthermore, libevent also support callbacks due
to signals or regular timeouts.

libevent is meant to replace the event loop found in event driven
network servers. An application just needs to call event_dispatch()
and then add or remove events dynamically without having to change
the event loop. Currently, libevent supports /dev/poll, kqueue(2),
select(2), poll(2) and epoll(4).

Recently, support for non-blocking DNS resolution was added to


A bug exists in the parsing of DNS responses in libevent, specifically
in the handling of label pointers. Label pointers in DNS are meant to
cut down on redundant information and overall response size by
allowing a label to reference an arbitrary byte offset in the packet.
If a pointer references its own offset, a pointer loop is formed.
libevent's parsing code does not properly handle such pointer loops.


A malicious resolver, authoritative server, or inline attacker can
send a DNS reply containing a pointer loop, causing libevent's DNS
parsing to enter an endless loop, effectively DoS'ing the service.


Applications utilizing the DNS resolution functionality of libevent
should upgrade to version >= 1.3.

Jon Oberheide <jon (at) oberheide (dot) org [email concealed]>
GnuPG Key: 1024D/F47C17FE
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Version: GnuPG v2.0.2 (GNU/Linux)


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