HAProxy 1.4 before 1.4.24 and 1.5 before 1.5-dev19, when configured to use hdr_ip or other "hdr_*" functions with a negative occurrence count, allows remote attackers to cause a denial of service (negative array index usage and crash) via an HTTP header with a certain number of values, related to the MAX_HDR_HISTORY variable.
https://bugzilla.redhat.com/show_bug.cgi?id=974259
http://www.ubuntu.com/usn/USN-1889-1
http://www.debian.org/security/2013/dsa-2711
http://secunia.com/advisories/54344
http://rhn.redhat.com/errata/RHSA-2013-1204.html