Bug 516990 (CVE-2009-2726)

A remote DoS (stack memory exhaustion) flaw was discovered in asterisk.  Quoting upstream security advisory for further details:

  On certain implementations of libc, the scanf family of functions uses an
  unbounded amount of stack memory to repeatedly allocate string buffers
  prior to conversion to the target type. Coupled with Asterisk's allocation
  of thread stack sizes that are smaller than the default, an attacker may
  exhaust stack memory in the SIP stack network thread by presenting
  excessively long numeric strings in various fields.

  Note that while this potential vulnerability has existed in Asterisk for a
  very long time, it is only potentially exploitable in 1.6.1 and above,
  since those versions are the first that have allowed SIP packets to exceed
  1500 bytes total, which does not permit strings that are large enough to
  crash Asterisk. (The number strings presented to us by the security
  researcher were approximately 32,000 bytes long.)

  Additionally note that while this can crash Asterisk, execution of
  arbitrary code is not possible with this vector.

  http://downloads.asterisk.org/pub/security/AST-2009-005.html
  http://labs.mudynamics.com/advisories/MU-200908-01.txt

Upstream patches for various versions are linked from upstream advisory.