CVE 2011-2691
The png_err function in pngerror.c in libpng 1.0.x before 1.0.55, 1.2.x before 1.2.45, 1.4.x before 1.4.8, and 1.5.x before 1.5.4 makes a function call using a NULL pointer argument instead of an empty-string argument, which allows remote attackers to cause a denial of service (application crash) via a crafted PNG image.
See the 
CVE page on Mitre.org
for more details.
