Off-by-one error in the png_formatted_warning function in pngerror.c in libpng 1.5.4 through 1.5.7 might allow remote attackers to cause a denial of service (application crash) and possibly execute arbitrary code via unspecified vectors, which trigger a stack-based buffer overflow.
http://www.libpng.org/pub/png/libpng.html
http://security.gentoo.org/glsa/glsa-201206-15.xml