libpng 1.0.6 through 1.0.32, 1.2.0 through 1.2.26, and 1.4.0beta01 through 1.4.0beta19 allows context-dependent attackers to cause a denial of service (crash) and possibly execute arbitrary code via a PNG file with zero length "unknown" chunks, which trigger an access of uninitialized memory.
CWE-189