A security advisory has been released for libpng, the "official PNG reference library". Libpng is used by ImageMagick, "a software suite to create, edit, and compose bitmap images". Some Rails applications use it to convert, resize or to create thumbnails. The original security advisory was issued by oCERT:

Applications using libpng that install unknown chunk handlers, or copy unknown chunks, may be vulnerable to a security issue which may result in incorrect output, information leaks, crashes, or arbitrary code execution. The issue involves libpng incorrectly handling zero length chunks which results in uninitialized memory affecting the control flow of the application.

The security advisory from libpng reads:

We believe this is a rare circumstance. It occurs in "pngtest" that is a part of the libpng distribution, in pngcrush, and in recent versions of ImageMagick (6.2.5 through 6.4.0-4). We are not aware of any other vulnerable applications. 

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.