Apple has released a new Security Update - 2004-08-09 - which it recommends for all users.
It repairs a flaw in which: "Malicious PNG images can cause application crashes and could execute arbitrary code," according to Apple.
Apple describes the problem: "A number of buffer overflows, null pointer de-references and integer overflows have been discovered in the reference library for reading and writing PNG images. These vulnerabilities have been corrected in libpng which is used by the CoreGraphics and AppKit frameworks in Mac OS X. After installing this update, applications that use the PNG image format via these frameworks will be protected against these flaws."