The gfxTextRun::SanitizeGlyphRuns function in gfx/thebes/src/gfxFont.cpp in the browser engine in Mozilla Firefox 3.6 before 3.6.2 on Mac OS X when the Core Text API is used does not properly perform certain deletions which allows remote attackers to cause a denial of service (memory corruption and application crash) and possibly execute arbitrary code via an HTML document containing invisible Unicode characters as demonstrated by the U+FEFF U+FFF9 U+FFFA and U+FFFB characters.