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.