-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Marc Ruef wrote: | III. EXPLOITATION | | The following proof-of-concept has been published in the articles "Wie | mit GIF-Bildern Cross Site Scripting-Angriffe im Internet Explorer | umgesetzt werden können" in scip monthly Security Summary Issue 19. | September 2005 (pp. 12-14)[1] and "GIF-Bug im Internet Explorer 6 - | Proof of Concept" at computec.ch[2]: | | 01 <GIF89aŸ 8 ÷™fÿ™™> The reason that this works in this case is that this is *not* a GIF header; GIF headers do not begin with "<". It is well known that IE will interpret files as HTML that contain certain HTML tags if a another type detection doesn't override it. For Microsoft's vague documentation on this process, see: http://msdn.microsoft.com/workshop/networking/moniker/overview/appendix_a.asp If you remove the "<" at the beginning, then IE will detect the GIF signature, overriding its HTML detection, and show a 'broken image' icon with no interpretation of JavaScript. (Tested MSIE 6.0.2900.2180.xpsp_sp2) However the advice is good; as a general rule sites accepting uploads should validate them as carefully as possible, as IE may not recognize all file types properly. Invalid image file headers and HTML-like tags near the start of a file should be considered suspicious. - -- brion vibber (brion @ pobox.com) -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.4 (Darwin) Comment: Using GnuPG with Thunderbird - http://enigmail.mozdev.org iD8DBQFDMyh1wRnhpk1wk44RAtoDAJ9QIJbNMXro7z3pFCzXuOy1Oz10gACfZxTd OSITbHzoYn+T8Ozq0d6ZfQ4= =Gm+c -----END PGP SIGNATURE-----
