Hello Andrey, Few comments on this - Correct me if I am wrong, "forged magic byte" might not always be able to fool the AV in real scenario (especially EXEs) unless you are talking about Static Virus scanners. In past few years the AV scanning technology has improved a lot and has gone even beyond "heuristic scanning techniques". >> The problem exists in the scanning engine - in the routine that determines the file type. >> If some file types (file types tested are .BAT, .HTML and .EML) changed to have the MAGIC BYTE >> of the EXE files (MZ) at the beginning, then many antivirus programs will be unable to detect >> the malicious file. It will break the normal flow of the antivirus scanning and many existent >> and future viruses will be undetected. Especially in case of EXEs, AFAIK not all EXEs has the same 'MAGIC BYTE' (MZ). MZ only appears in the first two bytes of Win32 executable files. Most older file types such as original .com files, any Linux/Mac files, and almost all scripting files do not contain MZ in the header. In fact the EICAR test virus which can be represented as a .txt or a .com file is one such file. It is a fully executable .com file that does not contain the MZ bytes and still executes on Win32. This implies that the AV scan engine doesn't just rely on the 'magic byte'. Changing the magic byte might fool the static AV scanners and maybe some current Avs but this might not work in case of real Viruses. As the scan engine do a heuristic scan and doesn't just rely upon the magic byte. I published a paper on similar topi "Anti-Virus Evasion Techniques" almost a year back which talks about various evasion techniques. It can be downloaded from here : http://hackingspirits.com/eth-hac/papers/whitepapers.asp As I haven't tested your finding on real viruses so can't say if at all I am wrong especially incase comments related to EXEs. However, in anycase if this exploit works for real viruses then this will imply that heuristic scan is a Joke ;-). Although heuristics can be fooled by many advance techniques (eg - obfuscation / polymorphism) but if it is fooled by this technique then I believe there are lot of work waiting for Guys @ AV Schools ;-) - Tr0y (www.hackingspirits.com) -----Original Message----- From: full-disclosure-bounces@xxxxxxxxxxxxxxxxx [mailto:full-disclosure-bounces@xxxxxxxxxxxxxxxxx] On Behalf Of Andrey Bayora Sent: Tuesday, October 25, 2005 8:38 AM To: full-disclosure@xxxxxxxxxxxxxxxxx Cc: bugtraq@xxxxxxxxxxxxxxxxx Subject: [Full-disclosure] Multiple Vendor Anti-Virus Software DetectionEvasion Vulnerability through forged magic byte Multiple Vendor Anti-Virus Software Detection Evasion Vulnerability through forged magic byte. AUTHOR: Andrey Bayora (www.securityelf.org) For more details, screenshots and examples please read my article "The Magic of magic byte" at www.securityelf.org . In addition, you will find a sample "triple headed" program which has 3 different 'execution entry points', depending on the extension of the file (exe, html or eml) - just change the extension and the SAME file will be executed by (at least) THREE DIFFERENT programs! (thanks to contributing author Wayne Langlois from www.diamondcs.com.au). DATE: October 25, 2005 VULNERABLE vendors and software (tested): 1. ArcaVir 2005 (engine 2005-06-03,vir def 2005-06-27, scanner ver 2005-03-06, package ver 2005-06-21) 2. AVG 7 (updates 24 June, ver.7.0.323, virus base 267.8.0/27) 3. eTrust CA (ver 7.0.1.4, engine 11.9.1, vir sig. 9229) 4. Dr.Web (v.4.32b, update 27.06.2005) 5. F-Prot (ver. 3.16c, update 6/24/2005) 6. Ikarus (latest demo version for DOS) 7. Kaspersky (update 24 June, ver. 5.0.372) 8. McAfee Internet Security Suite 7.1.5 (updates 25 June, ver 9.1.08, engine 4.4.00, dat 4.0.4519 6/22/2005) 9. McAfee Corporate (updates 25 June, ver. 8.0.0 patch 10, vir def 4521, engine 4400) 10. Norman ( ver 5.81, engine 5.83.02, update 2005/06/23) 11. TrendMicro PC-Cillin 2005 (ver 12.0.1244, engine 7.510.1002, pattern 2.701.00) 12. TrendMicro OfficeScan (ver7.0, engine 7.510.1002, vir pattern 2.701.00 6/23/2005) 13. Panda Titanium 2005 (updates 24 June, ver 4.02.01) 14. UNA - Ukrainian National Antivirus (ver. 1.83.2.16 kernel v.265) 15. Sophos 3.91 (engine 2.28.4, virData 3.91) IMPORTANT NOTE: Similar vulnerability may exist in many other antivirus\anti-spyware desktop and gateway products. In addition, various "file filter" solutions may be affected as well. NOT VULNERABLE vendors and software (tested): 1. F-Secure (updates 24 June, ver 5.56 b.10450) 2. Avast (ver. 4.6.655, vir databas 0525-5 06/25/2005) 3. BitDefender (ver. 8.0.200, update 6/24/2005, engine 7.01934) 4. ClamWin (ver. 0.86.1, upd 24 June 2005) 5. NOD32 (updates 24 June, ver 2.50.25, vir database 1.1152) 6. Symantec Corporate (ver 10.0.0.359, engine 103.0.2.7) 7. Norton Internet Security 2005 (ver 11.5.6.14) 8. VBA32 (ver 3.10.4, updates 27.06.2005) 9. HBEDV Antivir Personal (ver 6.31.00.01, engine 6.31.0.7, vir def 6.31.0.109 6/24/2005) 10. Sophos 5 (ver. 5.0.2, vir def 3.93, upd 6/30/2005) 11. Sophos 3.95 (engine 2.30.4) SEVERITY: critical DESCRIPTION: The problem exists in the scanning engine - in the routine that determines the file type. If some file types (file types tested are .BAT, .HTML and .EML) changed to have the MAGIC BYTE of the EXE files (MZ) at the beginning, then many antivirus programs will be unable to detect the malicious file. It will break the normal flow of the antivirus scanning and many existent and future viruses will be undetected. NOTE: In my test, I used the EXE headers (MZ), but it is possible to use other headers (magic byte) that will lead to the same effect. ANALYSIS: Some file types like .bat, .html and .eml can be properly executed even if they have some "unrelated" beginning. For example, in the case of .BAT files - it is possible to prepend some "junk" data at the beginning of the file without altering correct execution of the batch file. In my tests, I used the calc.exe headers (first 120 bytes - middle of the dosstub section) to change 5 different files of existing viruses. In addition, the simplest test of this vulnerability is to prepend only the magic byte (MZ) to the existing malicious file and check if this file is detected by antivirus program. NOTE, that this is NOT the case where the change of existing virus file resulted in the "broken" detection signature (see details and the test logic in "The Magic of magic byte" article at www.securityelf.org). WORKAROUND: I did not found any effective one besides of patching the vulnerable engine. CREDITS: The idea for this vulnerability came during discussions from Wayne Langlois at diamondcs.com.au, who hinted that JPEGs could probably be exploited in this way. TIME LINE: July 13, 2005 - Initial vendor notification July 16, 2005 - Second vendor notification .....Waiting.....Waiting.... October 24, 2005 - Public disclosure (uncoordinated) _______________________________________________ Full-Disclosure - We believe in it. Charter: http://lists.grok.org.uk/full-disclosure-charter.html Hosted and sponsored by Secunia - http://secunia.com/
