【警告】幾家防毒軟體無法檢測出內含病毒ZIP壓縮檔
McAfee VirusScan 6.0
Sophos Anti-Virus 3.78d
Sophos Anti-Virus 3.78
Computer Associates eTrust Antivirus EE 7.0
Computer Associates eTrust Antivirus EE 6.0
Kaspersky Antivirus Kaspersky Antivirus
RAV AntiVirus RAV AntiVirus
Remote exploitation of an exceptional condition error in multiple vendors' anti-virus software allows attackers to bypass security protections by evading virus detection.
The problem specifically exists in the parsing of .zip archive headers. The .zip file format stores information about compressed files in two locations - a local header and a global header. The local header exists just before the compressed data of each file, and the global header exists at the end of the .zip archive. It is possible to modify the uncompressed size of archived files in both the local and global header without affecting functionality. This has been confirmed with both WinZip and Microsoft Compressed Folders. An attacker can compress a malicious payload and evade detection by some anti-virus software by modifying the uncompressed size within the local and global headers to zero.
Successful exploitation allows remote attackers to pass malicious payloads within a compressed archive to a target without being detected. Most anti-virus engines have the ability to scan content packaged with compressed archives. As such, users with up-to-date anti-virus software are more likely to open attachments and files if they are under the false impression that the archive was already scanned and found to not contain a virus.
卡巴斯基實驗室消除對 zip 檔案做防毒掃描時的缺陷
2004.10.25.Kaspersky Labs
發佈詳細的潛在網路攻擊和安全弱點出版物的資訊安全專業公司iDefence已發現了一些防毒產品中存在的一個缺陷，這些產品包括McAfee、Computer Associates、Kaspersky Lab、Sophos、Eset 和 RAV。
卡巴斯基實驗室證實，上述缺陷確實存在。該缺陷來自解析zip格式檔案時的一個特殊條件，有被利用的可能，使駭客能逃避病毒檢測從而避開安全防護系統。
.zip格式的檔案在兩個位置保存壓縮檔案的資訊(local/ global header)，在header中包含解壓檔案的實際大小的資料。如果檔案可變的真實大小的資料被改變為零，防毒掃描程式會認為該檔案太小，不可能包含任何潛在的威脅，因而不予掃描，而且，改變某檔案可變的大小，並不影響該壓縮檔案的功能，檔案仍可被正確解壓。
卡巴斯基實驗室防毒研究部主任葉夫根尼.卡巴斯基說，“我們感謝iDefence公司提醒我們注意這一缺陷。儘管這在理論上存在安全風險，但我們尚未檢測到任何企圖利用這一缺陷的行為。我們3.x 、 4.x和 5.x版產品的每週升級缺陷修補即將發佈，以消除防毒掃描程式與.zip壓縮檔案間的這一矛盾”。
