CVE-2008-5523 in Avast Antivirus
Summary
by MITRE
avast! antivirus 4.8.1281.0, when Internet Explorer 6 or 7 is used, allows remote attackers to bypass detection of malware in an HTML document by placing an MZ header (aka "EXE info") at the beginning, and modifying the filename to have (1) no extension, (2) a .txt extension, or (3) a .jpg extension, as demonstrated by a document containing a CVE-2006-5745 exploit.
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Analysis
by VulDB Data Team • 12/02/2017
The vulnerability described in CVE-2008-5523 represents a significant security flaw in avast! antivirus version 4.8.1281.0 that specifically affects users running Internet Explorer 6 or 7. This issue demonstrates a sophisticated bypass mechanism that exploits the way antivirus software analyzes file extensions and headers to determine threat classification. The flaw allows attackers to craft malicious HTML documents that can evade detection by the antivirus system through manipulation of file naming conventions and binary headers.
The technical implementation of this vulnerability relies on the manipulation of executable file headers and the strategic use of file extensions to confuse the antivirus detection algorithms. By placing an MZ header at the beginning of an HTML document, attackers exploit the fact that avast! relies heavily on file extension analysis for threat detection. The MZ header is the signature that identifies executable files in the dos executable format, and when combined with specific filename modifications such as removing extensions entirely or using .txt and .jpg extensions, the antivirus system fails to properly identify the malicious content.
This vulnerability operates through a specific attack vector that leverages the interaction between the antivirus software and web browsers. When Internet Explorer 6 or 7 processes an HTML document containing malicious code with the modified headers and extensions, the antivirus software incorrectly classifies the file as benign due to its failure to properly analyze the binary content when the file name suggests it is a text or image file. This represents a classic case of heuristic bypass where the security solution's detection logic is circumvented through proper manipulation of file attributes.
The operational impact of this vulnerability is substantial as it allows remote attackers to deliver malware through seemingly harmless web content. The demonstration using CVE-2006-5745 exploit shows how this bypass technique could be used to deliver more sophisticated threats. This vulnerability particularly affects users running older versions of Internet Explorer, which were common in enterprise environments during that time period, making the attack surface quite broad.
The root cause of this vulnerability aligns with CWE-119, which addresses weak input validation and improper handling of file attributes. This weakness in the antivirus detection logic creates a condition where file headers and extensions are not properly cross-verified to determine file type. The flaw also demonstrates characteristics of techniques described in the ATT&CK framework under T1059 for command and scripting interpreter, where the bypass allows for more effective delivery of malicious code through web-based attack vectors. The vulnerability essentially creates a false positive detection scenario where legitimate-looking files are incorrectly classified as safe, allowing malware to execute without proper security screening.
Mitigation strategies for this vulnerability require multiple layers of defense including updating antivirus signatures, implementing more robust file type detection algorithms, and configuring web browsers with stricter content handling policies. Organizations should ensure that all antivirus software is updated to versions that properly address this header and extension validation issue, while also implementing network-based controls to monitor and block suspicious file transfers. The solution involves strengthening the antivirus detection engine to properly analyze file content regardless of extension manipulation and implementing additional security measures such as sandboxing for suspicious content analysis.