CVE-2008-5531 in Fortiguard Antivirus
Summary
by MITRE
Fortinet Antivirus 3.113.0.0, 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.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 12/02/2017
This vulnerability exists within Fortinet Antivirus version 3.113.0.0 which fails to properly analyze HTML documents when processed through Internet Explorer 6 or 7 browsers. The flaw stems from the antivirus engine's inability to correctly identify malicious content when specific file naming conventions are employed in conjunction with embedded executable headers. The technical implementation of this vulnerability involves the deliberate placement of an MZ header at the beginning of HTML documents, which is the signature that identifies executable files in the windows operating system. This header is typically found at the beginning of executable files and serves as a marker that helps operating systems identify and execute binary programs.
The operational impact of this vulnerability is significant as it allows threat actors to bypass security controls designed to detect and block malware. Attackers can craft HTML documents that contain embedded malicious code by manipulating the file extension and header placement to fool the antivirus system. The vulnerability specifically targets the filename extension handling mechanism where documents with no extension, .txt extension, or .jpg extension can bypass detection when they contain the MZ header. This technique exploits the heuristic analysis capabilities of the antivirus software, which relies on file extension and header validation to determine threat status.
The attack vector demonstrates a sophisticated approach to evading security controls by leveraging the trust placed in file extension validation and the assumption that HTML documents with standard extensions should not contain executable content. When Internet Explorer 6 or 7 processes these specially crafted documents, the antivirus system incorrectly identifies them as benign HTML files rather than potentially malicious executables. This vulnerability aligns with CWE-119 which addresses improper restriction of operations within a limited error handling scope, and represents a classic example of bypassing input validation controls. The technique used to exploit this vulnerability is consistent with ATT&CK tactic T1059 which involves executing malicious code through various application interfaces.
The security implications extend beyond simple file extension manipulation as this vulnerability exposes a fundamental flaw in how the antivirus system processes and validates file content. The system's failure to properly analyze file headers regardless of extension creates a pathway for attackers to deliver malware payloads that would otherwise be detected by standard antivirus signatures and heuristic analysis. Organizations using this version of Fortinet Antivirus are particularly vulnerable when users interact with web content through Internet Explorer 6 or 7 browsers, as these browsers are commonly used in legacy environments where security updates may not be regularly applied. The vulnerability underscores the importance of comprehensive file validation that does not rely solely on extension-based detection methods and emphasizes the need for robust content analysis regardless of file naming conventions. The exploitation of this vulnerability requires minimal technical skill and can be accomplished through simple HTML document manipulation, making it particularly dangerous in enterprise environments where users may encounter such malicious content through web browsing or email attachments.