CVE-2008-5541 in Sophos
Summary
by MITRE
Sophos Anti-Virus 4.33.0, 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 • 10/22/2018
The vulnerability described in CVE-2008-5541 represents a significant bypass mechanism within Sophos Anti-Virus 4.33.0 that specifically targets the detection capabilities when Internet Explorer 6 or 7 is in use. This flaw exploits the way security software interprets file signatures and extensions during malware analysis, creating a pathway for malicious actors to evade automated threat detection systems. The vulnerability operates through a sophisticated manipulation of file identification protocols that are commonly relied upon by endpoint protection solutions to determine file type and potential threat level.
The technical implementation of this vulnerability relies on the manipulation of the MZ header, which is the standard signature that identifies executable files in the windows operating system. By placing this MZ header at the beginning of an HTML document, the malicious file appears to contain executable content to the antivirus engine, even though it is actually a carefully crafted HTML file. The attack vector becomes more sophisticated when combined with filename manipulation techniques that remove or alter the traditional file extension. When filenames lack extensions or are given extensions like .txt or .jpg, the antivirus system's heuristic analysis becomes confused, as it cannot properly categorize the file based on standard identification methods. This manipulation directly impacts the way antivirus software performs file type recognition and threat assessment.
The operational impact of this vulnerability extends beyond simple detection bypass to potentially enable more sophisticated attack campaigns. When combined with known exploits like CVE-2006-5745, which targets vulnerabilities in internet explorer's handling of certain HTML elements, this vulnerability creates a multi-layered attack approach that can successfully deliver malicious payloads to users. The attack demonstrates how traditional signature-based detection methods can be circumvented through careful manipulation of file characteristics, highlighting weaknesses in heuristic analysis and file extension validation processes. This vulnerability particularly affects environments where users frequently interact with web content through older versions of internet explorer, creating a significant risk for organizations that have not fully migrated away from legacy systems.
This vulnerability aligns with several common attack patterns documented in the attack tactic, technique, and control framework, specifically targeting the execution and persistence phases of the attack lifecycle. The flaw represents a weakness in the input validation and file analysis processes that are fundamental to endpoint protection systems, as outlined in the common weakness enumeration standards. The ability to manipulate file signatures and extensions to bypass detection mechanisms directly relates to techniques described in the attack framework where adversaries attempt to evade security controls through file format manipulation. Organizations implementing security controls should recognize this vulnerability as an example of how legacy system compatibility requirements can create unexpected security gaps, particularly when older browsers continue to be used in enterprise environments.
The mitigation strategies for this vulnerability require a multi-faceted approach that addresses both the immediate detection bypass and the underlying system compatibility issues. Organizations should implement enhanced file analysis protocols that go beyond simple signature matching and extension validation, incorporating more sophisticated heuristic analysis that can detect anomalous file structures regardless of their apparent extension. The solution involves updating antivirus definitions and implementing stricter file validation processes that examine the complete file structure rather than relying solely on filename extensions. Additionally, organizations must consider the broader implications of supporting legacy browser versions and develop comprehensive migration strategies to reduce exposure to vulnerabilities like CVE-2008-5541. The vulnerability serves as a reminder that security controls must continuously evolve to address emerging attack techniques that exploit system compatibility requirements and legacy support considerations.