CVE-2008-5525 in ClamAV
Summary
by MITRE
ClamAV 0.94.1 and possibly 0.93.1, 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 • 05/10/2019
The vulnerability identified as CVE-2008-5525 represents a significant bypass issue within ClamAV antivirus software versions 0.93.1 and 0.94.1, specifically when analyzing HTML documents through Internet Explorer 6 or 7 browsers. This flaw exploits the way antivirus engines process file signatures and extensions, creating a pathway for malicious actors to evade detection mechanisms that rely heavily on file extension analysis. The vulnerability stems from ClamAV's insufficient validation of file headers when the filename extension does not match the actual file content, allowing attackers to disguise executable code within seemingly benign document formats.
The technical implementation of this vulnerability involves placing an MZ header at the beginning of an HTML document, which is the standard signature for windows executable files. This header contains the magic number 0x5a4d which identifies the file as a valid executable. By modifying the filename to lack an extension or to use .txt or .jpg extensions, attackers can trick ClamAV's heuristic analysis into treating the document as a legitimate text or image file rather than an executable. The MZ header effectively overrides the extension-based detection rules, allowing the embedded malware payload to bypass signature scanning and heuristic analysis. This technique specifically targets the file extension validation logic within ClamAV's engine, which fails to properly correlate the actual file content with the filename extension.
The operational impact of this vulnerability extends beyond simple malware evasion to potentially enable more sophisticated attack campaigns. When combined with other vulnerabilities such as CVE-2006-5745, which targets Internet Explorer vulnerabilities, attackers can create documents that appear harmless but contain executable code that will run when the document is opened in Internet Explorer. This creates a multi-layered attack vector where the first layer bypasses antivirus detection through the filename manipulation, while the second layer exploits browser vulnerabilities to execute malicious code. The vulnerability affects the core detection capabilities of ClamAV, potentially allowing attackers to deliver payloads that would otherwise be blocked by standard antivirus protection mechanisms, particularly in environments where ClamAV is the primary security tool.
This vulnerability aligns with several common attack patterns and security weaknesses documented in the CWE (Common Weakness Enumeration) catalog, specifically CWE-20, which covers "Improper Input Validation," and CWE-471, which addresses "Modification of Externally-Controllable Resource." The flaw demonstrates a classic case of insufficient validation of file metadata and content correlation, where the system fails to properly verify that the file type matches its actual content. From an ATT&CK framework perspective, this vulnerability maps to techniques involving T1059.005 (Command and Scripting Interpreter: Visual Basic) and T1070.004 (Indicator Removal on Host: File Deletion) as attackers can bypass detection to execute malicious code and potentially remove traces. Organizations using ClamAV in environments where HTML documents are frequently processed face elevated risk, particularly in scenarios involving web-based attacks or social engineering campaigns where attackers might manipulate file extensions to evade security controls. The vulnerability also highlights the importance of proper file type validation and the need for robust content analysis that goes beyond simple extension-based identification to ensure effective malware detection across all file formats.
The recommended mitigation strategies include immediate upgrading to ClamAV versions that address this vulnerability, implementing additional layers of security beyond signature-based detection, and establishing more robust file validation processes that verify content headers regardless of filename extensions. Network administrators should consider implementing file type restrictions and content analysis tools that can detect suspicious headers within documents, while also ensuring that email and web proxies have appropriate security measures to prevent the delivery of potentially malicious documents. Regular security assessments should verify that antivirus engines properly handle file content verification, particularly in environments where users interact with web-based content that may be manipulated to bypass detection mechanisms.