CVE-2008-5521 in AntiVir
Summary
by MITRE
Avira AntiVir 7.9.0.36 and possibly 7.8.1.28, 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-2008-5521 represents a significant bypass mechanism within Avira AntiVir 7.9.0.36 and potentially earlier versions when operating in conjunction with Internet Explorer 6 or 7. This flaw exploits the way security software interprets file signatures and extensions, creating a pathway for malicious actors to evade detection by manipulating the initial bytes of HTML documents. The vulnerability specifically targets the heuristic analysis capabilities of the antivirus software, which relies on file headers and extension patterns to identify potentially harmful content.
The technical implementation of this exploit involves placing an MZ header at the beginning of an HTML document, which is the standard signature for windows executable files. This header is typically the first two bytes of any windows executable file, consisting of the characters "MZ" followed by additional metadata. By embedding this signature at the start of an HTML document, the malicious payload can trick the antivirus software into treating the file as a legitimate executable rather than an HTML document. The attacker can then modify the filename to remove the extension entirely or append extensions such as .txt or .jpg, which are commonly associated with benign file types. This technique leverages the fact that many antivirus systems perform initial detection based on file signatures before examining the actual content and extension information.
The operational impact of this vulnerability extends beyond simple detection bypass, as it demonstrates a fundamental weakness in how antivirus systems handle file type identification and content analysis. When Internet Explorer 6 or 7 processes these manipulated documents, the browser's handling of file extensions and the antivirus software's signature-based detection mechanisms create a window of opportunity for attackers to deliver payloads that would otherwise be flagged as malicious. The exploit specifically references CVE-2006-5745, which indicates that this vulnerability could be used to deliver more sophisticated attacks that take advantage of previously discovered exploits within the browser or operating system. This creates a multi-layered threat scenario where both the antivirus system and the web browser become compromised through the manipulation of file metadata.
This vulnerability aligns with CWE-457, which describes the use of uninitialized variables, and more specifically relates to CWE-20, which covers "Improper Input Validation" in the context of file handling and content analysis. The attack pattern corresponds to techniques described in the ATT&CK framework under T1059 for command and script interpreters, and T1071 for application layer protocols, particularly when the malicious HTML document contains embedded scripts that leverage the browser's execution environment. The vulnerability also demonstrates characteristics of T1133, which covers external remote services, as the attack relies on the interaction between the local antivirus system and external web content. Organizations using affected versions of Avira AntiVir should consider immediate mitigation through software updates, implementation of additional network-based detection measures, and enhanced user education regarding suspicious file downloads and execution patterns.
The broader implications of this vulnerability highlight the challenges faced by traditional signature-based antivirus systems in dealing with polymorphic and obfuscated malware. Modern security approaches have evolved to address such limitations through behavioral analysis, machine learning algorithms, and multi-layered detection techniques that do not rely solely on file headers or extensions. This vulnerability serves as a historical example of why signature-based detection alone proves insufficient against determined attackers who understand the intricacies of security software operation. The issue also underscores the importance of comprehensive testing of security products across different browser environments and the need for robust file type identification mechanisms that can distinguish between legitimate and malicious content regardless of how the file is presented to the system.