CVE-2005-3382 in Sophos
Summary
by MITRE
Multiple interpretation error in Sophos 3.91 with the 2.28.4 engine allows remote attackers to bypass virus scanning via a file such as BAT, HTML, and EML with an "MZ" magic byte sequence which is normally associated with EXE, which causes the file to be treated as a safe type that could still be executed as a dangerous file type by applications on the end system, as demonstrated by a "triple headed" program that contains EXE, EML, and HTML content, aka the "magic byte bug."
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Analysis
by VulDB Data Team • 07/12/2018
The vulnerability described in CVE-2005-3382 represents a critical file type interpretation flaw within Sophos antivirus software version 3.91 running with the 2.28.4 engine. This issue stems from the software's improper handling of file magic bytes, specifically the MZ signature that traditionally identifies executable files. The flaw allows attackers to craft malicious files that appear benign to the antivirus scanner while retaining executable capabilities. The vulnerability is particularly dangerous because it exploits the fundamental trust users place in antivirus detection systems, creating a false sense of security that can be easily subverted.
The technical root cause of this vulnerability lies in the software's file type detection mechanism which relies on magic number identification to classify file types. When a file begins with the MZ magic byte sequence, Sophos incorrectly interprets it as a safe file type rather than recognizing it as an executable file that requires deeper inspection. This misclassification occurs because the antivirus engine fails to properly validate the file's actual content structure against its magic byte signature, instead applying a simplistic heuristic approach that doesn't account for the possibility of malicious file concatenation or obfuscation techniques. The vulnerability specifically affects file extensions such as BAT, HTML, and EML that can be crafted to contain MZ headers, allowing them to bypass standard scanning procedures.
The operational impact of this vulnerability is severe and demonstrates a sophisticated attack vector that can be exploited across multiple platforms and file types. Attackers can create what is referred to as "triple headed" programs that combine multiple file types into a single malicious payload, where the MZ header tricks the antivirus into treating the entire file as safe while preserving the underlying executable content. This allows malicious code to execute on target systems without detection, potentially leading to complete system compromise, data theft, or further attack propagation. The vulnerability affects the core security functionality of the antivirus software, undermining its primary purpose of protecting against malware threats.
This vulnerability aligns with CWE-20, which addresses "Improper Input Validation," and demonstrates characteristics consistent with ATT&CK technique T1059.007 for 'Command and Scripting Interpreter: JavaScript' and T1070.004 for 'Indicator Removal on Host: File Deletion.' The flaw represents a classic case of insufficient validation and sanitization of file content, where the antivirus engine fails to properly verify file integrity and content structure. Organizations affected by this vulnerability should implement immediate mitigations including updating to patched versions of Sophos software, implementing additional network-based detection measures, and establishing more robust file validation procedures. The incident highlights the critical importance of proper file type handling and validation in security software, as well as the need for comprehensive testing of heuristic detection systems against adversarial file construction techniques that exploit implementation weaknesses rather than just signature-based detection gaps.