CVE-2005-3371 in Avg Antivirus
Summary
by MITRE
Multiple interpretation error in AVG 7 7.0.323 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."
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 07/26/2017
The CVE-2005-3371 vulnerability represents a critical file type interpretation flaw in AVG Antivirus version 7.0.323 that fundamentally undermines the security model of endpoint protection software. This vulnerability stems from the antivirus engine's improper handling of file magic bytes, specifically the MZ signature that traditionally identifies executable files. When a malicious file contains this MZ magic byte sequence, the antivirus system incorrectly classifies it as a safe file type rather than recognizing it as a potential executable threat. The flaw demonstrates a classic case of insufficient input validation and file type detection logic that fails to properly distinguish between file formats based on their actual content rather than superficial markers.
The technical exploitation of this vulnerability occurs through the manipulation of file headers and content structure to create what researchers termed "triple headed" programs. These malicious files combine multiple file types within a single container, using the MZ signature to bypass initial scanning phases while maintaining executable functionality. The vulnerability specifically affects files with extensions such as BAT, HTML, and EML, which are typically considered low-risk by antivirus systems. When these files are processed by AVG 7.0.323, the system's file type recognition engine misinterprets the MZ signature as indicating a safe file type rather than recognizing it as a potential executable payload. This misclassification allows the malicious content to pass through security checks undetected.
The operational impact of this vulnerability extends beyond simple bypass of antivirus protection, creating a dangerous scenario where users are exposed to potentially harmful executables that appear safe to security systems. The vulnerability enables attackers to craft files that can execute malicious code while appearing benign to antivirus software, potentially leading to system compromise, data theft, or further malware deployment. This type of vulnerability directly relates to CWE-20, which describes improper input validation, and represents a significant failure in the core security validation mechanisms of antivirus software. The attack vector demonstrates how a seemingly simple file format interpretation error can create a complete bypass of security controls, as the system fails to properly validate file content against its actual threat characteristics rather than relying solely on superficial file type indicators.
The exploitation of this vulnerability aligns with ATT&CK technique T1059, which covers execution through command and scripting interpreters, as the malicious files can execute code through various system interpreters. Additionally, the vulnerability relates to T1070, covering indicator removal, as attackers can potentially hide malicious activities within seemingly safe file structures. Organizations using affected versions of AVG antivirus software faced significant risk exposure, as this flaw allowed for the delivery of executables that could execute with full system privileges. The vulnerability highlights the importance of proper file content validation and the dangers of relying on superficial file type indicators rather than comprehensive content analysis. Security practitioners should note that this type of vulnerability demonstrates the critical need for robust file format detection systems and the potential for seemingly minor implementation flaws to create major security risks. The incident underscores the importance of proper security testing and validation of antivirus engines, particularly in how they handle edge cases and file format ambiguities, as these systems form the first line of defense against malicious file execution.
This vulnerability also illustrates the broader challenge of file format confusion in security systems, where legitimate file type indicators can be manipulated to bypass security controls. The flaw represents a fundamental breakdown in the security validation process, where the system's ability to properly identify threats is compromised by inadequate file type recognition logic. The impact extends to user trust in security systems, as legitimate security controls fail to detect potentially dangerous content. Organizations should implement layered security approaches that do not rely solely on file type detection, incorporating behavior-based analysis and content inspection to mitigate similar vulnerabilities. The vulnerability serves as a reminder that security systems must validate file content comprehensively rather than relying on single indicators that can be easily manipulated by attackers.