CVE-2005-3373 in Dr. Web
Summary
by MITRE
Multiple interpretation error in Dr.Web 4.32b 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 identified as CVE-2005-3373 represents a critical file type interpretation flaw in Dr.Web 4.32b antivirus software that fundamentally undermines the security posture of systems relying on this protection. 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 malicious actors to craft files that appear benign to the antivirus scanner while retaining executable capabilities, creating a dangerous bypass mechanism that can evade detection and protection measures.
The technical implementation of this vulnerability exploits the conventional method by which antivirus systems identify file types through magic bytes - specific sequences of bytes that indicate the file format and type. When Dr.Web encounters a file containing the MZ magic byte sequence, it incorrectly classifies the file as safe, despite the file potentially containing malicious content in other formats such as HTML or EML. This misclassification occurs because the system fails to properly validate the complete file structure and content, instead relying on a single identifying byte pattern. The vulnerability specifically affects files that combine multiple formats within a single container, where the MZ header is embedded within a larger file structure that also contains HTML or EML components.
The operational impact of this vulnerability extends far beyond simple detection failure, as it creates a pathway for sophisticated attacks that can bypass multiple layers of security. Attackers can construct what is referred to as a "triple headed" program that combines EXE, EML, and HTML content within a single file, exploiting the magic byte bug to deliver malicious code that executes with the privileges of the target application. This approach demonstrates the principle of file format confusion and misidentification that is categorized under CWE-153, which addresses improper handling of file type identification. The vulnerability essentially allows an attacker to perform a form of file type confusion attack where the system's interpretation of file type differs from the actual content, creating a security gap that can be exploited by threat actors.
The implications of this vulnerability align with several techniques described in the MITRE ATT&CK framework, particularly those related to execution through file format confusion and evasion of security controls. The attack vector demonstrates how a single point of failure in file type detection can create a comprehensive bypass of endpoint protection mechanisms. Security researchers have noted that this type of vulnerability can enable attackers to perform privilege escalation, execute arbitrary code, and maintain persistence within targeted systems. The flaw also highlights the importance of proper input validation and file format parsing within security software, as the system's failure to properly validate file structures creates a dangerous condition where malicious files can be executed without proper security checks.
Mitigation strategies for this vulnerability require immediate patching of the affected Dr.Web version, but also necessitate broader architectural considerations in antivirus design. Organizations should implement multiple layers of file validation and content analysis beyond simple magic byte detection, including comprehensive file structure analysis and behavioral monitoring. The vulnerability underscores the critical need for security software to validate file content thoroughly rather than relying on single identifying markers, and demonstrates how seemingly minor implementation flaws can create significant security risks. Regular security assessments and penetration testing should specifically target file type interpretation mechanisms to identify similar vulnerabilities that could compromise system security.