CVE-2005-3381 in UNA
Summary
by MITRE
Multiple interpretation error in Ukrainian National Antivirus (UNA) 1.83.2.16 with kernel 265 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-2005-3381 represents a critical interpretation error within the Ukrainian National Antivirus version 1.83.2.16 that operates with kernel version 265. This flaw stems from improper file type recognition mechanisms that rely heavily on magic byte sequences for file classification. The core issue manifests when the antivirus system encounters files containing the "MZ" magic byte sequence, which conventionally identifies executable files in the dos executable format. This misinterpretation allows malicious actors to craft files that appear benign to the antivirus scanner while retaining executable capabilities, creating a dangerous discrepancy between the scanner's classification and the actual file behavior.
The technical exploitation of this vulnerability occurs through the manipulation of file headers and content structures to include the "MZ" signature while embedding other file types such as batch files, html documents, or email message files. This creates a deceptive file structure where the antivirus system classifies the file as safe due to its apparent executable signature, but the actual content remains capable of executing malicious code through the host system's native file execution mechanisms. The vulnerability specifically affects the kernel's file interpretation logic, which fails to properly validate file content beyond the initial magic byte recognition, leading to a false positive security assessment.
The operational impact of this vulnerability extends beyond simple bypass of antivirus protection, as it enables attackers to create what researchers have termed "triple headed" programs that combine multiple file types within a single container. These hybrid files can simultaneously contain executable code, email message structures, and web content, allowing for sophisticated attack vectors that exploit the trust relationships between different system components. The vulnerability essentially creates a backdoor through which malicious payloads can execute without detection, as the antivirus system treats the file as safe despite its potential to deliver harmful content through various execution paths. This type of vulnerability directly relates to CWE-20, which addresses improper input validation, and represents a classic example of how insufficient validation can lead to security bypass scenarios.
The attack surface for this vulnerability encompasses any system running the affected antivirus version where users might encounter email attachments, downloaded files, or executable content that has been crafted to exploit the magic byte interpretation error. Security professionals should note that this vulnerability demonstrates the importance of comprehensive file validation beyond simple signature matching, as the "MZ" byte sequence alone does not guarantee file safety. Mitigation strategies should include immediate patching of the antivirus software, implementation of additional file validation layers, and enhanced monitoring for suspicious file behavior patterns. The vulnerability also highlights the need for robust file type detection systems that consider multiple factors beyond magic bytes, aligning with ATT&CK technique T1059.007 for command and scripting interpreter usage, as attackers can leverage this bypass to execute malicious commands through seemingly safe file types. Organizations should implement layered security approaches that do not rely solely on signature-based detection methods, particularly in environments where the affected software is deployed, as the vulnerability creates a persistent risk for systems that fail to properly validate file content structure and execution potential.