CVE-2005-3400 in Fortinet
Summary
by MITRE
Multiple interpretation error in Fortinet 2.48.0.0 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-2005-3400 represents a critical file type interpretation flaw within Fortinet's antivirus software version 2.48.0.0 that fundamentally undermines the security model of network-based threat detection. This issue stems from an improper handling of file magic bytes, specifically the MZ signature that traditionally identifies executable files, leading to a dangerous misclassification that allows malicious payloads to evade detection. The flaw operates at the core of file type recognition mechanisms, where the software's inability to properly validate file content against its declared type creates a pathway for attackers to craft files that appear benign but contain executable components.
The technical implementation of this vulnerability exploits the concept of magic number detection, where software applications rely on specific byte sequences at the beginning of files to determine their type and apply appropriate processing. In this case, the MZ magic byte sequence, which should trigger strict antivirus scanning protocols, is being interpreted as a safe file type indicator instead of a potential executable marker. This misinterpretation creates a scenario where files with extensions like BAT, HTML, and EML can contain embedded executable code that bypasses traditional antivirus inspection mechanisms. The vulnerability demonstrates a classic case of improper input validation and type checking that aligns with CWE-20, which addresses improper input validation in software systems.
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 into a single payload. These composite files can contain executable code within HTML or email formats, exploiting the trust relationships between applications and file handlers on end systems. When users open these files, applications that recognize the MZ signature may execute the embedded executable content even though the file appears to be a safe format like HTML or EML. This creates a sophisticated attack vector that leverages the principle of least privilege and application trust models, allowing malicious code execution without triggering traditional security warnings or network-based detection mechanisms.
This vulnerability directly relates to ATT&CK technique T1059, which covers command and scripting interpreter usage, as attackers can leverage the bypassed scanning to execute malicious code through seemingly safe file formats. The attack chain typically involves crafting files with embedded executables that exploit the magic byte interpretation error, allowing the malicious payload to execute on target systems without triggering antivirus alerts. The impact is particularly severe in enterprise environments where network-based security appliances like Fortinet's antivirus solutions are deployed to protect against malware, as this vulnerability essentially allows attackers to neutralize the protective layer by exploiting the software's own interpretation logic. Organizations using affected Fortinet versions face significant risk of malware propagation, data exfiltration, and persistent threats that can establish footholds within networks through these cleverly crafted file delivery mechanisms.
The mitigation strategy for this vulnerability requires immediate patching of Fortinet antivirus software to version 2.48.0.1 or later, which addresses the magic byte interpretation error through proper file type validation and enhanced signature recognition mechanisms. Network administrators should also implement additional layers of protection including email filtering, web application firewalls, and network segmentation to reduce the attack surface. Organizations should conduct thorough vulnerability assessments to identify any systems running affected Fortinet software versions and ensure comprehensive testing of the patched software to prevent regression issues. The fix typically involves strengthening the file type identification logic to properly validate file content against expected signatures rather than relying solely on magic byte interpretation, which aligns with security best practices for input validation and type checking as recommended in industry standards and security frameworks.