CVE-2008-5540 in Webwasher
Summary
by MITRE
Secure Computing Secure Web Gateway (aka Webwasher), when Internet Explorer 6 or 7 is used, allows remote attackers to bypass detection of malware in an HTML document by placing an MZ header (aka "EXE info") at the beginning, and modifying the filename to have (1) no extension, (2) a .txt extension, or (3) a .jpg extension, as demonstrated by a document containing a CVE-2006-5745 exploit.
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Analysis
by VulDB Data Team • 12/05/2017
The vulnerability described in CVE-2008-5540 represents a significant security flaw in the Secure Computing Secure Web Gateway appliance, specifically affecting its content inspection capabilities when processing web traffic through Internet Explorer 6 or 7 browsers. This weakness stems from the gateway's inadequate file type detection mechanisms that fail to properly analyze file content based on actual binary signatures rather than relying solely on filename extensions or superficial headers. The Secure Web Gateway is designed to protect enterprise networks by inspecting and filtering web content to prevent malware delivery, but this vulnerability creates a critical bypass opportunity for threat actors. The flaw manifests when attackers exploit the gateway's failure to properly identify executable content by manipulating the file structure and naming conventions, effectively circumventing the security controls meant to protect against malicious downloads and code execution attempts. This vulnerability directly impacts the integrity of the gateway's security posture by allowing potentially harmful code to pass undetected through the inspection process.
The technical implementation of this vulnerability exploits the fundamental weakness in how the Secure Web Gateway performs content analysis and file type determination. The MZ header, which is the standard signature for windows executable files, is placed at the beginning of HTML documents to deceive the gateway's inspection engine. This header is typically used by Windows operating systems to identify executable files, but the gateway fails to properly correlate this signature with the actual content type being inspected. The attacker can modify the filename in several ways to achieve the bypass - removing the extension entirely, changing it to .txt, or using a .jpg extension while maintaining the malicious content structure. This approach leverages the common practice of browsers and security systems that often rely on filename extensions as primary indicators of file type, rather than performing deep content analysis. The technique demonstrates a classic case of content-type confusion where the system's decision-making process is manipulated through header manipulation, allowing the malicious payload to appear as benign content during inspection.
The operational impact of CVE-2008-5540 is severe for organizations relying on the Secure Web Gateway for network protection, as it enables attackers to deliver malware through seemingly safe file transfers that bypass the gateway's security controls. This vulnerability creates a significant risk for enterprise environments where the gateway serves as a critical defense layer against web-based threats and malware delivery. The ability to bypass detection using a simple filename manipulation technique means that organizations may experience successful malware infections without any indication that the security controls failed. The specific exploitation scenario involving CVE-2006-5745 demonstrates how this bypass can be used to deliver more sophisticated attacks that take advantage of additional vulnerabilities within the target systems. Network administrators may be misled into believing their security infrastructure is functioning properly while malicious content successfully penetrates their defenses, potentially leading to data breaches, system compromise, and extended attack lifecycles. The vulnerability essentially undermines the gateway's core functionality as a content inspection and malware detection device.
The remediation and mitigation strategies for this vulnerability should focus on implementing more robust content analysis mechanisms that rely on actual binary signatures rather than filename extensions or superficial header inspection. Organizations should ensure that the Secure Web Gateway firmware is updated to the latest available version that addresses this specific bypass technique. The implementation of deep content inspection capabilities that analyze the complete file structure, including header validation and content type correlation, is essential to prevent similar attacks. Security policies should be updated to require more comprehensive file type validation processes that do not rely solely on filename extensions. Network administrators should consider implementing additional layers of security controls, including email filtering, network-based intrusion detection systems, and endpoint protection mechanisms that can detect anomalous behavior regardless of the gateway's inspection failures. This vulnerability aligns with CWE-502 which addresses "Deserialization of Untrusted Data" and potentially CWE-20 which covers "Improper Input Validation" in the context of content analysis systems. From an ATT&CK perspective, this vulnerability maps to T1071.004 for application layer protocol and T1059.001 for command and scripting interpreter, as it allows for the delivery of malicious payloads that can execute commands and scripts within the target environment, potentially leading to further exploitation opportunities.