CVE-2012-1458 in Sophos
Summary
by MITRE
The Microsoft CHM file parser in ClamAV 0.96.4 and Sophos Anti-Virus 4.61.0 allows remote attackers to bypass malware detection via a crafted reset interval in the LZXC header of a CHM file. NOTE: this may later be SPLIT into multiple CVEs if additional information is published showing that the error occurred independently in different CHM parser implementations.
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Analysis
by VulDB Data Team • 03/22/2021
The vulnerability described in CVE-2012-1458 represents a critical flaw in how CHM (Compiled HTML Help) files are parsed by antivirus software, specifically affecting ClamAV 0.96.4 and Sophos Anti-Virus 4.61.0 implementations. This issue stems from improper handling of the LZXC compression header within CHM files, creating a bypass mechanism that allows malicious actors to evade detection by security systems that rely on these parsers. The vulnerability specifically targets the reset interval parameter within the LZXC header structure, which is used during decompression operations to manage internal state and buffer management. When a crafted reset interval is included in a CHM file, it causes the parser to misinterpret the decompression process, leading to incomplete or incorrect analysis of the file contents. This flaw demonstrates a fundamental weakness in the parser's validation logic and state management mechanisms, allowing attackers to craft malicious CHM files that appear benign to the antivirus engine while containing hidden malicious payloads.
The technical exploitation of this vulnerability occurs through manipulation of the LZXC compression header structure within CHM files, specifically targeting the reset interval field that controls how decompression state is maintained during the decompression process. When the parser encounters a malformed reset interval value, it fails to properly validate the decompression state transitions, causing the parser to skip or misinterpret portions of the compressed data. This behavior creates a detection gap where the antivirus engine may not fully decompress or analyze the complete file content, allowing malicious code to remain hidden within the compressed structure. The vulnerability falls under the CWE-129 weakness category, which deals with improper validation of the length of input data, and specifically relates to improper handling of data structures within compression algorithms. The flaw represents a classic case of insufficient input validation and improper error handling in decompression routines, where the parser does not adequately verify that the reset interval value falls within acceptable ranges or logical parameters.
The operational impact of this vulnerability extends beyond simple malware evasion to potentially enable more sophisticated attack vectors. Attackers can craft CHM files that exploit this parsing error to hide malicious executables, scripts, or other harmful content within seemingly legitimate compressed archives. This capability undermines the fundamental security premise of antivirus systems that rely on comprehensive file analysis, as the parser's inability to correctly process the LZXC header structure creates blind spots in threat detection. The vulnerability affects not only the specific versions mentioned but potentially represents a broader class of issues in CHM parsing implementations across different antivirus vendors, as evidenced by the note suggesting this may be split into multiple CVEs for different implementations. Organizations using affected antivirus versions face significant risk as attackers can now bypass security controls by leveraging this specific parsing weakness in CHM file handling. The impact is particularly concerning for enterprise environments where CHM files may be encountered in legitimate business contexts, such as help documentation, training materials, or software installation packages, making the attack surface more expansive.
Mitigation strategies for CVE-2012-1458 should focus on immediate software updates and implementation of additional security controls. Organizations must upgrade to patched versions of ClamAV and Sophos Anti-Virus that address the LZXC header parsing error, ensuring that the updated parsers properly validate reset interval values and handle decompression state transitions correctly. System administrators should implement network-level controls to restrict CHM file execution and scanning, particularly in environments where such files are frequently encountered. The implementation of multiple layers of defense is crucial, including network firewalls that block CHM file transfers, endpoint protection that monitors for suspicious CHM file behavior, and regular security assessments to identify any remaining vulnerabilities in file parsing implementations. Additionally, security teams should consider implementing behavioral analysis systems that can detect anomalous decompression patterns or file access behaviors that might indicate exploitation attempts. The vulnerability highlights the importance of maintaining up-to-date antivirus signatures and parser implementations, as well as the need for regular security audits of core security components that handle file format parsing operations. Organizations should also consider implementing principle of least privilege controls for CHM file handling processes and establish monitoring protocols to detect potential exploitation attempts through anomalous parsing behaviors.