CVE-2016-9296 in p7zip
Summary
by MITRE
A null pointer dereference bug affects the 16.02 and many old versions of p7zip. A lack of null pointer check for the variable folders.PackPositions in function CInArchive::ReadAndDecodePackedStreams in CPP/7zip/Archive/7z/7zIn.cpp, as used in the 7z.so library and in 7z applications, will cause a crash and a denial of service when decoding malformed 7z files.
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Analysis
by VulDB Data Team • 05/30/2019
The vulnerability identified as CVE-2016-9296 represents a critical null pointer dereference flaw within the p7zip archive utility suite, specifically affecting version 16.02 and numerous older iterations. This security weakness resides within the CInArchive::ReadAndDecodePackedStreams function located in the CPP/7zip/Archive/7z/7zIn.cpp source file, making it a fundamental component of the decompression process for 7z format archives. The flaw manifests when the application fails to properly validate the folders.PackPositions variable, which serves as a critical data structure for managing packed stream positions during archive extraction operations.
The technical implementation of this vulnerability stems from inadequate input validation within the archive parsing mechanism. When processing malformed 7z files, the application attempts to dereference a null pointer that should have been validated before use, creating an immediate crash condition. This null pointer dereference occurs specifically during the parsing of packed streams, where the software expects certain data structures to be properly initialized but encounters uninitialized or improperly structured folder information. The vulnerability operates at the level of the 7z.so shared library and affects all applications utilizing this library for archive processing, making it particularly dangerous as it can be exploited across multiple software implementations that depend on p7zip functionality.
The operational impact of CVE-2016-9296 constitutes a reliable denial of service condition that can be triggered by any user capable of providing a maliciously crafted 7z file to a vulnerable system. This vulnerability falls under CWE-476 which specifically addresses null pointer dereference issues, and aligns with ATT&CK technique T1203 which involves exploiting software vulnerabilities to achieve system compromise. The crash occurs during the normal archive extraction process, making it particularly insidious as legitimate users may inadvertently trigger the vulnerability when opening what appears to be a normal archive file. The service disruption affects not only individual applications but potentially entire systems that rely on 7z archive processing capabilities, including backup systems, file sharing platforms, and automated processing workflows.
Mitigation strategies for this vulnerability require immediate patching of affected p7zip installations to version 16.02 or later, where the null pointer validation has been properly implemented. System administrators should conduct comprehensive inventory checks to identify all systems utilizing vulnerable p7zip versions and ensure proper update deployment across all affected environments. Network security controls should include implementation of file validation mechanisms that scan archive files for known malicious patterns before processing, while also implementing sandboxing techniques for archive extraction operations. Organizations should also consider implementing file type restrictions and access controls to limit exposure to potentially malicious archive files. The fix implemented in the patched versions addresses the core issue by ensuring proper null pointer validation before dereferencing the folders.PackPositions variable, thereby preventing the crash condition and maintaining system availability. This vulnerability demonstrates the importance of robust input validation in archive processing utilities and highlights the need for comprehensive security testing of file format parsers to prevent similar issues in other software components.