CVE-2019-3902 in Mercurial
Summary
by MITRE
A flaw was found in Mercurial before 4.9. It was possible to use symlinks and subrepositories to defeat Mercurial's path-checking logic and write files outside a repository.
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Analysis
by VulDB Data Team • 09/06/2023
The vulnerability identified as CVE-2019-3902 represents a critical path traversal flaw in the Mercurial distributed version control system affecting versions prior to 4.9. This security issue stems from insufficient validation of symbolic links and subrepository configurations during file operations within the repository structure. The flaw allows attackers to manipulate the path resolution logic through carefully crafted symbolic links and subrepository definitions, enabling unauthorized file system access beyond the intended repository boundaries.
The technical implementation of this vulnerability exploits the way Mercurial handles path resolution when processing symbolic links and subrepositories. When Mercurial processes repository operations, it fails to adequately sanitize or validate the paths associated with symbolic links that point outside the repository directory structure. This occurs particularly when subrepositories are involved, as the system does not properly enforce path restrictions that should prevent file operations from escaping the designated repository boundaries. The flaw specifically manifests when the system resolves symbolic links without proper canonicalization checks, allowing attackers to create or modify files in arbitrary locations on the filesystem.
The operational impact of this vulnerability is severe and potentially devastating for systems using affected Mercurial versions. An attacker with access to a repository could exploit this flaw to write malicious files to sensitive system locations, potentially leading to privilege escalation, data corruption, or system compromise. The vulnerability is particularly dangerous in environments where Mercurial is used for code management and where repository access might be granted to untrusted users or where automated processes interact with repositories. The ability to write files outside the repository boundaries provides attackers with a persistent means of establishing footholds or executing malicious code within the target environment.
The vulnerability maps to CWE-22 Path Traversal and aligns with several ATT&CK techniques including T1059 Command and Scripting Interpreter and T1566 Phishing with Social Engineering. Organizations should immediately upgrade to Mercurial version 4.9 or later to address this vulnerability. Additionally, administrators should implement strict access controls on repository directories, disable unnecessary symbolic link usage, and monitor for suspicious file operations. The mitigation strategy should include regular security audits of repository configurations and the implementation of file system access controls that prevent unauthorized write operations to critical system locations. Organizations relying on Mercurial for source code management should also consider implementing automated scanning tools to detect potential symbolic link configurations that could lead to similar path traversal vulnerabilities.