CVE-2016-3630 in Mercurial
Summary
by MITRE
The binary delta decoder in Mercurial before 3.7.3 allows remote attackers to execute arbitrary code via a (1) clone, (2) push, or (3) pull command, related to (a) a list sizing rounding error and (b) short records.
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Analysis
by VulDB Data Team • 07/24/2022
The vulnerability identified as CVE-2016-3630 represents a critical security flaw within Mercurial's binary delta decoder implementation, affecting versions prior to 3.7.3. This weakness exists in the core functionality that handles binary file differences during version control operations, specifically when processing data received from remote repositories. The vulnerability stems from improper handling of binary delta encoding data structures that are commonly used in Mercurial's network protocols for efficient data transfer between repositories.
The technical root cause of this vulnerability involves two interconnected issues within the binary delta decoder's memory management and data processing logic. The first issue manifests as a list sizing rounding error that occurs when the decoder calculates memory allocation for processing binary data structures. This rounding error leads to insufficient memory allocation for data buffers, creating potential buffer overflow conditions when the actual data exceeds the allocated space. The second vulnerability involves short records processing, where malformed or truncated binary delta records are not properly validated before processing, allowing attackers to craft malicious data sequences that exploit the memory allocation flaws.
Attackers can leverage this vulnerability through three distinct Mercurial operations that involve network communication with remote repositories. The clone operation allows remote attackers to execute arbitrary code when downloading repository data from a malicious source. The push operation enables attackers to inject malicious binary delta data during repository updates, while the pull operation permits code execution when fetching changes from remote repositories. All three attack vectors exploit the same underlying memory handling flaw, making the vulnerability particularly dangerous as it can be triggered through any normal Mercurial workflow involving remote repository operations.
The operational impact of this vulnerability extends beyond simple code execution, as it provides attackers with complete system compromise capabilities through the binary delta decoder's processing chain. When successfully exploited, the vulnerability enables attackers to execute arbitrary commands with the privileges of the Mercurial process, potentially leading to full system compromise if the process runs with elevated permissions. The vulnerability affects both client-side operations during repository synchronization and server-side operations during repository updates, making it particularly dangerous in collaborative development environments where multiple users interact with shared repositories.
Organizations using Mercurial should prioritize immediate remediation through version upgrades to 3.7.3 or later, as this represents the official fix for the vulnerability. The mitigation strategy should include comprehensive network monitoring to detect potential exploitation attempts, particularly around Mercurial repository operations. Security teams should implement network segmentation to limit access to Mercurial repositories, especially those accessible from untrusted networks. Additionally, the vulnerability aligns with CWE-122 (Heap-based Buffer Overflow) and CWE-129 (Improper Validation of Array Index) classifications, while the attack vectors correspond to ATT&CK techniques involving remote code execution through protocol manipulation and data injection attacks. The vulnerability demonstrates the critical importance of proper input validation and memory management in version control systems, as the binary delta protocol is fundamental to efficient repository operations and the security implications extend far beyond individual repository security to encompass entire development infrastructure integrity.