CVE-2017-9462 in Mercurial
Summary
by MITRE
In Mercurial before 4.1.3, "hg serve --stdio" allows remote authenticated users to launch the Python debugger, and consequently execute arbitrary code, by using --debugger as a repository name.
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Analysis
by VulDB Data Team • 12/08/2022
The vulnerability identified as CVE-2017-9462 represents a critical security flaw in the Mercurial distributed version control system affecting versions prior to 4.1.3. This vulnerability specifically impacts the hg serve command when executed with the --stdio option, creating a pathway for remote authenticated attackers to escalate privileges and execute arbitrary code on the target system. The flaw stems from improper input validation and argument handling within the command execution flow, particularly when repository names are processed through the stdio interface.
The technical exploitation mechanism involves leveraging the --debugger flag as a repository name parameter, which causes the system to invoke the Python debugger instead of performing normal repository operations. This behavior creates an unintended execution path where attacker-controlled input directly influences the Python interpreter's debugging functionality. The vulnerability is classified under CWE-20 as a "Improper Input Validation" issue, where the application fails to properly sanitize user-supplied repository names before processing them in a context that allows code execution. The flaw demonstrates characteristics of command injection and privilege escalation, as the authenticated user can leverage their access rights to manipulate the execution environment.
From an operational perspective, this vulnerability poses significant risks to organizations relying on Mercurial for source code management and version control. The remote authenticated nature of the exploit means that attackers with valid credentials can potentially compromise systems without requiring additional attack vectors or elevated privileges. The ability to launch the Python debugger and subsequently execute arbitrary code creates a complete compromise scenario where attackers can access system resources, modify code repositories, or establish persistent backdoors. This vulnerability directly impacts the integrity and confidentiality of version control systems, potentially exposing sensitive source code and development artifacts.
The attack surface for this vulnerability is primarily limited to systems where Mercurial is configured to run with the --stdio option and where authenticated users have access to repository operations. The remediation strategy involves upgrading to Mercurial version 4.1.3 or later, which includes proper input validation and sanitization of repository names. Organizations should also consider implementing network segmentation and access controls to limit exposure of the affected command interface. Additionally, security monitoring should be enhanced to detect unusual patterns in repository name usage that might indicate exploitation attempts. The vulnerability aligns with ATT&CK technique T1059.006 for "Command and Scripting Interpreter: Python", as it leverages Python interpreter functionality to execute malicious code through debugger invocation. System administrators should also review and restrict the use of --stdio mode where possible, as this interface presents additional attack surfaces that may not be necessary for typical operations.