Synopsis
The claude-code-action GitHub Action checks out the PR head branch when operating in a pull request context, making the working directory attacker-controlled. Combined with the action unconditionally setting ‘enableAllProjectMcpServers’ to ‘true’ in Claude Code’s user settings and loading settings from project and local source by default ‘(settingsSource: [“user”, “project”, “local”])’, an attacker can supply a malicious ‘.mcp.json’ file in his PR branch.
When a privileged user triggers the GitHub Action (via an ‘issue_comment’ event for example), the MCP server defined in the attacker-controlled configuration is automatically started without approval, resulting in arbitrary command execution in the runner with access to all workflow secrets.
Disclosure Timeline
February 12, 2026: Tenable reports the finding to Anthropic.
February 13, 2026: HackerOne Bot triages the vulnerability as informative stating it’s a configuration issue rather than a vulnerability.
February 13, 2026: Tenable confirms it’s not a misconfiguration but a vulnerability.
February 13, 2026: Anthropic reopens the report and triages it with low severity.
February 14, 2026: Tenable asks for details about the severity assessment (high vs low)
February 22, 2026: Anthropic reevaluates the severity to medium.
March 9, 2026: Tenable requests for an update about fix release expectations.
March 13, 2026: Anthropic confirms fix is in progress.
March 16, 2026: Tenable requests for a new severity review.
March 24, 2026; Anthropic confirms the medium severity and confirms a fix has been released.
March 25, 2026: Tenable confirms that the fix seems to be working.
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