CVE-2026-42073: Gitlawb OpenClaude CSRF and Denial of Service in MCP authentication

Executive brief

A vulnerability in the OpenClaude authentication process allows an attacker to disrupt a user's login session. By sending a specially crafted web request, an attacker can force the local authentication server to shut down prematurely. This results in a denial of service for the user attempting to log in, preventing them from completing the authentication flow.

Technical details

A logic flaw in the OAuth callback handler of OpenClaude MCP allows for a CSRF state check bypass. The application uses a conditional check that only validates the 'state' parameter if an 'error' parameter is absent. By providing any value in the 'error' query parameter, an attacker can bypass the state validation entirely. This triggers the error-handling block which calls a cleanup function, shutting down the local HTTP callback server and terminating the authentication session. This can be exploited remotely via cross-origin requests without knowledge of the valid state token. The issue is fixed in version 0.5.1 by decoupling the state validation from the error check.

Affected products

• Gitlawb openclaude < 0.5.1

Timeline

• 2026-05-12: advisory: GitHub Advisory published
• 2026-05-12: patched: Fix released in version 0.5.1

References

• https://api.github.com/users/xancyber
• https://github.com/xancyber
• https://api.github.com/users/xancyber/gists%7B/gist_id%7D
• https://api.github.com/users/xancyber/repos
• https://avatars.githubusercontent.com/u/272321005?v=4
• https://api.github.com/users/xancyber/events%7B/privacy%7D