CVE-2026-42073

Description

# OAuth State Validation Bypass via `error` Parameter Causes Local Server DoS in MCP Auth Callback --- ## Description The OpenClaude MCP authentication flow starts a temporary local HTTP server to handle OAuth callbacks. To prevent CSRF attacks, the server validates a `state` parameter against an internally stored value. However, due to a logic flaw in the order of conditionals, an attacker can completely bypass this check and force the server to shut down — without knowing the `state` value at all. The vulnerable code looks like this: ```typescript if (!error && state !== oauthState) { rejectOnce(new Error('OAuth state mismatch - possible CSRF attack')) return } if (error) { cleanup() rejectOnce(new Error(errorMessage)) return } ``` When a request arrives with an `error` query parameter (e.g., `?error=anything`), the first condition becomes `false` because `!error` evaluates to `false`. This means the CSRF check is **never reached**. Execution falls through to the second block, where `cleanup()` is called — shutting down the local server and terminating the user's active authentication session. The attacker does not need to know the `state` value. Any request containing an `error` parameter is enough to trigger the shutdown. --- ## Impact - The user's OAuth flow is silently terminated mid-session - The local callback server is shut down (Denial of Service) - Can be triggered remotely via a malicious web page using a cross-origin request (CSRF) - No authentication or prior knowledge of the `state` value is required --- ## Steps to Reproduce Save the following as `poc.js` and run with Node.js: ```javascript import { createServer } from 'http'; import { parse } from 'url'; const expectedState = "secure_state_abc123"; const server = createServer((req, res) => { const parsedUrl = parse(req.url || '', true); const { pathname, query } = parsedUrl; const { state, error } = query; if (pathname === '/callback') { // Vulnerable: error param causes state check to be skipped entirely if (!error && state !== expectedState) { res.writeHead(400); res.end('State mismatch'); console.log('[-] CSRF attempt blocked.'); return; } if (error) { res.writeHead(200); res.end(`Error: ${error}`); console.log(`[!] Server shutting down. Triggered by: ${error}`); server.close(); return; } } }); server.listen(12345, '127.0.0.1', () => { console.log('Listening on http://127.0.0.1:12345'); }); ``` **Terminal 1 — start the server:** ```bash node poc.js ``` **Terminal 2 — trigger the bypass:** ```bash curl "http://127.0.0.1:12345/callback?error=triggered" ``` **Expected result:** Server shuts down immediately. The `state` value was never checked. --- ## Root Cause The CSRF protection is conditioned on `!error`, meaning it is silently disabled whenever an `error` parameter is present. The two checks need to be decoupled — state validation must happen first, independently of any other parameters. --- ## Fix Move the `state` check before the `error` check, and remove the dependency on `!error`: ```typescript // Fixed if (state !== oauthState) { cleanup() rejectOnce(new Error('OAuth state mismatch - possible CSRF attack')) return } if (error) { cleanup() rejectOnce(new Error(errorMessage)) return } ``` With this change, any request — whether it contains an `error` parameter or not — must first pass the state validation before any further processing occurs. --- Credit: Xanlar Agamalizade