CVE-2026-52830

Summary fast-mcp-telegram validates HTTP Bearer tokens by joining the raw token string into a session-file path. The verifier rejects the exact reserved token telegram, but it does not reject path separators or normalize the path before checking whether the session file exists. A remote HTTP client can therefore authenticate as the default legacy session with a token such as ../fast-mcp-telegram/telegram when the documented default session file ~/.config/fast-mcp-telegram/telegram.session exists. This bypasses the reserved session name control that is intended to prevent HTTP multi-user sessions from colliding with the default stdio or legacy account. With account-prefixed MCP tools enabled, the attacker still sees and calls the prefixed tools for the default account, so the prefix middleware does not stop the session selection bypass. Impact An unauthenticated network client can access the Telegram account represented by the default telegram.session file without knowing a generated bearer token, if that legacy or default session file is present on a server running HTTP auth. The attacker can then call Telegram MCP tools as that account, including message reading, message sending, MTProto API calls, and attachment-producing tool surfaces available to the session. Technical details SessionFileTokenVerifier.verify_token() strips whitespace and rejects exact reserved names: if token.lower() in RESERVED_SESSION_NAMES: return None It then appends .session to the raw token and checks the resulting path: session_path = self._session_directory / f"{token}.session" if not session_path.is_file(): return None No check rejects /, \\, .., absolute paths, or resolved paths outside the configured session directory. The session client path is built the same way in src/client/connection.py: session_path = SESSION_DIR / f"{token}.session" client = await _build_telegram_client_for_token(session_path,

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