Critical Path-Traversal Flaw in fast-mcp-telegram Exposes Telegram Accounts to Unauthorized Access A severe vulnerability (CVE-2026-52830, GHSA-rxw2-pc8j-vxwm) in fast-mcp-telegram a Telegram MCP server bridging accounts to AI assistants and HTTP clients allows attackers to bypass authentication and gain full access to a Telegram session by exploiting a path-traversal flaw in session file resolution. ### Vulnerability Details The flaw affects versions ≤ 0.19.0 of fast-mcp-telegram, where bearer tokens are improperly validated. Instead of treating tokens as opaque identifiers, the server constructs session file paths by directly appending the token to a directory (e.g., `session_dir/{token}.session`). While the code blocks reserved names like telegram, it fails to sanitize path separators (`/`, `\`, `..`), enabling attackers to craft malicious tokens (e.g., `../fast-mcp-telegram/telegram`) that resolve to the default `telegram.session` file. This bypasses the intended high-entropy token requirement, granting unauthorized access to the default Telegram account including message read/write capabilities, MTProto API calls, and exposed tools without requiring a valid token. ### Impact & Exploitation - Authentication Bypass: Attackers can impersonate the default Telegram account by submitting a traversal sequence in the `Authorization: Bearer` header. - Privileged Access: Once authenticated, attackers gain full control over the account’s messages, API functions, and configured tools. - Downstream Risks: Middleware tool prefixing (e.g., account-specific restrictions) fails to mitigate the issue, as the flaw occurs during session selection, not tool enforcement. ### Remediation & Response The vulnerability was reported by David Carliez and patched in version 0.19.1, which enforces stricter token validation. Recommended actions include: - Immediate upgrades to the latest version. - Rotation of default/legacy session files to prevent lingering exposure. - Log audits for suspicious bearer tokens containing traversal sequences. The flaw underscores the risks of improper path handling in authentication systems, where filesystem-dependent logic can undermine security boundaries.