CVE-2026-52830
CriticalCVSS 9.4Exploitation Probability (EPSS)
Low risk34th percentile — higher than 34% of all known CVEs
Summary
A vulnerability in fast-mcp-telegram before version 0.19.1 allows a remote HTTP client to bypass authentication by manipulating the session file path. The Bearer token validation does not reject path separators or normalize the path before checking if the session file exists, enabling a token like '../fast-mcp-telegram/telegram' to authenticate as the default legacy session.
Risk Assessment
An attacker can gain unauthorized access to the default legacy session, leading to account takeover and potential execution of MCP operations on behalf of that account, despite session name collision protections.
Recommendation
Immediately upgrade fast-mcp-telegram to version 0.19.1 or later, which contains the fix for this vulnerability.
Original NVD description (English source)
fast-mcp-telegram is a Telegram MCP Server. Prior to 0.19.1, 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. This vulnerability is fixed in 0.19.1.