CVE-2026-52830 fast-mcp-telegram: Bearer token path traversal bypasses reserved Telegram session protection
Remote attackers can bypass reserved session protections via path traversal in Bearer token validation, accessing default account sessions.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-52830?
CVE-2026-52830 is a critical path traversal vulnerability in leshchenko1979 fast-mcp-telegram, published on 2 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Which versions are affected by CVE-2026-52830?
3 open source packages ship an affected version, including fast-mcp-telegram.
| Package | Affected | Fixed in |
|---|---|---|
fast-mcp-telegram PyPI | All versions | 0.19.1 |
fast-mcp-telegram PyPI | All versions | 0.19.1 |
fast-mcp-telegram PyPI | All versions | 0.19.1 |
How do I fix CVE-2026-52830?
Upgrade fast-mcp-telegram to 0.19.1 or later.
Upgrade, and resolve every user-supplied path against an allowed base folder before opening it. More about Path traversal.
Is CVE-2026-52830 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-52830 exists.
Sources
Also known as GHSA-rxw2-pc8j-vxwm, PYSEC-2026-2473.