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Vulnerability · Published Jul 2, 2026 · Assigned by GitHub_M

CVE-2026-52830fast-mcp-telegram: Bearer token path traversal bypasses reserved Telegram session protection

Critical 9.4

In plain English

A critical-severity path traversal in fast-mcp-telegram from leshchenko1979. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 51% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)9.4Critical
Exploit odds (30 days)0.6%top 48.8% of all CVEs
Exploited in the wildNo reportsCISA SSVC: poc
Attack toolingNoneno public tooling known

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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
PyPIfast-mcp-telegram0.19.1GHSA-rxw2-pc8j-vxwm
PyPIfast-mcp-telegram0.19.1PYSEC-2026-2473

Affected products (as reported)

VendorProductVersions
leshchenko1979 fast-mcp-telegram < 0.19.1

Weakness: CWE-22, CWE-287 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L

References