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Vulnerability record · CVE-2026-52830 · published 2 July 2026

CVE-2026-52830: Path traversal vulnerability

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.

9.4 CVSS 3.1 Critical EPSS 0.65% · top 51.0% CWE-22 · Path traversalCWE-287 · Improper authentication Deferred
9.4CVSS 3.1 base score
0.65%EPSS exploitation probability, 30 days
NoNot in CISA KEV
0Affected product versions listed by NVD
3References
6 Jul 2026Last modified by NVD

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.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L

References

LinkTags
https://github.com/advisories/GHSA-rxw2-pc8j-vxwm
https://web.archive.org/web/20250926152207/https://github.com/leshchenko1979/fast-mcp-telegram
https://github.com/advisories/GHSA-rxw2-pc8j-vxwm

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Source: NIST National Vulnerability Database (record CVE-2026-52830), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.