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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-52830: 0.65% estimated exploitation activity in the next 30 days; percentile 51.10 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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. Impact An unauthenticated network client can access the Telegram account represented by the default telegram.session file without knowing a generated…

Published by the source

2 Jul 2026, 20:38 UTC

Last modified by the source

13 Aug 2026, 17:00 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-rxw2-pc8j-vxwm

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-rxw2-pc8j-vxwm

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-rxw2-pc8j-vxwm

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# Patch Next advisory brief: CVE-2026-52830

Advisory: fast-mcp-telegram: Bearer token path traversal bypasses reserved Telegram session protection
IDs: CVE-2026-52830, GHSA-rxw2-pc8j-vxwm
Record: https://patchnext.com/advisories/cve-2026-52830 (revision 3, source checked 26 Oct 2026, 13:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fast-mcp-telegram 0.19.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI fast-mcp-telegram: affected <= 0.19.0; fixed in 0.19.1

## Suggested inspection (read-only)
- `python -m pip show fast-mcp-telegram`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on fast-mcp-telegram.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-rxw2-pc8j-vxwm (GitHub-reviewed advisory GHSA-rxw2-pc8j-vxwm lists affected ranges: PyPI fast-mcp-telegram >=0 <=0.19.0)
- Verified: https://osv.dev/vulnerability/GHSA-rxw2-pc8j-vxwm (OSV record GHSA-rxw2-pc8j-vxwm lists affected ranges: PyPI fast-mcp-telegram >=0 <0.19.1 (fixed))

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