---
title: "CVE-2026-52830: fast-mcp-telegram: Bearer token path traversal bypasses reserved Telegram session protection"
url: https://erroneo.us/security/cve/CVE-2026-52830/
description: "CVE-2026-52830 (fast-mcp-telegram (PyPI)): fast-mcp-telegram is a Telegram MCP Server. Prior to 0.19.1, fast-mcp-telegram validates HTTP Bearer tokens by…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-07-02
date_modified: 2026-08-13
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-52830"
kev: false
cvss: 9.4
epss: 0.00648
license: All rights reserved. Quote with attribution and a link.
---

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

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Not in CISA KEV , but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 0.6% , percentile 50: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 50% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes fast-mcp-telegram (PyPI) <= 0.19.0, fixed in 0.19.1 . Check your lockfile, including transitive dependencies.

## What it is

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

- PyPI `fast-mcp-telegram` <= 0.19.0 → fixed: 0.19.1

## Affected products (CVE record)

- leshchenko1979 fast-mcp-telegram: < 0.19.1

## Severity

- GitHub_M CVSS 3.1: 9.4 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L`
- GitHub CVSS 3.1: 9.4 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc , automatable yes , technical impact partial .
- FIRST EPSS (model of 2026-10-21): 0.65% probability of exploitation activity in the next 30 days; percentile 50.

## Sources

- [GitHub Advisory GHSA-rxw2-pc8j-vxwm](https://github.com/advisories/GHSA-rxw2-pc8j-vxwm)
- [CVE.org record CVE-2026-52830](https://www.cve.org/CVERecord?id=CVE-2026-52830)
- [NVD entry CVE-2026-52830](https://nvd.nist.gov/vuln/detail/CVE-2026-52830)
- [web.archive.org](https://web.archive.org/web/20250926152207/https://github.com/leshchenko1979/fast-mcp-telegram)
- [github.com](https://github.com/leshchenko1979/fast-mcp-telegram/security/advisories/GHSA-rxw2-pc8j-vxwm)
- [github.com](https://github.com/pypa/advisory-database/tree/main/vulns/fast-mcp-telegram/PYSEC-2026-2473.yaml)
- [pypi.org](https://pypi.org/project/fast-mcp-telegram)

---
Source page: https://erroneo.us/security/cve/CVE-2026-52830/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt