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

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• 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.

From the GitHub advisory

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 bearer token, if that legacy or default session file is present on a server running HTTP auth. The attacker can then call Telegram MCP tools as that account, including message reading, message sending, MTProto API calls, and attachment-producing tool surfaces available to the session.

Weakness type: CWE-22, CWE-287.

Product: fast-mcp-telegram (PyPI) · Assigned by: GitHub_M · Published: 2026-07-02

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• 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.

Timeline

• 2026-07-02 — CVE record published by GitHub_M
• 2026-07-02 — GitHub-reviewed advisory GHSA-rxw2-pc8j-vxwm published (critical)
• 2026-07-06 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-rxw2-pc8j-vxwm github.com
2. CVE.org record CVE-2026-52830 cve.org
3. NVD entry CVE-2026-52830 nvd.nist.gov

Other references from the CVE record and advisory

1. web.archive.org web.archive.org
2. github.com github.com
3. github.com github.com
4. pypi.org pypi.org

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