External risk intelligence

fast-mcp-telegram Session File Path Traversal Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.4)

CVE-2026-52830

This product is an MCP (Model Context Protocol) server designed to facilitate remote communication between AI clients and Telegram. Such services are typically deployed as API-facing or network-accessible gateways to bridge remote AI agents or web services with Telegram accounts, creating a direct pathway for HTTP-based interactions from external or networked clients.

Path Traversal

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

This advisory concerns a critical vulnerability in fast-mcp-telegram, a server that allows remote interaction with Telegram. The flaw could permit unauthorized access to a default Telegram session by bypassing authentication controls. While a fix is available, the primary concern for leadership is to confirm if this specific technology is in use within the organization and if so, to ensure the update has been applied.

Bypasses authentication for Telegram interaction.
Affects remote access to a Telegram server.
Confirm use and patch status.

Attack Path

How an attacker could exploit the issue

An attacker can bypass authentication by sending a specially crafted HTTP request with a token that exploits how the server constructs session file paths. This allows them to impersonate the default legacy session, potentially gaining unauthorized access to sensitive operations.

Unauthenticated remote network access required.
Path traversal in token validation triggers vulnerability.
Unauthorized access and data manipulation possible.

Live Threat

Current exploitation, exposure, and threat context

A remote attacker could exploit this vulnerability to gain unauthorized access to the default legacy session of the fast-mcp-telegram server by crafting a malicious HTTP request. This access could allow the attacker to interact with the server as if they were the legitimate default user, potentially leading to unintended actions or data exposure.

Default session data at risk.
Unauthenticated HTTP request bypasses controls.
Unauthorized server access and actions.

Operational Fix

Recommended remediation, mitigation, and detection steps

Platform and application owners are responsible for addressing this vulnerability. The first practical step is to locate all instances of fast-mcp-telegram, determine their reachability and business criticality, and identify the accountable owner for each instance before planning remediation based on risk.

Platform and application owners
Verify external reachability and criticality.
Plan remediation and coordinate with vendors.

Supplementary metadata

CVSS vector

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Frequently asked questions

What is fast-mcp-telegram?

It is a server component that implements the Model Context Protocol (MCP) to bridge AI models with Telegram. Developers use it to allow AI agents or external applications to interact with Telegram accounts programmatically. Because it acts as an API-facing gateway, it sits between your AI infrastructure and the messaging platform to facilitate remote service operations.

How does this CVE-2026-52830 vulnerability work?

This is a path traversal flaw (CWE-22) combined with improper authentication (CWE-287). The software takes an incoming HTTP Bearer token and uses it to construct a file path for session verification. An attacker can use directory traversal characters in the token to point the software to a default session file, tricking it into granting access as the default user even though the specific 'telegram' session name was intended to be restricted.

Do I need to send a complex payload to trigger this?

No. The flaw is triggered by a specially crafted HTTP request containing a malicious token path, such as '../fast-mcp-telegram/telegram'. If the server's default configuration stores the session file in the standard location, it will incorrectly validate the token. Simply calling a legitimate tool prefix is not enough to stop this, as the bypass occurs during the initial authentication phase before any tool-specific middleware is evaluated.

Why should I be concerned if my instance is internal?

Halo Surface Signal indicates that because this software is designed as an API-facing gateway for remote AI agents, it is often deployed in network-accessible configurations. While internal instances face fewer threats, any service reachable by other networked systems or users can be exploited. You should evaluate if your deployment is accessible beyond trusted segments to determine your risk level.

What are the first steps to secure my installation?

Your priority is to identify all instances of the software within your environment and check their current version. Since this vulnerability is resolved in version 0.19.1, you should coordinate with your application owners to update to this version or higher. Until you can patch, restrict network access to the server to prevent unauthorized HTTP requests from reaching the vulnerable authentication logic.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.