Critical fast-mcp-telegram CVE-2023-4589 lets attackers access sensitive files

Key Takeaways

• A critical path traversal vulnerability, CVE-2026-52830, has been identified in the fast-mcp-telegram package.
• The flaw allows unauthenticated attackers to bypass security measures and access sensitive Telegram session files.
• Successful exploitation grants attackers full control over Telegram accounts associated with the compromised session, enabling message reading, sending, and API interaction.
• All versions of fast-mcp-telegram up to 0.19.0 are affected; users must upgrade to version 0.19.1 immediately.

Critical Flaw in fast-mcp-telegram Exposes Sensitive User Data

A severe security vulnerability has been uncovered in the fast-mcp-telegram package, presenting a significant risk of unauthorized access to Telegram session data and subsequent account compromise. This critical flaw, identified as CVE-2026-52830, could enable remote attackers to execute arbitrary actions on affected Telegram accounts.

The vulnerability impacts all fast-mcp-telegram versions up to and including 0.19.0. Developers have addressed the issue in version 0.19.1, and users are strongly advised to update without delay.

Path Traversal Enables Session File Access

The root cause of CVE-2026-52830 lies in inadequate validation of HTTP Bearer tokens, which the fast-mcp-telegram application utilizes to construct file paths for session data. While the system attempts to block the specific session name “telegram,” it critically fails to properly sanitize or normalize user-provided input. This oversight creates a window for attackers to leverage path traversal techniques, circumventing existing security restrictions.

Specifically, a malicious actor can craft a token, such as “../fast-mcp-telegram/telegram”, which resolves to the default session file path on the server. Despite the direct “telegram” token being blocked, this traversal variant successfully points to the same critical file and is accepted by the system. Consequently, an unauthenticated attacker can gain access to the default Telegram session without requiring a legitimate token.

This vulnerability is particularly dangerous in environments where the default session file, typically located at ~/.config/fast-mcp-telegram/telegram.session, is present. Once authenticated through this bypass, the attacker gains full control over the Telegram MCP tools linked to that account.

Compromised accounts could be used to read and send messages, invoke MTProto API calls, and access attachments exposed via the tool’s interface, leading to significant privacy breaches and potential misuse.

According to the GitHub Advisory Database, the vulnerability stems from a classic path traversal flaw exacerbated by weak input validation. The application neglects to restrict characters like “/”, “..”, or absolute paths, nor does it verify that the resolved file path remains within the intended directory. This effectively breaks the authentication boundary, allowing filesystem paths to dictate access control decisions.

Security researchers have demonstrated the issue with a controlled proof-of-concept, confirming the bypass. The tests showed that while valid tokens functioned correctly and invalid paths were rejected, tokens employing traversal sequences successfully authenticated and mapped to the protected default session. Crucially, existing account-prefix controls offer no mitigation against this fundamental authentication bypass.

What You Should Do

• Upgrade Immediately: All users of fast-mcp-telegram must upgrade to version 0.19.1 without delay. This is the most critical step to mitigate the vulnerability.
• Validate Bearer Tokens: Developers should implement strict validation of bearer tokens, treating them as opaque identifiers and limiting them to a safe character set, such as alphanumeric strings.
• Secure File Path Resolution: Ensure applications normalize and securely resolve file paths, strictly enforcing that they remain within designated session directories. Reject any tokens containing path traversal sequences or invalid characters outright.
• Review Deployments: Pay particular attention to deployments exposing HTTP authentication endpoints, as these are at higher risk of exploitation.

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