A significant security vulnerability has been identified in the fast-mcp-telegram package, which enables remote attackers to gain unauthorized access to sensitive Telegram session data. This vulnerability is particularly dangerous as it can lead to unauthorized actions within the application.
Details of the Vulnerability
Registered as CVE-2026-52830, the flaw affects all versions up to 0.19.0 and has been addressed in version 0.19.1. The core issue arises from inadequate validation of HTTP Bearer tokens, which are crucial for authentication processes. In this application, these tokens are directly utilized to form file paths to session files, leading to potential security risks.
The vulnerability becomes evident due to the application’s failure to properly sanitize or normalize user inputs. Although the session name “telegram” is restricted, attackers can exploit this oversight by using path traversal techniques.
Exploit Methodology
The exploit involves attackers providing specially crafted tokens, such as “../fast-mcp-telegram/telegram,” which bypass the application’s restrictions by resolving to the default session file path on the server. Consequently, unauthorized attackers can access these session files without needing a valid token.
This issue is especially critical in environments where the default session file is present, typically found at ~/.config/fast-mcp-telegram/telegram.session. Once an attacker gains access, they can perform actions such as reading and sending messages or executing MTProto API calls.
Mitigation and Recommendations
According to the GitHub Advisory Database, this vulnerability is a combination of a path traversal flaw and weak input validation. The application fails to restrict characters like “../” or absolute paths, inadvertently allowing path-based access-control decisions.
Developers are urged to implement stringent validation of bearer tokens, treating them as opaque identifiers limited to safe characters. Moreover, applications should securely resolve file paths to ensure they remain within designated directories. Any tokens with path traversal sequences or invalid characters should be outright rejected.
To prevent potential exploitation, maintainers have released version 0.19.1. Users are strongly advised to upgrade immediately, especially if their deployments expose HTTP authentication endpoints, to mitigate the risk of unauthorized access.
