The fast-mcp-telegram server is vulnerable to an authentication bypass attack that permits unauthorized access to legacy session data, posing a critical risk to system integrity.

This vulnerability involves improper path validation during HTTP Bearer token processing, where an attacker can use path separators to traverse directories. This allows an unauthenticated remote user to authenticate as the default legacy session by manipulating the session-file path.

Successful exploitation permits unauthorized access to Telegram session data and associated MCP tools. Given the high CVSS score of 9.4, this vulnerability could lead to significant unauthorized interaction with Telegram accounts, potentially resulting in data exfiltration or unauthorized command execution within the application context.

Immediate Action: Upgrade to version 0.19.1 or later immediately to incorporate the necessary path normalization and validation logic.

Proactive Monitoring: Review server access logs for anomalous Bearer token strings containing path traversal characters such as "../" or unexpected session file references.

Compensating Controls: Implement strict network-level access controls to restrict access to the MCP server to known, trusted IP addresses until the patch is applied.

Public Exploit Available: Unknown

The severity of this flaw necessitates immediate attention. Administrators must prioritize updating the fast-mcp-telegram package to version 0.19.1 to eliminate the path traversal vector. Failure to remediate could allow attackers to hijack sessions and gain unauthorized control over managed Telegram accounts.