Unauthenticated Remote Access and Credential Exposure in Telnyx MCP Server

CVE-2026-81098 affects the Telnyx MCP server, where the HTTP transport listener was erroneously configured to bind to all available network interfaces rather than the local loopback. Additionally, the server's authentication middleware failed to validate the presence of caller credentials, allowing any unauthenticated remote request to reach the server's initialization and tool dispatch logic.

When a request is processed, the server automatically forwards its own internal credentials, including the Telnyx API key, client secret, and code-execution keys, to the upstream endpoint. This flaw allows an unauthenticated remote attacker with network access to the server port to act on behalf of the server, leading to unauthorized tool execution and the potential compromise of sensitive API credentials. The vulnerability has been addressed by restricting the host binding to the loopback interface and enforcing mandatory credential validation within the authentication middleware.

Impact

Successful exploitation allows unauthenticated remote callers to interact with the Telnyx MCP server, potentially resulting in unauthorized code execution and the exposure of the server's hardcoded Telnyx API keys and client secrets to an attacker-controlled upstream endpoint. This could lead to a complete compromise of the Telnyx API integration and further unauthorized actions within the victim's cloud environment.

Recommendation

• Update the Telnyx MCP server to the version that enforces loopback binding and mandatory API key authentication.
• Audit the Telnyx API keys and client secrets previously configured in the affected MCP server instances, as they may have been exposed to upstream endpoints.
• Restrict network access to the MCP server port via host-based firewalls or network security groups to ensure it is only accessible from trusted local processes.