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CVE-2026-81098 – Missing Authentication / Credential Exposure – Telnyx MCP Server (telnyx-mcp) ≤ 6.8

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Summary

CVE-2026-81098 is a critical missing-authentication vulnerability (CWE-306) in the Telnyx MCP Server (telnyx-mcp npm package), affecting all versions through 6.83.0. The server’s Streamable HTTP transport listened on all network interfaces and accepted tool-dispatch requests even when no authentication header was supplied, allowing an unauthenticated network attacker to invoke server tools that acted using the server’s own stored Telnyx API key and client secret. The flaw has a CVSS v3.1 base score of 9.1 (v4.0: 9.3), rated Critical.

Technical details

  • Root cause: packages/mcp-server/src/http.ts bound the HTTP listener to all interfaces (0.0.0.0) by default and parsed caller authentication headers in a mode that did not fail/reject when the headers were absent.
  • Trigger conditions: A request reaching the exposed MCP HTTP endpoint with no credential still completed session initialization and was permitted to dispatch tool calls.
  • Attack vector: Network — no authentication, no user interaction, and low attack complexity are required (AV:N/AC:L/PR:N/UI:N).
  • Impact: Tool dispatch forwarded the server’s own stored secrets — the Telnyx API key, client secret, and the code-execution key — to the upstream Telnyx endpoint on behalf of the unauthenticated caller. This gives an attacker who can reach the exposed port the ability to act with the full privileges of the server’s Telnyx account (high confidentiality and integrity impact; no availability impact per the CVSS vector).

Affected software

  • Product: Telnyx MCP Server (telnyx-mcp, npm package, part of team-telnyx/telnyx-node)
  • Versions: All versions from 0 up to and including 6.83.0
  • Fixed in: Versions after 6.83.0 (the current code defaults the host to loopback, requires a server API key, and enforces it in middleware)

Severity

  • CVSS v3.1 Base Score: 9.1 (Critical) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
  • CVSS v4.0 Base Score: 9.3 (Critical) — CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade telnyx-mcp to a version newer than 6.83.0, which binds the HTTP listener to loopback by default, requires a server API key, and enforces authentication in middleware.
  • If unable to patch immediately:
    • Ensure the MCP server’s HTTP interface is bound to localhost/loopback rather than exposed on all network interfaces.
    • Restrict network access to the MCP server port using firewall rules or network segmentation so it is not reachable from untrusted networks or the public internet.
    • Rotate the Telnyx API key, client secret, and any code-execution keys configured on the affected server, since any exposed instance should be considered potentially compromised.
    • Audit logs for unexpected/unauthenticated tool-dispatch requests to the MCP HTTP endpoint prior to patching.

References

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