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CVE-2026-81094 – Missing Authentication / Unauthenticated Network Exposure – mcp-router before 0.6.3

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Summary

CVE-2026-81094 is a missing-authentication vulnerability (CWE-306) affecting mcp-router, an open-source MCP (Model Context Protocol) aggregator/CLI. In vulnerable versions, the serve command bound the aggregator to all network interfaces by default and only enforced authentication when an operator explicitly configured it, leaving the service reachable and usable by anyone who could route traffic to the listening port. The issue carries a CVSS v3.1 base score of 9.1 (CVSS v4.0: 9.3), reflecting network-exploitable, low-complexity, unauthenticated access with a high confidentiality impact.

Technical details

  • Root cause: The mcp-router serve command defaulted to binding on all network interfaces (0.0.0.0) rather than localhost, and did not mandate an authentication token unless the operator explicitly configured one.
  • Trigger conditions: Any deployment running mcp-router serve with default settings on a host reachable over a network (e.g., not restricted by firewall/NAT) is exposed; no special configuration is required to trigger exposure — it is the out-of-the-box behavior.
  • Attack vector: Network (AV:N) — a remote, unauthenticated attacker who can reach the exposed port can connect directly to the MCP aggregator.
  • Impact: Unauthorized access to the MCP aggregator allows an attacker to interact with aggregated MCP servers/tools without credentials, resulting in high confidentiality impact (potential exposure of aggregated tool data/configuration); integrity/availability impact is not indicated in the scoring (I:N/A:N per CVSS v3.1 vector observed in source data, C:H).

Affected software

  • Product: mcp-router (https://github.com/mcp-router/mcp-router)
  • Versions affected: All versions prior to 0.6.3
  • Fixed version: 0.6.3 and later (fix commit 4c4642cfd274097ec8b33ecd3047390829c79d35)

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 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
  • CWE-306: Missing Authentication for Critical Function

Mitigation and recommended actions

  • Immediate: Upgrade mcp-router to version 0.6.3 or later, which changes the serve command’s default bind address to 127.0.0.1 (localhost only) and enforces that a --token be supplied whenever a non-loopback --host is specified, ensuring network-exposed instances require authentication.
  • If immediate patching is not possible:
    • Restrict network access to the mcp-router listening port using host-based firewalls, security groups, or network segmentation so it is reachable only from trusted hosts.
    • Do not run serve bound to a public or non-loopback interface without explicitly configuring an authentication token.
    • Audit existing deployments for instances currently bound to 0.0.0.0 or other non-loopback interfaces without authentication configured, and reconfigure them to bind to localhost or require a token immediately.
    • Review logs/network traffic for unexpected inbound connections to the mcp-router port as an indicator of possible exploitation prior to remediation.

References

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