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CVE-2026-79537 – Cross-Tenant Session Hijacking / Data Exfiltration – MetaMCP through 2.4.22

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Summary

CVE-2026-79537 is a critical Insecure Direct Object Reference (IDOR) / authorization bypass vulnerability in MetaMCP, an MCP aggregator and gateway product from metatool-ai, affecting all versions through 2.4.22. The flaw allows an unauthenticated network attacker to hijack another tenant’s active MCP session and use it to list and execute that tenant’s private MCP tools, exfiltrating data via the victim’s forwarded credentials. The issue is rated CRITICAL with a CVSS v3.1 base score of 9.1.

Technical details

  • Root cause: MetaMCP’s session store keys sessions solely by the client-supplied mcp-session-id HTTP header, with no binding to an owner, tenant/namespace, or endpoint.
  • The per-endpoint authorization middleware only validates that the caller is permitted to use the requested URL endpoint — it never verifies that the session ID presented actually belongs to that caller.
  • An unauthenticated endpoint, GET /metamcp/health/sessions, discloses active session IDs and associated namespace UUIDs, giving attackers the values needed to impersonate other tenants.
  • Attack vector: network, no authentication or user interaction required, low attack complexity — an attacker only needs to reach the exposed health endpoint and then replay a harvested session ID against the MCP transport dispatch mechanism.
  • Impact: cross-tenant access to private MCP tools and data, with the attacker’s requests forwarded using the victim’s own credentials, resulting in high confidentiality and integrity impact.

Affected software

  • MetaMCP (metatool-ai/metamcp), all versions through 2.4.22

Severity

  • CVSS v3.1 Base Score: 9.1 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Mitigation and recommended actions

  • Immediate: No patched version has been confirmed as of publication; monitor the metatool-ai/metamcp repository for a fixed release and upgrade as soon as one is available.
  • Network mitigations: Restrict access to MetaMCP deployments to trusted networks only; do not expose the gateway or its health endpoints directly to the internet.
  • Restrict or disable the GET /metamcp/health/sessions endpoint, or require authentication on it and strip session ID/namespace data from its response.
  • Treat the mcp-session-id header as untrusted client input — do not rely on it alone for authorization; ensure session lookups are bound to an authenticated owner and namespace.
  • Audit MCP session logs for cross-tenant access patterns or requests presenting session IDs not issued to the requesting party.

References

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