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Description

Vulnerable to DNS rebinding attacks when using SSE (http://b/499408790). During the beta phase, we implemented `allowed-origins` and `allowed-hosts` flags to align with MCP security guidelines. However, the hardcoded `Access-Control-Allow-Origin: *` header in the SSE initialization handler was inadvertently retained. This vulnerability specifically impacts users connecting via Toolbox using SSE under specification v2024-11-05.

PUBLISHED Reserved 2026-05-27 | Published 2026-05-27 | Updated 2026-05-28 | Assigner Google




CRITICAL: 9.4CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H

Problem types

CWE-942: Permissive Cross-domain Policy with Untrusted Domains

Product status

Default status
unaffected

Any version before PR 3054 (Fix CORS bypass)
affected

References

github.com/googleapis/mcp-toolbox/issues/3053

github.com/googleapis/mcp-toolbox/pull/3054

cve.org (CVE-2026-9739)

nvd.nist.gov (CVE-2026-9739)

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