CVE-2026-9739

Severity: critical (CVSS 9.4 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-05-27 · updated 2026-10-26
Weakness: CWE-942

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.

Affected products

Google MCP Toolbox for Databases (googleapis/mcp-toolbox): before PR 3054 (Fix CORS bypass) — fixed in PR 3054 (Fix CORS bypass)

How to fix

Upgrade to a fixed version: Google MCP Toolbox for Databases (googleapis/mcp-toolbox) PR 3054 (Fix CORS bypass). Check the vendor advisory for mitigations.

References

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