# CVE-2026-9739

## Summary

- **CVE ID:** CVE-2026-9739
- **Severity:** CRITICAL
- **CVSS Score:** 9.4 (CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H)
- **CWE:** CWE-942
- **Published:** May 27, 2026
- **Last Modified:** May 28, 2026

## 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 (0)

## References

- [CNA](https://github.com/googleapis/mcp-toolbox/issues/3053)
- [CNA](https://github.com/googleapis/mcp-toolbox/pull/3054)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.28%
- **EPSS Percentile:** 20.3

---
_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-11._