---
cve: "CVE-2026-9739"
severity: "CRITICAL"
cvss: 9.4
epss: "28%"
vendor: "Google"
kev: false
exploited: false
published: "2026-05-27 23:16:48"
tags: [cve, security, critical]
source: tsecurity.de CVE-Dossier
exported: "2026-09-02T15:57:42+02:00"
---

# CVE-2026-9739

> 9.4 CRITICAL · 🧪 PoC

## Beschreibung

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.

## CVSS-Vektor

```
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
```

| Metrik | Wert | Bewertung |
|---|---|---|
| AV Angriffsvektor | Netzwerk | bad |
| AC Komplexität | Gering | bad |
| PR Privilegien | Keine | bad |

## Referenzen

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

---
_Exportiert aus dem [tsecurity.de CVE-Dossier](https://tsecurity.de/cve?cve=CVE-2026-9739) · Datenquellen: EUVD (ENISA), NVD, OSV, CISA KEV, FIRST EPSS, Exploit-DB, BSI BITS_