CVE-2026-9739

CRITICAL EPSS 19.7%

Published May 27, 20261mo ago · Modified Jun 17, 20261w ago

9.4 CVSS 4.0

Critical

Published May 27, 2026 1mo ago

Last Modified Jun 17, 2026 1w ago

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.

CVSS Details

Base Score

9.4

Exploitability

—

Impact

—

Vector string

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Attack Vector Network

Attack Complexity Low

Privileges Required None

User Interaction A

Scope X

Threat Intelligence

EPSS Exploit Probability

19.7% percentile

Exploit & Patch Status

No Known Exploit

No Patch Available

Weaknesses 1

CWE-942

References 2

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

Remediation

No remediation data recorded yet

Check vendor advisories and the NVD entry for patch availability.