CVE-2026-9739CWE-942

Vulnerable to DNS rebinding attacks when using SSE (http://b/499408790)

Critical · published May 27, 2026

CVSS v4.0
9.4
EPSS
0%
Percentile
13.9
In the wild
Unconfirmed
What it is

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.

The record
Technical detail
CVSS v4.0
9.4 · CRITICAL
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
EPSS
0.00237 · 13.9th percentile
Weakness
CWE-942 · Permissive Cross-domain Security Policy with Untrusted Domains
Published
2026-05-27T21:38Z
EPSS history
Timeline
  • 27 MAY 21:38Z
    Vulnerable to DNS rebinding attacks when using SSE (http://b/499408790)
    cvelistv5