CVE-2026-9739
CWE-942Published: May 27, 2026· Updated: May 29, 2026
Official 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.
Risk Analysis
This critical vulnerability in systems using SSE under specification v2024-11-05 is susceptible to DNS rebinding attacks due to a hardcoded 'Access-Control-Allow-Origin: *' header. With a CVSSv4 score of 9.4, this flaw presents a significant risk, allowing attackers to bypass origin restrictions. The absence of an EPSS score means the current exploitation likelihood is not publicly assessed, but the high severity warrants immediate action.
No public exploit is currently known for this vulnerability. Exploitation requires user interaction, specifically when connecting via Toolbox using SSE, and is remotely exploitable.
Update affected systems to versions that properly enforce 'allowed-origins' and 'allowed-hosts' flags and remove the hardcoded 'Access-Control-Allow-Origin: *' header. Ensure that all web applications are configured to prevent DNS rebinding attacks.
Technical Analysis
CVE-2026-9739 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.
Exploitation does not require any privileges, though user interaction (A) is needed, which slightly reduces the risk of mass automated attacks.
CVSS v3.1 Vector Breakdown
Exploit & PoC Resources
All References (2)
Quick Facts
Related CVEs (CWE-942)
Recommended Actions
- →Apply vendor patches immediately
- →Monitor CVE-2026-9739 in threat intel feeds
- →Review IDS/IPS signatures for exploitation attempts