How severe is CVE-2026-9739?

CVE-2026-9739 has a CVSS 4.0 base score of 9.4, rated critical severity.

Is CVE-2026-9739 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (20th percentile), an estimate of the probability of exploitation in the next 30 days.

How do I fix CVE-2026-9739?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

Does CVE-2026-9739 have an EU (EUVD) identifier?

Yes. CVE-2026-9739 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-32672.

When was CVE-2026-9739 published?

CVE-2026-9739 was published on 2026-05-27 and last updated on 2026-06-17.

CVE-2026-9739

CVE-2026-9739 is a critical-severity vulnerability with a CVSS 4.0 base score of 9.4. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-942.

Key facts

Severity: Critical (CVSS 4.0 base score 9.4)
EPSS exploit prediction: 0% (20th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-32672
Weakness: CWE-942
Published: 2026-05-27
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2026-9739?: 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.
How severe is CVE-2026-9739?: CVE-2026-9739 has a CVSS 4.0 base score of 9.4, rated critical severity.
Is CVE-2026-9739 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (20th percentile), an estimate of the probability of exploitation in the next 30 days.
How do I fix CVE-2026-9739?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
Does CVE-2026-9739 have an EU (EUVD) identifier?: Yes. CVE-2026-9739 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-32672.
When was CVE-2026-9739 published?: CVE-2026-9739 was published on 2026-05-27 and last updated on 2026-06-17.

References

Other CWE-942 vulnerabilities

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CVE-2026-8948 — Critical (CVSS 9.1): Same-origin policy bypass in the DOM: Networking component. This vulnerability was fixed in Firefox 151 and Thunderbird…
CVE-2023-38125 — High (CVSS 8.8): Softing edgeAggregator Permissive Cross-domain Policy with Untrusted Domains Remote Code Execution Vulnerability. This…

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