How severe is CVE-2025-6427?

CVE-2025-6427 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.

Is CVE-2025-6427 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (22nd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2025-6427?

CVE-2025-6427 affects Mozilla Firefox. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2025-6427?

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-2025-6427 have an EU (EUVD) identifier?

Yes. CVE-2025-6427 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-21379.

When was CVE-2025-6427 published?

CVE-2025-6427 was published on 2025-06-24 and last updated on 2026-06-17.

CVE-2025-6427

CVE-2025-6427 is a critical-severity vulnerability in Mozilla Firefox with a CVSS 3.x base score of 9.1. 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-693.

Key facts

Severity: Critical (CVSS 3.x base score 9.1)
EPSS exploit prediction: 0% (22nd percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2025-21379
Weakness: CWE-693
Affected product: Mozilla Firefox
Published: 2025-06-24
Last modified: 2026-06-17

Description

An attacker was able to bypass the `connect-src` directive of a Content Security Policy by manipulating subdocuments. This would have also hidden the connections from the Network tab in Devtools. This vulnerability was fixed in Firefox 140 and Thunderbird 140.

Frequently asked questions

What is CVE-2025-6427?: An attacker was able to bypass the `connect-src` directive of a Content Security Policy by manipulating subdocuments. This would have also hidden the connections from the Network tab in Devtools. This vulnerability was fixed in Firefox 140 and Thunderbird 140.
How severe is CVE-2025-6427?: CVE-2025-6427 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.
Is CVE-2025-6427 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (22nd percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2025-6427?: CVE-2025-6427 affects Mozilla Firefox. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2025-6427?: 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-2025-6427 have an EU (EUVD) identifier?: Yes. CVE-2025-6427 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-21379.
When was CVE-2025-6427 published?: CVE-2025-6427 was published on 2025-06-24 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:mozilla:firefox:*:*:*:*:-:*:*:*

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