How severe is CVE-2025-0239?

CVE-2025-0239 has a CVSS 3.x base score of 4.0, rated medium severity. It is exploitable over local access with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity low, and availability none.

Is CVE-2025-0239 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (13th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2025-0239?

CVE-2025-0239 primarily affects Mozilla Firefox. In total, 3 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2025-0239?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2025-0239 have an EU (EUVD) identifier?

Yes. CVE-2025-0239 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-1571.

When was CVE-2025-0239 published?

CVE-2025-0239 was published on 2025-01-07 and last updated on 2026-06-17.

CVE-2025-0239

CVE-2025-0239 is a medium-severity vulnerability in Mozilla Firefox with a CVSS 3.x base score of 4.0. 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-295.

Key facts

Severity: Medium (CVSS 3.x base score 4.0)
EPSS exploit prediction: 0% (13th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2025-1571
Weakness: CWE-295
Affected product: Mozilla Firefox
Published: 2025-01-07
Last modified: 2026-06-17

Description

When using Alt-Svc, ALPN did not properly validate certificates when the original server is redirecting to an insecure site. This vulnerability was fixed in Firefox 134, Firefox ESR 128.6, Thunderbird 134, and Thunderbird 128.6.

Frequently asked questions

What is CVE-2025-0239?: When using Alt-Svc, ALPN did not properly validate certificates when the original server is redirecting to an insecure site. This vulnerability was fixed in Firefox 134, Firefox ESR 128.6, Thunderbird 134, and Thunderbird 128.6.
How severe is CVE-2025-0239?: CVE-2025-0239 has a CVSS 3.x base score of 4.0, rated medium severity. It is exploitable over local access with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity low, and availability none.
Is CVE-2025-0239 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (13th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2025-0239?: CVE-2025-0239 primarily affects Mozilla Firefox. In total, 3 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2025-0239?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2025-0239 have an EU (EUVD) identifier?: Yes. CVE-2025-0239 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-1571.
When was CVE-2025-0239 published?: CVE-2025-0239 was published on 2025-01-07 and last updated on 2026-06-17.

References

Affected products (3)

cpe:2.3:a:mozilla:firefox:*:*:*:*:esr:*:*:*
cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:*
cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:*

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