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CVE-2025-0239: mozilla firefox Improper Certificate Validation

CVSS: 4.0v3.1MediumModified
AliasesUBUNTU-CVE-2025-0239DEBIAN-CVE-2025-0239CGA-2fcf-j2wr-p4h9
Published 07-01-2025Last modified 13-04-2026
Open JSON

Vulnerability summary

CVE-2025-0239 is a medium (CVSS 4.0) Improper Certificate Validation vulnerability in mozilla firefox. EPSS estimates a 0.23% chance of exploitation in the next 30 days, higher than 12.4% of scored vulnerabilities.

Overall risk
16/100
0100
Low · default modelHow is this calculated?
CVSS score
4.0/10
010
Medium · v3.1 (CVE.org)
EPSS score
0.23%
0%100%
Low · 0.00 pp in 30 days
KEVNot listedIn no KEV catalog
RansomwareNone recordedNo known use recorded
Public exploits0No public exploits recorded

Advisory lineage

Upstream 0 · Downstream 33 · Related 12
DLA-4011-1DLA-4012-1DSA-5839-1DSA-5841-1MGASA-2025-0009MGASA-2025-0010

Timeline

  1. 07-01-2025
    PublishedVulnerability first disclosed.
  2. 13-04-2026
    Last modifiedVulnerability information updated.

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.

CVSS metrics

6 scores · 6 sources · 1 version

EPSS trend

Probability

0.23%

Chance of exploitation in the next 30 days

Percentile

12%

Versus all other CVEs

30-day change

0.00 pp

From 0.23% on 18-09-2026

18-09-202618-10-2026
EPSS score
0.23%

Known exploited

Compare KEV catalogs

Not in any of the 3 tracked catalogs.

Public exploits

0 found

No public exploits indexed.

Weaknesses

CWE · root cause

Affected systems

Ubuntu9 products
  • firefox< 134.0+build1-0ubuntu0.20.04.1
  • mozjs102All versions (No fix recorded)
  • mozjs115All versions (No fix recorded)
  • mozjs38All versions (No fix recorded)
  • mozjs52All versions (No fix recorded)
Chainguard2 products
  • firefox< 134.0-r0
  • firefox-esr< 128.6.0-r0
Debian2 products
  • firefox-esr< 128.6.0esr-1~deb12u1 | < 128.6.0esr-1
  • thunderbird< 1:128.6.0esr-1~deb12u1 | < 1:128.6.0esr-1
Mozilla2 products
Wolfi1 product
  • firefox< 134.0-r0

5 vendors · 16 products

References

13 links
Show all 13 references