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CVE-2025-5271VulnerabilityPublished Updated

CVE-2025-5271: Mozilla Firefox · CWE-116

Affects Mozilla FirefoxSource CVE Database V5

0

Not known to be exploited · CVSS 6.5 Medium : upgrade to 139 when convenient.

Data as of 26 Oct 2026, 08:30 UTC

CVSS 3.1 base score
6.5Medium
How severe the flaw is on the 0–10 CVSS scale.
EPSS
0.3% chance of exploitation in 30 days – higher than 23% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch
Available – fixed in 139
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 27 May 2025

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponised: no evidence on record.

    No public exploit code indexed in Metasploit or Exploit-DB

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

What to do

Upgrade Mozilla Firefox to 139.

Affected software

ProductAffectedFixed in
Mozilla FirefoxAffected
  • <139
Fixed in
  • 139

AI-suggested

This vulnerability is fixed in Mozilla Firefox version 139. Users and administrators should update to Firefox 139 or later to remediate this issue. No additional mitigation steps are required as the vendor advisory confirms the official fix is available.

Description

Previewing a response in Devtools ignored CSP headers, which could have allowed content injection attacks. This vulnerability was fixed in Firefox 139 and Thunderbird 139.

References

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