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CVE-2026-8956 Mozilla CVE debrief

CVE-2026-8956 is a critical integer overflow in Mozilla’s Networking: JAR component. Mozilla fixed it in Firefox 151, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11; the CVSS 3.1 vector indicates network reachability, no privileges, no user interaction, and high impact.

Vendor
Mozilla
Product
Firefox
CVSS
CRITICAL 9.8
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-19
Original CVE updated
2026-05-20
Advisory published
2026-05-19
Advisory updated
2026-05-20

Who should care

Security teams and administrators responsible for Firefox and Thunderbird fleets, especially managed desktops, enterprise endpoints, and any systems that rely on Mozilla ESR builds or standardized browser/mail deployments.

Technical summary

NVD lists CVE-2026-8956 as a CWE-190 integer overflow in Mozilla’s Networking: JAR component with CVSS 3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. The affected CPE ranges in NVD include Firefox releases earlier than 151.0.0, Firefox ESR releases earlier than 140.11.0, Thunderbird releases earlier than 151.0.0, and Thunderbird ESR releases earlier than 140.11. Mozilla’s referenced advisories and bug record are the official sources cited by NVD for this issue.

Defensive priority

Critical. This issue is rated 9.8 and should be treated as urgent patching for exposed and broadly deployed Mozilla client software.

Recommended defensive actions

  • Upgrade Firefox to 151 or later, or Firefox ESR to 140.11 or later.
  • Upgrade Thunderbird to 151 or later, or Thunderbird ESR to 140.11 or later.
  • Inventory all Mozilla browser and mail client installations to confirm affected versions are covered.
  • Prioritize managed endpoints and any internet-connected systems for accelerated deployment and verification.
  • Validate patch rollout by checking installed versions after update and monitoring vendor advisories/NVD for any follow-on guidance.

Evidence notes

The CVE was published on 2026-05-19 and last modified on 2026-05-20. NVD marks the record as analyzed and cites Mozilla’s Bugzilla entry plus four Mozilla security advisories as references. NVD also maps the weakness to CWE-190 and provides the affected version boundaries and CVSS vector used here.

Official resources

Publicly disclosed in the official CVE record on 2026-05-19 and updated on 2026-05-20. This debrief uses only the supplied NVD and Mozilla reference metadata and does not infer any unlisted exploit details.