CVE-2026-6759 Mozilla CVE debrief

Who should care

Defenders responsible for managing Firefox, Firefox ESR, Thunderbird, and Thunderbird ESR deployments should assess exposure and apply patches or upgrades as available. Defenders should prioritize verifying exposure of affected systems and apply patches or upgrades as available due to the high CVSS score of 7.5 and potential for high impact attacks. Defenders should review the supplied official advisory or CVE record to validate affected scope, severity,

Why it matters

Defenders should prioritize verifying exposure of Firefox, Firefox ESR, Thunderbird, and Thunderbird ESR versions prior to 150, 140.10, 150, and 140.10 respectively, and apply patches or upgrades as available due to the high CVSS score of 7.5 and potential for high impact attacks.

• Potential high impact attacks against unpatched systems.
• Verification of exposure and application of patches or upgrades is required.

Technical summary

The CVE record describes a use-after-free vulnerability in the Widget: Cocoa component of Firefox, Firefox ESR, Thunderbird, and Thunderbird ESR. This vulnerability was fixed in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10. The high CVSS score of 7.5 indicates a significant risk, and defenders should review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance. The vulnerability affects Firefox, Firefox ESR, Thunderbird, and Thunderbird ESR, and defenders should prioritize verifying exposure of affected systems and apply patches or upgrades as available.

Defensive priority

Defenders should prioritize verifying exposure of Firefox, Firefox ESR, Thunderbird, and Thunderbird ESR versions prior to 150, 140.10, 150, and 140.10 respectively, and apply patches or upgrades as available.

Recommended defensive actions

• Verify exposure of Firefox, Firefox ESR, Thunderbird, and Thunderbird ESR versions prior to 150, 140.10, 150, and 140.10 respectively.
• Apply patches or upgrades as available from Mozilla.
• Monitor for potential high impact attacks targeting this vulnerability.
• Review compensating controls for exposed systems while remediation is scheduled and verified.
• Check relevant monitoring, detection, and logs for exposed assets that need extra review.
• Track exceptions, retest remediated assets, and close the item only after evidence is documented.
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.

Evidence notes

The CVE record and NVD entry provide details on the vulnerability, including its CVSS score of 7.5 and severity of HIGH. Vendor advisories are available from Mozilla. The vulnerability affects Firefox, Firefox ESR, Thunderbird, and Thunderbird ESR, and was fixed in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10. Defenders should verify exposure of affected systems and apply patches or upgrades as available. The CVE record was published on 2026-04-21T13:16:21.857Z and has not been modified since then. The NVD

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.