CVE-2026-109776 Mozilla CVE debrief

Who should care

Defenders responsible for managing Firefox installations, particularly those using affected versions, should assess exposure and prioritize verification and remediation efforts. This includes IT personnel, security teams, and system administrators who oversee Firefox deployments. They should review the CVE record and source item for further guidance and take immediate action to verify and patch vulnerable systems.

Why it matters

A race condition vulnerability in the XPCOM component of Firefox was fixed in Firefox 158 and Firefox ESR 153.5. Defenders should assess exposure, particularly for systems using affected versions, and prioritize verification and remediation efforts.

• Verification of affected systems requires immediate attention to prevent potential exploitation
• Patching of vulnerable systems should be prioritized to prevent potential exploitation
• Defenders should assess exposure of systems using affected Firefox versions

Technical summary

A race condition vulnerability exists in the XPCOM component of Firefox. This vulnerability was fixed in Firefox 158 and Firefox ESR 153.5. The CVE record and source item provide limited information about the vulnerability. Defenders should prioritize verification of affected systems, particularly those using Firefox versions prior to 158 or Firefox ESR versions prior to 153.5, and apply patches as soon as possible. The technical details of the vulnerability are not fully disclosed, but it is clear that the race condition could have significant operational impacts if exploited.

Defensive priority

Defenders should prioritize verification of affected systems, particularly those using Firefox versions prior to 158 or Firefox ESR versions prior to 153.5, and apply patches as soon as possible.

Recommended defensive actions

• Verify Firefox version and update to 158 or Firefox ESR 153.5 if necessary
• Assess exposure of systems using affected Firefox versions
• Prioritize patching of vulnerable systems
• 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 source item provide limited information about the vulnerability, confirming a race condition exists in the XPCOM component and was fixed in Firefox 158 and Firefox ESR 153.5. Defenders should verify the vulnerability's impact on their systems, especially those using affected Firefox versions prior to 158 or Firefox ESR versions prior to 153.5. Evidence is limited, and further verification is necessary to determine the full scope of the vulnerability.

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.