CVE-2026-109778 Mozilla CVE debrief

Who should care

Defenders responsible for managing Firefox deployments, particularly those using WebExtensions, should assess exposure and apply patches to prevent potential privilege escalation. This includes reviewing WebExtensions component configurations and ensuring proper security settings. Monitoring for exploitation attempts and anomalies in Firefox usage is recommended. Defenders should also verify exposure and apply patches for Firefox versions prior to 158 and

Why it matters

CVE-2026-109778 is a privilege escalation vulnerability in the WebExtensions component of Mozilla Firefox. Defenders should verify exposure, apply patches, and monitor for potential exploitation attempts. The vulnerability was fixed in Firefox 158 and Firefox ESR 153.5. Further verification is needed to determine the full scope of affected systems and potential impact.

• Potential privilege escalation in Firefox WebExtensions component
• Need to verify exposure and apply patches for Firefox versions prior to 158 and Firefox ESR versions prior to 153.5
• Monitoring for exploitation attempts and anomalies in Firefox usage is recommended

Technical summary

The WebExtensions component in Mozilla Firefox is vulnerable to privilege escalation. This issue was fixed in Firefox 158 and Firefox ESR 153.5. The vulnerability affects Firefox deployments using WebExtensions. Defenders should assess exposure and apply patches to prevent potential exploitation. Review of WebExtensions component configurations and security settings is recommended. Monitoring for exploitation attempts and anomalies in Firefox usage is also advised. Further verification is needed to determine the full scope of affected systems and potential impact.

Defensive priority

Defenders should prioritize verifying exposure and applying patches for Firefox versions prior to 158 and Firefox ESR versions prior to 153.5.

Recommended defensive actions

• Verify Firefox version and apply patches for versions prior to 158 and Firefox ESR versions prior to 153.5
• Review WebExtensions component configurations and ensure proper security settings
• Monitor for potential exploitation attempts and anomalies in Firefox usage
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed
• Check relevant monitoring, detection, and logs for exposed assets that need extra review

Evidence notes

The CVE record and Mozilla advisories provide limited information about the vulnerability. Further verification is needed to determine the full scope of affected systems and potential impact. Defenders should verify exposure and apply patches for Firefox versions prior to 158 and Firefox ESR versions prior to 153.5. The vulnerability was fixed in Firefox 158 and Firefox ESR 153.5. Evidence limits and source grounding indicate a need for additional review of WebExtensions component configurations and security settings.

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.