CVE-2026-16405 Mozilla CVE debrief

Who should care

Users of Mozilla Firefox, Firefox ESR, and Thunderbird should apply updates to prevent potential information disclosure. This vulnerability has a high CVSS score of 7.5, indicating a high severity. System administrators and security teams responsible for these products should prioritize applying updates. Additionally, developers and users who utilize WebSocket components in their applications should review the vulnerability details and take necessary precautions.

Technical summary

CVE-2026-16405 is an information disclosure vulnerability in the Networking: WebSockets component of Mozilla Firefox, Firefox ESR, and Thunderbird. The vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13. This vulnerability has a high CVSS score of 7.5, indicating a high severity. The vulnerability allows for information disclosure, which could potentially lead to further exploitation. Users of these products should apply updates to prevent potential information disclosure.

Defensive priority

High priority due to high CVSS score of 7.5 and potential for information disclosure.

Recommended defensive actions

• Apply updates to Firefox, Firefox ESR, and Thunderbird to the latest versions.
• Ensure that WebSocket components are properly configured and secured.
• Monitor for any suspicious activity related to WebSocket connections.
• 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 for CVE-2026-16405 was published on 2026-07-21T13:17:15.910Z and has not been modified since then. The vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13. The information disclosure vulnerability in the Networking: WebSockets component of Mozilla Firefox, Firefox ESR, and Thunderbird was addressed. However, specific details about the vulnerability, such as the source of the disclosure and the exact nature of the information disclosure, are limited. Defenders should verify the affected systems and apply updates accordingly.

Official resources