PatchSiren cyber security CVE debrief
CVE-2026-16358 Mozilla CVE debrief
A site isolation issue was found in the Graphics: WebRender component of Firefox, Firefox ESR, and Thunderbird. This vulnerability, tracked as CVE-2026-16358, could potentially allow an attacker to bypass site isolation, a security feature designed to prevent malicious scripts from accessing sensitive data on other websites. The issue was addressed in Firefox 153, Firefox ESR 115.38, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13. Users of these products should update to the latest versions to ensure they are protected against this vulnerability.
- Vendor
- Mozilla
- Product
- Firefox
- CVSS
- CRITICAL 9.8
- CISA KEV
- Not listed in stored evidence
- Original CVE published
- 2026-07-21
- Original CVE updated
- 2026-07-22
- Advisory published
- 2026-07-21
- Advisory updated
- 2026-07-22
Who should care
Users of Firefox, Firefox ESR, and Thunderbird should update to the latest versions to ensure they are protected against this vulnerability. This includes users of Firefox versions prior to 153, Firefox ESR versions prior to 115.38 and 140.13, and Thunderbird versions prior to 153 and 140.13. Additionally, security teams and vulnerability management teams should review the affected scope and severity to prioritize and plan updates or mitigations.
Technical summary
The site isolation issue in the Graphics: WebRender component could potentially allow an attacker to bypass site isolation, which is a security feature designed to prevent malicious scripts from accessing sensitive data on other websites. This vulnerability was assigned a CVSS score of 9.8, indicating a critical severity level. The vulnerability affects Firefox, Firefox ESR, and Thunderbird products. The issue was addressed in Firefox 153, Firefox ESR 115.38, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.
Defensive priority
High
Recommended defensive actions
- Update Firefox to version 153 or later
- Update Firefox ESR to version 115.38 or later
- Update Firefox ESR to version 140.13 or later
- Update Thunderbird to version 153 or later
- Update Thunderbird to version 140.13 or later
- Review compensating controls for exposed systems while remediation is scheduled and verified
- Track exceptions, retest remediated assets, and close the item only after evidence is documented
Evidence notes
The CVE record was published on 2026-07-21T13:17:04.310Z and was last modified on 2026-07-22T20:16:51.360Z. The NVD entry is currently Undergoing Analysis. This information is based on the CVE record and NVD entry. Further verification is recommended to confirm affected scope and severity.
Official resources
AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record was published on 2026-07-21T13:17:04.310Z and has not been modified since then. The NVD entry is currently Undergoing Analysis.