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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.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-16358 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-16358

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-16358 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-16358

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental 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.