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CVE-2026-5731 Mozilla CVE debrief

CVE-2026-5731 is a critical vulnerability affecting multiple Mozilla products, including Firefox ESR 115.34.0, Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1, and Thunderbird 149.0.1. The vulnerability is caused by memory safety bugs, some of which showed evidence of memory corruption. If exploited, these bugs could potentially allow attackers to run arbitrary code. The vulnerability was fixed in Firefox 149.0.2, Firefox ESR 115.34.1, Firefox ESR 140.9.1, Thunderbird 149.0.2, and Thunderbird 140.9.1. Users are advised to update their software to the latest versions to mitigate this vulnerability.

Vendor
Mozilla
Product
Firefox
CVSS
CRITICAL 9.8
CISA KEV
Not listed in stored evidence
Original CVE published
2026-04-07
Original CVE updated
2026-07-15
Advisory published
2026-04-07
Advisory updated
2026-07-15

Who should care

This vulnerability affects users of Mozilla's Firefox ESR, Thunderbird ESR, and Firefox products. Specifically, users of Firefox ESR 115.34.0, Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1, and Thunderbird 149.0.1 are at risk. Given the critical severity and potential for arbitrary code execution, administrators and users of these products should prioritize patching.

Technical summary

The CVE-2026-5731 vulnerability is caused by multiple memory safety bugs in Mozilla products. These bugs were found in Firefox ESR 115.34.0, Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1, and Thunderbird 149.0.1. Some of these bugs demonstrated memory corruption, indicating a high risk of potential exploitation for arbitrary code execution. The Common Vulnerability Scoring System (CVSS) score for this vulnerability is 9.8, indicating a critical severity level. The CVSS vector is CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H, reflecting the vulnerability's high impact on confidentiality, integrity, and availability.

Defensive priority

Given the critical severity of CVE-2026-5731 and its potential for arbitrary code execution, patching affected systems should be prioritized. Administrators should ensure that Firefox ESR, Thunderbird ESR, and Firefox are updated to their latest versions: Firefox 149.0.2, Firefox ESR 115.34.1, Firefox ESR 140.9.1, Thunderbird 149.0.2, and Thunderbird 140.9.1.

Recommended defensive actions

  • Update Firefox to version 149.0.2 or later.
  • Update Firefox ESR to version 115.34.1 or later.
  • Update Firefox ESR to version 140.9.1 or later.
  • Update Thunderbird to version 149.0.2 or later.
  • Update Thunderbird ESR to version 140.9.1 or later.

Evidence notes

The CVE-2026-5731 vulnerability was publicly disclosed on April 7, 2026, and last modified on June 30, 2026. The vulnerability was fixed in various Mozilla products, including Firefox, Firefox ESR, and Thunderbird. Multiple references, including vendor advisories and bug reports, are available for further information.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-5731 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-5731

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

  • CVE-2026-5731 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-5731

    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.