CVE-2026-5731 — CRITICAL (9.8)

Vulnerability Description

Memory safety bugs present 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 showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This 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.

CVSS Score

9.8

CRITICAL

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Attack Vector

NETWORK

Attack Complexity

LOW

Privileges Required

NONE

User Interaction

NONE

Scope

UNCHANGED

Confidentiality

HIGH

Integrity

HIGH

Availability

HIGH

Affected Products

Vendor	Product	Versions
Mozilla	Firefox	115.34.0
Mozilla	Thunderbird	140.9.0

Related Weaknesses (CWE)

CWE-119

References

https://bugzilla.mozilla.org/buglist.cgi?bug_id=2021894%2C2022225%2C2022252%2C20Broken Link
https://www.mozilla.org/security/advisories/mfsa2026-25/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-26/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-27/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-28/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-29/Vendor Advisory

FAQ

What is CVE-2026-5731?

CVE-2026-5731 is a vulnerability with a CVSS score of 9.8 (CRITICAL). Memory safety bugs present 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 showed evidence of memory corruption and w...

How severe is CVE-2026-5731?

CVE-2026-5731 has been rated CRITICAL with a CVSS base score of 9.8/10. This is considered a critical vulnerability requiring immediate attention.

Is there a patch for CVE-2026-5731?

Check the references section above for vendor advisories and patch information. Affected products include: Mozilla Firefox, Mozilla Thunderbird.