CVE-2026-5731: Mozilla Firefox

Critical severity, CVSS 9.8. EPSS: 0.6% chance of exploitation in the next 30 days.

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.

Affected products

• Mozilla Firefox: version 115.34.0 only; version 140.9.0 only; version 149.0.1 only
• Mozilla Thunderbird: version 140.9.0 only; version 149.0.1 only

Published 2026-04-07. Last modified 2026-07-15.