CVE-2026-5731
Critical
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.
SeverityCritical
CVSS9.8
Exploit probability—
Product
Mozilla Firefox
PublishedApr 7, 2026
First seenApr 7, 2026
How to fix
Vendor advisories
- access.redhat.com/errata/RHSA-2026:13977
- access.redhat.com/errata/RHSA-2026:13665
- access.redhat.com/errata/RHSA-2026:11813
- access.redhat.com/errata/RHSA-2026:7672
- access.redhat.com/errata/RHSA-2026:9638
- access.redhat.com/errata/RHSA-2026:8052
- access.redhat.com/errata/RHSA-2026:9345
- access.redhat.com/errata/RHSA-2026:11805
- access.redhat.com/errata/RHSA-2026:13600
- access.redhat.com/errata/RHSA-2026:14303
- access.redhat.com/errata/RHSA-2026:13683
- access.redhat.com/errata/RHSA-2026:15889
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| mozilla firefox | 115.34.0, 140.9.0, 149.0.1 | — |
| mozilla thunderbird | 140.9.0, 149.0.1 | — |
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