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CVE-2026-5731: Memory safety bugs fixed in Firefox ESR 115.34.1, Firefox ESR 140.9.1, Thunderbird ESR 140.9.1, Firefox 149.0.2 and Thunderbird 149.0.2

• Severity: critical (CVSS 9.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-04-07 · updated 2026-10-24
• Weakness: CWE-119

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.

Affected products

• Mozilla Firefox: 115.34.0, 140.9.0, 149.0.1
• Mozilla Thunderbird: 140.9.0, 149.0.1

References

• https://bugzilla.mozilla.org/buglist.cgi?bug_id=2021894%2C2022225%2C2022252%2C2022294%2C2023007%2C2023130%2C2023191%2C2023364%2C2023829%2C2024074%2C2024417%2C2024433%2C2024436%2C2024437%2C2024453%2C2024461%2C2024462%2C2024472%2C2024474%2C2024477%2C2025364%2C2025401%2C2025402%2C2025472%2C2026287%2C2026299%2C2026305%2C2026426
• https://www.mozilla.org/security/advisories/mfsa2026-25/
• https://www.mozilla.org/security/advisories/mfsa2026-26/
• https://www.mozilla.org/security/advisories/mfsa2026-27/
• https://www.mozilla.org/security/advisories/mfsa2026-28/
• https://www.mozilla.org/security/advisories/mfsa2026-29/
• https://access.redhat.com/errata/RHSA-2026:11805
• https://access.redhat.com/errata/RHSA-2026:11813
• https://access.redhat.com/errata/RHSA-2026:12264
• https://access.redhat.com/errata/RHSA-2026:13342
• https://access.redhat.com/errata/RHSA-2026:13412
• https://access.redhat.com/errata/RHSA-2026:13533

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