CVE-2026-5731

moderate-risk
Published 2026-04-07

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.

Do I need to act?

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0.07% chance of exploitation
EPSS score — low exploit probability
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Not on CISA KEV list
No confirmed active exploitation reported to CISA
?
Patch status unknown
Check vendor advisories for fix availability and mitigation guidance
9
CVSS 9.8/10 Critical
NETWORK / LOW complexity

Affected Products (5)

Affected Vendors

Mozilla

References (6)

Broken Link https://bugzilla.mozilla.org/buglist.cgi?bug_id=2021894%2C2022225%2C2022252%2C20...
Vendor Advisory 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/
44
/ 100
moderate-risk
Severity 32/34 · Critical
Exploitability 0/34 · Minimal
Exposure 12/34 · Low