CVE-2026-5731: Mozilla Firefox

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

CVSS 9.8 CriticalCriticalEPSS 0.0035 · 29th percentile

Critical, because:

CVSS score 9.8: 9.0 or more.

How we decide what’s critical

Published by the CVE Program (CNA: mozilla) at 7 Apr 2026, 12:43 UTC. agentprime has not analysed this yet, so whether it affects any app is not known. This CVE names a vendor and product rather than a package, so the lockfile check doesn’t match it.

Affected products, as the CNA wrote them
Vendor	Product	Versions
Mozilla	Firefox	unaffected 115.34.1 to 115., inclusive rpm unaffected 140.9.1 to 140., inclusive rpm unaffected 149.0.2 to *, inclusive rpm
Mozilla	Thunderbird	unaffected 140.9.1 to 140., inclusive rpm unaffected 149.0.2 to , inclusive rpm

Identifiers

CVE-2026-5731

CNA

mozilla

Weakness (CWE)

CWE-119 · CWE-787

Not analysed yet

We haven’t published our own analysis of this CVE. If a person reviews it, this page will say what’s affected and what has to be true for it to matter.

Severity and exploitation

Score	Rating	CVSS	Vector	Source
9.8	Critical	3.1	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H	ADP (CISA-ADP)Used for this record
8.8	High	3.1	CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H	ADP (redhat-SADP)

CISA KEV Not listed.
EPSS 0.0035, the 29th percentile, on 9 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

This record came from the full history of the CVE Program, published 7 Apr 2026. New records show when each source confirmed them.

Sources

References from the CNA

Facts on this page come from the CVE List as the CNA published them, with CVSS from the sources named above, CISA KEV and FIRST EPSS. Vendors, products and versions are shown as the CNA wrote them. We don’t reproduce the CVE’s description.