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Vulnerability · Published Apr 7, 2026 · Assigned by mozilla

CVE-2026-5731Memory 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

Critical 9.8

In plain English

A critical-severity memory corruption in Firefox from Mozilla. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 29% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)9.8Critical
Exploit odds (30 days)0.4%top 70.9% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official 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 packages & fixed versions

EcosystemPackageFixed inAdvisory
AlmaLinux:10firefox140.9.1-1.el10_1ALSA-2026:7672
AlmaLinux:8firefox140.9.1-1.el8_10.alma.1ALSA-2026:8052
AlmaLinux:9firefox140.9.1-1.el9_7.alma.1ALSA-2026:7671
AlmaLinux:9firefox-x11140.9.1-1.el9_7.alma.1ALSA-2026:7671
AlmaLinux:10thunderbird140.9.1-1.el10_1.alma.1ALSA-2026:9638
AlmaLinux:8thunderbird140.9.1-1.el8_10.alma.1ALSA-2026:9345
AlmaLinux:9thunderbird140.9.1-1.el9_7.alma.1ALSA-2026:8459
Debian:12firefox-esr140.9.1esr-1~deb12u1DEBIAN-CVE-2026-5731
Debian:13firefox-esr140.9.1esr-1~deb13u1DEBIAN-CVE-2026-5731
Debian:14firefox-esr140.9.1esr-1DEBIAN-CVE-2026-5731
Debian:12thunderbird1:140.9.1esr-1~deb12u1DEBIAN-CVE-2026-5731
Debian:13thunderbird1:140.9.1esr-1~deb13u1DEBIAN-CVE-2026-5731
Debian:14thunderbird1:140.9.1esr-1DEBIAN-CVE-2026-5731
Rocky Linux:10firefox0:140.9.1-1.el10_1RLSA-2026:7672
Rocky Linux:8firefox0:140.9.1-1.el8_10RLSA-2026:8052
Rocky Linux:9firefox0:140.9.1-1.el9_7RLSA-2026:7671
Rocky Linux:10thunderbird0:140.9.1-1.el10_1RLSA-2026:9638
Rocky Linux:8thunderbird0:140.9.1-1.el8_10RLSA-2026:9345
Rocky Linux:9thunderbird0:140.9.1-1.el9_7RLSA-2026:8459
Ubuntu:22.04:LTSmozjs102no fix listedUBUNTU-CVE-2026-5731
Ubuntu:24.04:LTSmozjs102no fix listedUBUNTU-CVE-2026-5731
Ubuntu:24.04:LTSmozjs115no fix listedUBUNTU-CVE-2026-5731
Ubuntu:22.04:LTSmozjs78no fix listedUBUNTU-CVE-2026-5731
Ubuntu:22.04:LTSmozjs91no fix listedUBUNTU-CVE-2026-5731
Ubuntu:22.04:LTSthunderbirdno fix listedUBUNTU-CVE-2026-5731

Affected products (as reported)

VendorProductVersions
Mozilla Firefox see references
Mozilla Thunderbird see references

Weakness: CWE-119, CWE-787 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

References