CVE-2026-5734

Memory safety bugs fixed in Firefox ESR 140.9.1, Thunderbird ESR 140.9.1, Firefox 149.0.2 and Thunderbird 149.0.2

Published Apr 7, 2026 CVSS 3.1 · 8.1 High EPSS < 1% KEV · Not listed
Description
Memory safety bugs present in 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 140.9.1, Thunderbird 149.0.2, and Thunderbird 140.9.1.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Debian DLA Debian DLA DLA-4526-1 firefox-esr security update
Debian DLA Debian DLA DLA-4534-1 thunderbird security update
Debian DSA Debian DSA DSA-6202-1 firefox-esr security update
Debian DSA Debian DSA DSA-6211-1 thunderbird security update
References
Link Providers
https://access.redhat.com/errata/RHSA-2026:11805 cve-icon
https://access.redhat.com/errata/RHSA-2026:11813 cve-icon
https://access.redhat.com/errata/RHSA-2026:12264 cve-icon
https://access.redhat.com/errata/RHSA-2026:13342 cve-icon
https://access.redhat.com/errata/RHSA-2026:13412 cve-icon
https://access.redhat.com/errata/RHSA-2026:13533 cve-icon
https://access.redhat.com/errata/RHSA-2026:13582 cve-icon
https://access.redhat.com/errata/RHSA-2026:13583 cve-icon
https://access.redhat.com/errata/RHSA-2026:13596 cve-icon
https://access.redhat.com/errata/RHSA-2026:13600 cve-icon
https://access.redhat.com/errata/RHSA-2026:13665 cve-icon
https://access.redhat.com/errata/RHSA-2026:13682 cve-icon
https://access.redhat.com/errata/RHSA-2026:13683 cve-icon
https://access.redhat.com/errata/RHSA-2026:13922 cve-icon
https://access.redhat.com/errata/RHSA-2026:13977 cve-icon
https://access.redhat.com/errata/RHSA-2026:14223 cve-icon
https://access.redhat.com/errata/RHSA-2026:14303 cve-icon
https://access.redhat.com/errata/RHSA-2026:15889 cve-icon
https://access.redhat.com/errata/RHSA-2026:7671 cve-icon
https://access.redhat.com/errata/RHSA-2026:7672 cve-icon
https://access.redhat.com/errata/RHSA-2026:8052 cve-icon
https://access.redhat.com/errata/RHSA-2026:8459 cve-icon
https://access.redhat.com/errata/RHSA-2026:9345 cve-icon
https://access.redhat.com/errata/RHSA-2026:9638 cve-icon
https://access.redhat.com/security/cve/CVE-2026-5734 cve-icon
https://bugzilla.mozilla.org/buglist.cgi?bug_id=2022369%2C2023026%2C2023545%2C2023555%2C2023958%2C2025422%2C2025468%2C2025492%2C2025505 cve-icon cve-icon
https://bugzilla.redhat.com/show_bug.cgi?id=2455897 cve-icon
https://nvd.nist.gov/vuln/detail/CVE-2026-5734 cve-icon
https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-5734.json cve-icon
https://www.cve.org/CVERecord?id=CVE-2026-5734 cve-icon
https://www.mozilla.org/security/advisories/mfsa2026-25/ cve-icon cve-icon
https://www.mozilla.org/security/advisories/mfsa2026-27/ cve-icon cve-icon
https://www.mozilla.org/security/advisories/mfsa2026-27/#CVE-2026-5734 cve-icon
https://www.mozilla.org/security/advisories/mfsa2026-28/ cve-icon cve-icon
https://www.mozilla.org/security/advisories/mfsa2026-29/ cve-icon cve-icon
https://www.mozilla.org/security/advisories/mfsa2026-29/#CVE-2026-5734 cve-icon
History
Type Values Removed Values Added
Description Memory safety bugs present in 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 affects Firefox < 149.0.2, Firefox ESR < 140.9.1, Thunderbird < 149.0.2, and Thunderbird < 140.9.1. Memory safety bugs present in 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 140.9.1, Thunderbird 149.0.2, and Thunderbird 140.9.1.
Type Values Removed Values Added
Weaknesses CWE-120
Metrics cvssV3_1

{'score': 9.8, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H'}

ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'total'}, 'version': '2.0.3'}

cvssV3_1

{'score': 8.1, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H'}

Type Values Removed Values Added
First Time appeared Mozilla firefox Esr
Vendors & Products Mozilla firefox Esr
Type Values Removed Values Added
Description Memory safety bugs present in 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 affects Firefox < 149.0.2 and Firefox ESR < 140.9.1. Memory safety bugs present in 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 affects Firefox < 149.0.2, Firefox ESR < 140.9.1, Thunderbird < 149.0.2, and Thunderbird < 140.9.1.
First Time appeared Mozilla
Mozilla firefox
Mozilla thunderbird
Weaknesses CWE-787
CPEs cpe:2.3:a:mozilla:firefox:*:*:*:*:-:*:*:*
cpe:2.3:a:mozilla:firefox:*:*:*:*:esr:*:*:*
cpe:2.3:a:mozilla:thunderbird:*:*:*:*:-:*:*:*
cpe:2.3:a:mozilla:thunderbird:*:*:*:*:esr:*:*:*
Vendors & Products Mozilla
Mozilla firefox
Mozilla thunderbird
References
Metrics cvssV3_1

{'score': 9.8, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H'}

Type Values Removed Values Added
Description Memory safety bugs present in 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 affects Firefox < 149.0.2 and Firefox ESR < 140.9.1.
Title Memory safety bugs fixed in Firefox ESR 140.9.1, Thunderbird ESR 140.9.1, Firefox 149.0.2 and Thunderbird 149.0.2
References
Weaknesses

Sources

Status
PUBLISHED
Assigner
mozilla
Published
Updated
Jul 15, 2026, 12:42 AM
Reserved
Apr 7, 2026, 12:43 PM
Status
Modified
Published
Apr 7, 2026, 1:16 PM
Modified
Oct 23, 2026, 11:10 PM
Severity
Important
Public date
Apr 7, 2026, 12:43 PM