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

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.

Published: 2026-04-07 Last update: 2026-04-16 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-5731 is rated Moderate Risk (45.2/100): CVSS Critical severity, with low exploitation likelihood (EPSS 0.06%). Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2026-5731

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-04-08 — 0.06% —

Full EPSS history (1 record total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-5731

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
9.8 3.1 CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 5.9 134c704f-9b21-4f2e-91b3-4a467353bcc0

Weakness enumeration for CVE-2026-5731

OS Trackers for CVE-2026-5731

vendor priority summary link
debian not yet assigned CVE-2026-5731 not yet assigned priority: Debian including 3 source packages (firefox, firefox-esr, thunderbird), 11 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 11. https://security-tracker.debian.org/tracker/CVE-2026-5731
redhat high — https://access.redhat.com/security/cve/CVE-2026-5731
suse high CVE-2026-5731 severity important: SUSE including 32 source package names (MozillaFirefox-140.9.1-112.307.1, MozillaFirefox-140.9.1-150200.152.228.1, …), 85 product×package rows across 22 product lines (SUSE Liberty Linux 10, SUSE Liberty Linux 8, … (22 product lines)): Fixed 81, First Fixed 4. https://www.suse.com/security/cve/CVE-2026-5731/
ubuntu medium CVE-2026-5731 medium priority: Ubuntu including 9 source packages (firefox, mozjs102, …), 49 status rows across 7 suites (bionic, focal, jammy, noble, questing, resolute, upstream): DNE 23, needs-triage 10, ignored 8, not-affected 7, needed 1. https://ubuntu.com/security/CVE-2026-5731

Affected software / configurations for CVE-2026-5731

Vendor Product Version Raw CPE
mozilla firefox 115.34.0 cpe:2.3:a:mozilla:firefox:115.34.0:*:*:*:esr:*:*:*
mozilla firefox 140.9.0 cpe:2.3:a:mozilla:firefox:140.9.0:*:*:*:esr:*:*:*
mozilla firefox 149.0.1 cpe:2.3:a:mozilla:firefox:149.0.1:*:*:*:-:*:*:*
mozilla thunderbird 140.9.0 cpe:2.3:a:mozilla:thunderbird:140.9.0:*:*:*:esr:*:*:*
mozilla thunderbird 149.0.1 cpe:2.3:a:mozilla:thunderbird:149.0.1:*:*:*:-:*:*:*

References for CVE-2026-5731

cvelogic Threat Intelligence