CVE-2026-5731
published 2026-04-07CVE-2026-5731: 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…
PriorityP351critical9.8CVSS 3.1
AVNACLPRNUINSUCHIHAH
EPSS
0.34%
25.8th percentile
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
9 ranges
| Vendor | Product | Version range | Fixed in |
|---|---|---|---|
| debian | firefox | < firefox 149.0.2-1 (sid) | firefox 149.0.2-1 (sid) |
| debian | firefox-esr | < firefox 149.0.2-1 (sid) | firefox 149.0.2-1 (sid) |
| debian | thunderbird | < firefox 149.0.2-1 (sid) | firefox 149.0.2-1 (sid) |
| mozilla | firefox | — | — |
| mozilla | firefox | — | — |
| mozilla | firefox | — | — |
| mozilla | firefox | — | — |
| mozilla | thunderbird | — | — |
| mozilla | thunderbird | — | — |
CVSS provenance
nvdv3.19.8CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
osv9.8CRITICAL
vendor_debian9.8CRITICAL
vendor_redhat9.8CRITICAL
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Red Hat
thunderbird: 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
vendor_redhat·2026-04-07·CVSS 9.8
CVE-2026-5731 [CRITICAL] CWE-787 thunderbird: 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
thunderbird: 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
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 affects Firefox < 149.0.2, Firefox ESR < 115.34.1, Firefox ESR < 140.9.1, Thunderbird < 149.0.2, and Thunderbird < 140.9.1.
A flaw was found in Firefox and Thunderbird. The Mozilla Foundation's Security Advisory describes the following issue:
Memory safety bugs present in Firefox ESR 115.34.0, Firefox ESR 140.9.0, Thunderb
Debian
CVE-2026-5731: firefox - Memory safety bugs present in Firefox ESR 115.34.0, Firefox ESR 140.9.0, Thunder...
vendor_debian·2026·CVSS 9.8
CVE-2026-5731 [CRITICAL] CVE-2026-5731: firefox - Memory safety bugs present in Firefox ESR 115.34.0, Firefox ESR 140.9.0, Thunder...
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 affects Firefox < 149.0.2, Firefox ESR < 115.34.1, Firefox ESR < 140.9.1, Thunderbird < 149.0.2, and Thunderbird < 140.9.1.
Scope: local
sid: resolved (fixed in 149.0.2-1)
Mozilla
Mozilla Foundation Security Advisory 2026-28: CVE-2026-5731
vendor_mozilla·CVSS 9.8
CVE-2026-5731 [CRITICAL] Mozilla Foundation Security Advisory 2026-28: CVE-2026-5731
Mozilla Foundation Security Advisory 2026-28
CVE: CVE-2026-5731
Product: Thunderbird
Impact: high
Fixed in: Thunderbird 149.0.2
Mozilla
Mozilla Foundation Security Advisory 2026-29: CVE-2026-5731
vendor_mozilla·CVSS 9.8
CVE-2026-5731 [CRITICAL] Mozilla Foundation Security Advisory 2026-29: CVE-2026-5731
Mozilla Foundation Security Advisory 2026-29
CVE: CVE-2026-5731
Product: Thunderbird
Impact: high
Fixed in: Thunderbird 140.9.1
Mozilla
Mozilla Foundation Security Advisory 2026-25: CVE-2026-5731
vendor_mozilla·CVSS 9.8
CVE-2026-5731 [CRITICAL] Mozilla Foundation Security Advisory 2026-25: CVE-2026-5731
Mozilla Foundation Security Advisory 2026-25
CVE: CVE-2026-5731
Product: Firefox
Impact: high
Fixed in: Firefox 149.0.2
Mozilla
Mozilla Foundation Security Advisory 2026-27: CVE-2026-5731
vendor_mozilla·CVSS 9.8
CVE-2026-5731 [CRITICAL] Mozilla Foundation Security Advisory 2026-27: CVE-2026-5731
Mozilla Foundation Security Advisory 2026-27
CVE: CVE-2026-5731
Product: Firefox ESR
Impact: high
Fixed in: Firefox ESR 140.9.1
Mozilla
Mozilla Foundation Security Advisory 2026-26: CVE-2026-5731
vendor_mozilla·CVSS 9.8
CVE-2026-5731 [CRITICAL] Mozilla Foundation Security Advisory 2026-26: CVE-2026-5731
Mozilla Foundation Security Advisory 2026-26
CVE: CVE-2026-5731
Product: Firefox ESR
Impact: high
Fixed in: Firefox ESR 115.34.1
VulDB
Mozilla Firefox up to 149.0.1 memory corruption (Nessus ID 306064 / WID-SEC-2026-0997)
vuldb·2026-05-02·CVSS 9.8
CVE-2026-5731 [CRITICAL] Mozilla Firefox up to 149.0.1 memory corruption (Nessus ID 306064 / WID-SEC-2026-0997)
A vulnerability described as critical has been identified in Mozilla Firefox up to 149.0.1. This impacts an unknown function. Executing a manipulation can lead to memory corruption.
This vulnerability appears as CVE-2026-5731. The attack may be performed from remote. There is no available exploit.
Upgrading the affected component is recommended.
GHSA
GHSA-fwrw-mfrr-q8px: Memory safety bugs present in Firefox ESR 115
ghsa_unreviewed·2026-04-07
CVE-2026-5731 [CRITICAL] CWE-119 GHSA-fwrw-mfrr-q8px: Memory safety bugs present in Firefox ESR 115
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 affects Firefox < 149.0.2, Firefox ESR < 115.34.1, and Firefox ESR < 140.9.1.
OSV
CVE-2026-5731: Memory safety bugs present in Firefox ESR 115
osv·2026-04-07·CVSS 9.8
CVE-2026-5731 [CRITICAL] CVE-2026-5731: Memory safety bugs present in Firefox ESR 115
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 affects Firefox < 149.0.2, Firefox ESR < 115.34.1, Firefox ESR < 140.9.1, Thunderbird < 149.0.2, and Thunderbird < 140.9.1.
No detection rules found.
No public exploits indexed.
Bugzilla
CVE-2026-5731 thunderbird: 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
bugzilla·2026-04-07·CVSS 9.8
CVE-2026-5731 [CRITICAL] CVE-2026-5731 thunderbird: 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
CVE-2026-5731 thunderbird: 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
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 affects Firefox < 149.0.2, Firefox ESR < 115.34.1, and Firefox ESR < 140.9.1.
Wiz
CVE-2026-5731 Impact, Exploitability, and Mitigation Steps | Wiz
blogs_wiz·CVSS 9.8
CVE-2026-5731 [CRITICAL] CVE-2026-5731 Impact, Exploitability, and Mitigation Steps | Wiz
## CVE-2026-5731 :
Mozilla Firefox vulnerability analysis and mitigation
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 affects Firefox < 149.0.2, Firefox ESR < 115.34.1, Firefox ESR < 140.9.1, Thunderbird < 149.0.2, and Thunderbird < 140.9.1.
Source : NVD
## 9.8
Score
Published April 7, 2026
Severity CRITICAL
CNA Score 9.8
Affected Technologies
Mozilla Firefox
Mozilla Thunderbird
Has Public Exploit No
Has CISA KEV Exploit No
CISA KEV Release Date N/A
CISA KEV Due Date N/A
Exploitation Probability Percentile
https://bugzilla.mozilla.org/buglist.cgi?bug_id=2021894%2C2022225%2C2022252%2C2022294%2C2023007%2C2023130%2C2023191%2C2023364%2C2023829%2C2024074%2C2024417%2C2024433%2C2024436%2C2024437%2C2024453%2C2024461%2C2024462%2C2024472%2C2024474%2C2024477%2C2025364%2C2025401%2C2025402%2C2025472%2C2026287%2C2026299%2C2026305%2C2026426https://www.mozilla.org/security/advisories/mfsa2026-25/https://www.mozilla.org/security/advisories/mfsa2026-26/https://www.mozilla.org/security/advisories/mfsa2026-27/https://www.mozilla.org/security/advisories/mfsa2026-28/https://www.mozilla.org/security/advisories/mfsa2026-29/https://access.redhat.com/errata/RHSA-2026:11805https://access.redhat.com/errata/RHSA-2026:11813https://access.redhat.com/errata/RHSA-2026:12264https://access.redhat.com/errata/RHSA-2026:13342https://access.redhat.com/errata/RHSA-2026:13412https://access.redhat.com/errata/RHSA-2026:13533https://access.redhat.com/errata/RHSA-2026:13582https://access.redhat.com/errata/RHSA-2026:13583https://access.redhat.com/errata/RHSA-2026:13596https://access.redhat.com/errata/RHSA-2026:13600https://access.redhat.com/errata/RHSA-2026:13665https://access.redhat.com/errata/RHSA-2026:13682https://access.redhat.com/errata/RHSA-2026:13683https://access.redhat.com/errata/RHSA-2026:13922https://access.redhat.com/errata/RHSA-2026:13977https://access.redhat.com/errata/RHSA-2026:14223https://access.redhat.com/errata/RHSA-2026:14303https://access.redhat.com/errata/RHSA-2026:15889https://access.redhat.com/errata/RHSA-2026:7671https://access.redhat.com/errata/RHSA-2026:7672https://access.redhat.com/errata/RHSA-2026:8052https://access.redhat.com/errata/RHSA-2026:8459https://access.redhat.com/errata/RHSA-2026:9345https://access.redhat.com/errata/RHSA-2026:9638https://access.redhat.com/security/cve/CVE-2026-5731https://bugzilla.redhat.com/show_bug.cgi?id=2455901https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-5731.json
2026-04-07
Published