CVE-2026-4691
published 2026-03-24CVE-2026-4691: Use-after-free in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird…
PriorityP350critical9.8CVSS 3.1
AVNACLPRNUINSUCHIHAH
EPSS
0.48%
38.6th percentile
Use-after-free in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.
Affected
11 ranges
| Vendor | Product | Version range | Fixed in |
|---|---|---|---|
| debian | firefox | < firefox 149.0-1 (sid) | firefox 149.0-1 (sid) |
| debian | firefox-esr | < firefox 149.0-1 (sid) | firefox 149.0-1 (sid) |
| debian | thunderbird | < firefox 149.0-1 (sid) | firefox 149.0-1 (sid) |
| mozilla | firefox | < 115.34.0 | 115.34.0 |
| mozilla | firefox | < 149.0 | 149.0 |
| mozilla | firefox | — | — |
| mozilla | firefox | >= 128.0 < 140.9.0 | 140.9.0 |
| mozilla | thunderbird | >= 0 < 1:140.9.0esr-1~deb11u1 | 1:140.9.0esr-1~deb11u1 |
| mozilla | thunderbird | >= 0 < 1:140.9.0esr-1~deb12u1 | 1:140.9.0esr-1~deb12u1 |
| mozilla | thunderbird | >= 0 < 1:140.9.0esr-1~deb13u1 | 1:140.9.0esr-1~deb13u1 |
| mozilla | thunderbird | >= 0 < 1:140.9.0esr-1 | 1:140.9.0esr-1 |
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
firefox: thunderbird: Use-after-free in the CSS Parsing and Computation component
vendor_redhat·2026-03-24·CVSS 9.8
CVE-2026-4691 [CRITICAL] CWE-825 firefox: thunderbird: Use-after-free in the CSS Parsing and Computation component
firefox: thunderbird: Use-after-free in the CSS Parsing and Computation component
Use-after-free in the CSS Parsing and Computation component. This vulnerability affects Firefox < 149, Firefox ESR < 115.34, Firefox ESR < 140.9, Thunderbird < 149, and Thunderbird < 140.9.
A flaw was found in Firefox and Thunderbird. The Mozilla Foundation's Security Advisory describes the following issue:
Use-after-free in the CSS Parsing and Computation component
Statement: Red Hat Product Security rates the severity of this flaw as determined by the Mozilla Foundation Security Advisory.
Package: rhel10/firefox-flatpak (Red Hat Enterprise Linux 10) - Affected
Package: rhel10/thunderbird-flatpak (Red Hat Enterprise Linux 10) - Affected
Package: firefox (Red Hat Enterprise Linux 6) - Out of support sco
Debian
CVE-2026-4691: firefox - Use-after-free in the CSS Parsing and Computation component. This vulnerability ...
vendor_debian·2026·CVSS 9.8
CVE-2026-4691 [CRITICAL] CVE-2026-4691: firefox - Use-after-free in the CSS Parsing and Computation component. This vulnerability ...
Use-after-free in the CSS Parsing and Computation component. This vulnerability affects Firefox < 149, Firefox ESR < 115.34, Firefox ESR < 140.9, Thunderbird < 149, and Thunderbird < 140.9.
Scope: local
sid: resolved (fixed in 149.0-1)
Mozilla
Mozilla Foundation Security Advisory 2026-23: CVE-2026-4691
vendor_mozilla·CVSS 9.8
CVE-2026-4691 [CRITICAL] Mozilla Foundation Security Advisory 2026-23: CVE-2026-4691
Mozilla Foundation Security Advisory 2026-23
CVE: CVE-2026-4691
Product: Thunderbird
Impact: high
Fixed in: Thunderbird 149
Mozilla
Mozilla Foundation Security Advisory 2026-20: CVE-2026-4691
vendor_mozilla·CVSS 9.8
CVE-2026-4691 [CRITICAL] Mozilla Foundation Security Advisory 2026-20: CVE-2026-4691
Mozilla Foundation Security Advisory 2026-20
CVE: CVE-2026-4691
Product: Firefox
Impact: high
Fixed in: Firefox 149
Mozilla
Mozilla Foundation Security Advisory 2026-24: CVE-2026-4691
vendor_mozilla·CVSS 9.8
CVE-2026-4691 [CRITICAL] Mozilla Foundation Security Advisory 2026-24: CVE-2026-4691
Mozilla Foundation Security Advisory 2026-24
CVE: CVE-2026-4691
Product: Thunderbird
Impact: high
Fixed in: Thunderbird 140.9
Mozilla
Mozilla Foundation Security Advisory 2026-22: CVE-2026-4691
vendor_mozilla·CVSS 9.8
CVE-2026-4691 [CRITICAL] Mozilla Foundation Security Advisory 2026-22: CVE-2026-4691
Mozilla Foundation Security Advisory 2026-22
CVE: CVE-2026-4691
Product: Firefox ESR
Impact: high
Fixed in: Firefox ESR 140.9
Mozilla
Mozilla Foundation Security Advisory 2026-21: CVE-2026-4691
vendor_mozilla·CVSS 9.8
CVE-2026-4691 [CRITICAL] Mozilla Foundation Security Advisory 2026-21: CVE-2026-4691
Mozilla Foundation Security Advisory 2026-21
CVE: CVE-2026-4691
Product: Firefox ESR
Impact: high
Fixed in: Firefox ESR 115.34
VulDB
Mozilla Firefox up to 148 CSS Parser use after free (Nessus ID 304234 / WID-SEC-2026-0850)
vuldb·2026-04-16·CVSS 9.8
CVE-2026-4691 [CRITICAL] Mozilla Firefox up to 148 CSS Parser use after free (Nessus ID 304234 / WID-SEC-2026-0850)
A vulnerability was found in Mozilla Firefox up to 148. It has been rated as critical. Affected by this vulnerability is an unknown functionality of the component CSS Parser. Performing a manipulation results in use after free.
This vulnerability is reported as CVE-2026-4691. The attack is possible to be carried out remotely. No exploit exists.
Upgrading the affected component is advised.
OSV
CVE-2026-4691: Use-after-free in the CSS Parsing and Computation component
osv·2026-03-24·CVSS 9.8
CVE-2026-4691 [CRITICAL] CVE-2026-4691: Use-after-free in the CSS Parsing and Computation component
Use-after-free in the CSS Parsing and Computation component. This vulnerability affects Firefox < 149, Firefox ESR < 115.34, Firefox ESR < 140.9, Thunderbird < 149, and Thunderbird < 140.9.
GHSA
GHSA-h895-88fj-h89r: Use-after-free in the CSS Parsing and Computation component
ghsa_unreviewed·2026-03-24
CVE-2026-4691 [CRITICAL] CWE-416 GHSA-h895-88fj-h89r: Use-after-free in the CSS Parsing and Computation component
Use-after-free in the CSS Parsing and Computation component. This vulnerability affects Firefox < 149, Firefox ESR < 115.34, and Firefox ESR < 140.9.
No detection rules found.
No public exploits indexed.
Bugzilla
CVE-2026-4691 firefox: thunderbird: Use-after-free in the CSS Parsing and Computation component
bugzilla·2026-03-24·CVSS 9.8
CVE-2026-4691 [CRITICAL] CVE-2026-4691 firefox: thunderbird: Use-after-free in the CSS Parsing and Computation component
CVE-2026-4691 firefox: thunderbird: Use-after-free in the CSS Parsing and Computation component
Use-after-free in the CSS Parsing and Computation component. This vulnerability affects Firefox < 149, Firefox ESR < 115.34, and Firefox ESR < 140.9.
Discussion:
This issue has been addressed in the following products:
Red Hat Enterprise Linux 10
Via RHSA-2026:5931 https://access.redhat.com/errata/RHSA-2026:5931
---
This issue has been addressed in the following products:
Red Hat Enterprise Linux 9
Via RHSA-2026:5930 https://access.redhat.com/errata/RHSA-2026:5930
---
This issue has been addressed in the following products:
Red Hat Enterprise Linux 8
Via RHSA-2026:5932 https://access.redhat.com/errata/RHSA-2026:5932
---
This issue has been addressed in the following products:
Red
Wiz
CVE-2026-4691 Impact, Exploitability, and Mitigation Steps | Wiz
blogs_wiz·CVSS 7.5
CVE-2026-4691 [HIGH] CVE-2026-4691 Impact, Exploitability, and Mitigation Steps | Wiz
## CVE-2026-4691 :
NixOS vulnerability analysis and mitigation
Use-after-free in the CSS Parsing and Computation component. This vulnerability affects Firefox < 149, Firefox ESR < 115.34, Firefox ESR < 140.9, Thunderbird < 149, and Thunderbird < 140.9.
Source : NVD
## 9.8
Score
Published March 24, 2026
Severity CRITICAL
CNA Score 9.8
Affected Technologies
NixOS
Mozilla Firefox
Has Public Exploit No
Has CISA KEV Exploit No
CISA KEV Release Date N/A
CISA KEV Due Date N/A
Exploitation Probability Percentile (EPSS) 10.1
Exploitation Probability (EPSS) N/A
Affected packages and libraries
MozillaThunderbird-translations-common
rhel10::thunderbird-flatpak
Sources
AlmaLinux 8 Severity HIGH Has Fix Added at: Mar 31, 2026
AlmaLinux 9 Severity HIGH Has Fix Added at: Mar 31, 2
https://bugzilla.mozilla.org/show_bug.cgi?id=2017512https://www.mozilla.org/security/advisories/mfsa2026-20/https://www.mozilla.org/security/advisories/mfsa2026-21/https://www.mozilla.org/security/advisories/mfsa2026-22/https://www.mozilla.org/security/advisories/mfsa2026-23/https://www.mozilla.org/security/advisories/mfsa2026-24/https://access.redhat.com/errata/RHSA-2026:5930https://access.redhat.com/errata/RHSA-2026:5931https://access.redhat.com/errata/RHSA-2026:5932https://access.redhat.com/errata/RHSA-2026:6188https://access.redhat.com/errata/RHSA-2026:6342https://access.redhat.com/errata/RHSA-2026:6917https://access.redhat.com/errata/RHSA-2026:7837https://access.redhat.com/errata/RHSA-2026:7838https://access.redhat.com/errata/RHSA-2026:7839https://access.redhat.com/errata/RHSA-2026:7840https://access.redhat.com/errata/RHSA-2026:7841https://access.redhat.com/errata/RHSA-2026:7842https://access.redhat.com/errata/RHSA-2026:7843https://access.redhat.com/errata/RHSA-2026:7845https://access.redhat.com/errata/RHSA-2026:7858https://access.redhat.com/errata/RHSA-2026:8284https://access.redhat.com/errata/RHSA-2026:8285https://access.redhat.com/errata/RHSA-2026:8286https://access.redhat.com/errata/RHSA-2026:8287https://access.redhat.com/errata/RHSA-2026:8288https://access.redhat.com/errata/RHSA-2026:8289https://access.redhat.com/errata/RHSA-2026:8290https://access.redhat.com/errata/RHSA-2026:8315https://access.redhat.com/errata/RHSA-2026:8427https://access.redhat.com/errata/RHSA-2026:8850https://access.redhat.com/security/cve/CVE-2026-4691https://bugzilla.redhat.com/show_bug.cgi?id=2450738https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-4691.json
2026-03-24
Published