CVE-2026-4691
CriticalCVSS 9.8Exploitation Probability (EPSS)
Low risk38th percentile - higher than 38% of all known CVEs
Summary
Use-after-free vulnerability in the CSS Parsing and Computation component of Firefox and Thunderbird. The flaw was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.
Risk Assessment
An attacker could exploit this vulnerability to execute arbitrary code or cause a denial of service, potentially compromising data confidentiality and integrity.
Recommendation
Immediately update Firefox and Thunderbird to the patched versions listed (Firefox 149, Firefox ESR 115.34/140.9, Thunderbird 149/140.9).
Other vulnerabilities in Firefox
See all- CVE-2026-113470Medium
Mozilla blocked compromised intermediate eMudhra certificates via OneCRL on October 17, and the list was distributed to clients via RemoteSettings the same day. This update contains the updated OneCRL block list, ensuring clients will block the compromised certificates even if RemoteSettings is disabled by policy or otherwise unavailable in the user's network environment. The vulnerability was fixed in Firefox 158.0.1, Firefox ESR 153.5.1, Firefox ESR 115.43.1, Thunderbird 158.0.1, and Thunderbird 153.5.1.
- CVE-2026-113401Medium
A denial-of-service vulnerability exists in the PDF Viewer component. The vulnerability was fixed in Firefox 158.0.1 and Thunderbird 158.0.1.
- CVE-2026-109780Critical
Vulnerability in the DOM: Core & HTML component allows mitigation bypass. This issue was fixed in Firefox 158 and Thunderbird 158.
- CVE-2026-109774Critical
Invalid pointer and undefined behavior in the DOM: Notifications component. Fixed in Firefox 158 and Thunderbird 158.
- CVE-2026-109772Critical
Same-origin policy bypass in the DevTools component. Fixed in Firefox 158 and Thunderbird 158.
- CVE-2026-109755Critical
A vulnerability in the DOM: Content Processes component allows sandbox escape. It was fixed in Firefox 158 and Thunderbird 158.
- CVE-2026-109750Critical
A vulnerability in the DOM: Web Authentication component causes a site isolation issue. It was fixed in Firefox 158 and Thunderbird 158.
- CVE-2026-109745Critical
Sandbox escape due to uninitialized memory in the DOM: Web Serial component. It was fixed in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5.
- CVE-2026-109740Critical
Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. The flaw was fixed in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5.
- CVE-2026-106016Critical
Mitigation bypass in the File Handling component. This vulnerability was fixed in Firefox 157.0.1 and Thunderbird 158.
Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS