Critical Severity Vulnerability
This vulnerability has been rated as Critical severity. Immediate action is recommended.
CVE-2026-5731
CriticalVulnerability Description
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.
CVSS Metrics
Common Vulnerability Scoring System
Vector String:
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Known Affected Software
5 configuration(s) from 1 vendor(s)
cpe:2.3:a:mozilla:firefox:149.0.1:*:*:*:-:*:*:*
cpe:2.3:a:mozilla:firefox:140.9.0:*:*:*:esr:*:*:*
cpe:2.3:a:mozilla:firefox:115.34.0:*:*:*:esr:*:*:*
cpe:2.3:a:mozilla:thunderbird:149.0.1:*:*:*:-:*:*:*
cpe:2.3:a:mozilla:thunderbird:140.9.0:*:*:*:esr:*:*:*
CVE-2026-5731
CVE-2026-5731
References & Resources
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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%2C2026426security@mozilla.org Broken Link
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https://www.mozilla.org/security/advisories/mfsa2026-25/security@mozilla.org Vendor Advisory
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https://www.mozilla.org/security/advisories/mfsa2026-26/security@mozilla.org Vendor Advisory
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https://www.mozilla.org/security/advisories/mfsa2026-27/security@mozilla.org Vendor Advisory
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https://www.mozilla.org/security/advisories/mfsa2026-28/security@mozilla.org Vendor Advisory
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https://www.mozilla.org/security/advisories/mfsa2026-29/security@mozilla.org Vendor Advisory
Severity Details
Weakness Type (CWE)
Improper Restriction of Operations within the Bounds of a Memory Buffer
- Description
- The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to…
- Exploit Likelihood
- High
- Typical Severity
- High
- Abstraction Level
- Class
Key Information
- Published Date
- April 07, 2026
