CVE-2026-0891

What is CVE-2026-0891?

Memory safety bugs present in Firefox ESR 140.6, Thunderbird ESR 140.6, Firefox 146 and Thunderbird 146. 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 147, Firefox ESR 140.7, Thunderbird 147, and Thunderbird 140.7.

Timeline

• 2026-01-13Published to the U.S. National Vulnerability Database (NVD)
• 2026-03-20First covered in a defend.network daily briefing
• 2026-07-15NVD record last updated

Affected product

Mozilla Firefox

NVD also lists CPE entries for: Mozilla Firefox, Mozilla Thunderbird

References

• https://www.mozilla.org/security/advisories/mfsa2026-01/
• https://www.mozilla.org/security/advisories/mfsa2026-03/
• https://www.mozilla.org/security/advisories/mfsa2026-04/
• https://nvd.nist.gov/vuln/detail/CVE-2026-0891

Referenced in our briefings & reports

• VMware ESXi ransomware exploit; BlackSuit healthcare breach (2026-03-20)

Related vulnerabilities

• CVE-2026-88779 – Citrix NetScaler (same weakness: CWE-119)
• CVE-2026-88772 – Citrix NetScaler (same weakness: CWE-119)
• CVE-2026-8452 – Citrix NetScaler ADC And NetScaler Gateway (same weakness: CWE-119)

Mozilla: 13 known exploited vulnerabilities (CISA KEV) →