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CVE-2026-74987: Internally found bugs fixed in Firefox ESR 140.14, Firefox ESR 153.1 and Firefox 154

• Severity: critical (CVSS 9.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.7% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-18 · updated 2026-10-28
• Weakness: CWE-119

Description

Internally found bugs present in Firefox ESR 140.13, Firefox ESR 153.0 and Firefox 153. Some of these bugs showed evidence of memory corruption or another security-relevant defect and we presume that with enough effort some of these could have been exploited. This vulnerability was fixed in Firefox 154, Firefox ESR 140.14, Firefox ESR 153.1, Thunderbird 154, Thunderbird 140.14, and Thunderbird 153.1.

Affected products

• Mozilla Firefox: before 140.14.0; from 141.0 before 153.1.0 — fixed in 140.14.0, 153.1.0
• Mozilla Thunderbird: before 140.14.0; from 141.0 before 153.1.0 — fixed in 140.14.0, 153.1.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 140.14.0, 153.1.0; Mozilla Thunderbird 140.14.0, 153.1.0. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/buglist.cgi?bug_id=1500946%2C1788109%2C2045379%2C2045380%2C2049339%2C2049393%2C2053580%2C2054662%2C2054665%2C2054673%2C2054785%2C2058645
• https://bugzilla.mozilla.org/buglist.cgi?bug_id=2048797%2C2050536%2C2053272%2C2053579%2C2057115%2C2057116%2C2057130%2C2057991%2C2057995%2C2058002%2C2058008%2C2058032%2C2058102%2C2058667
• https://bugzilla.mozilla.org/show_bug.cgi?id=2059424
• https://www.mozilla.org/security/advisories/mfsa2026-74/
• https://www.mozilla.org/security/advisories/mfsa2026-76/
• https://www.mozilla.org/security/advisories/mfsa2026-77/
• https://www.mozilla.org/security/advisories/mfsa2026-78/
• https://www.mozilla.org/security/advisories/mfsa2026-79/
• https://www.mozilla.org/security/advisories/mfsa2026-80/

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