CVE-2026-74987

CRITICAL 9.8

Internally found bugs fixed in Thunderbird ESR 140.14, Thunderbird ESR 153.1 and Thunderbird 154

CVSS Score

9.8

EPSS Score

0.5%

EPSS Percentile

42th

Internally found bugs present in Thunderbird ESR 140.13, Thunderbird ESR 153.0 and Thunderbird 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.

Vendor	mozilla
Product	firefox
Ecosystems	Browser JavaScript
Industries	Technology
Published	Aug 18, 2026
Last Updated	Sep 1, 2026

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Affected Versions

Mozilla / Firefox

All versions affected

Mozilla / Thunderbird

All versions affected

References

NVD ↗ CVE.org ↗ EPSS Data ↗

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

Credits

Nicolas Silva, Tom Ritter and the Mozilla Fuzzing Team