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Critical severity · published Aug 18, 2026
CVE-2026-74987
Internally found bugs fixed in Thunderbird ESR 140.14, Thunderbird ESR 153.1 and Thunderbird 154
9.8
CVSS
0.7%
EPSS
No
On KEV
Description
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
High
Integrity
High
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.7% · higher than 54% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
FI
Firefox · Mozilla
before 140.14.0; 141.0 before 153.1.0 · source: NVD
TH
Thunderbird · Mozilla
before 140.14.0; 141.0 before 153.1.0 · source: NVD
Affected product
FI
Firefox
Mozilla
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-74987
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-74987
Broken Link
https://bugzilla.mozilla.org/buglist.cgi?bug_id=1500946%2C1788109%2C2045379%2C2045380%2C2049339%2C2049393%2C2053580%2C2054662%2C2054665%2C2054673%2C2054785%2C2058645
Broken Link
https://bugzilla.mozilla.org/buglist.cgi?bug_id=2048797%2C2050536%2C2053272%2C2053579%2C2057115%2C2057116%2C2057130%2C2057991%2C2057995%2C2058002%2C2058008%2C2058032%2C2058102%2C2058667
Permissions Required
https://bugzilla.mozilla.org/show_bug.cgi?id=2059424
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-74/
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-76/
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-77/
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