CVE-2026-103500: Heap buffer overflow opening large email

Severity: medium (CVSS 6.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-30 · updated 2026-10-16
Weakness: CWE-787

Description

An attacker could cause a heap buffer overflow by getting a user to open an email that is greater than or equal to 2GB in size. This vulnerability was fixed in Thunderbird 157, Thunderbird 140.17, and Thunderbird 153.4.

Affected products

Mozilla Thunderbird: before 140.17.0; from 141.0 before 153.4.0; from 154.0 before 157.0 — fixed in 140.17.0, 153.4.0, 157.0

How to fix

Upgrade to a fixed version: Mozilla Thunderbird 140.17.0, 153.4.0, 157.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/show_bug.cgi?id=2070267
https://www.mozilla.org/security/advisories/mfsa2026-101/
https://www.mozilla.org/security/advisories/mfsa2026-102/
https://www.mozilla.org/security/advisories/mfsa2026-103/

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