What’s Vulnerable › Vulnerabilities

CVE-2026-84640: One byte overflow read in mail parser

• Severity: high (CVSS 7.5 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: 2026-09-01 · updated 2026-09-03
• Weakness: CWE-126

Description

A maliciously constructed mail header could lead to a one byte read past the end of a buffer. This vulnerability was fixed in Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

Affected products

• Mozilla Thunderbird: before 140.15.0; from 141.0 before 153.2.0; from 154.0 before 155.0 — fixed in 140.15.0, 153.2.0, 155.0

How to fix

Upgrade to a fixed version: Mozilla Thunderbird 140.15.0, 153.2.0, 155.0. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=2063964
• https://www.mozilla.org/security/advisories/mfsa2026-86/
• https://www.mozilla.org/security/advisories/mfsa2026-87/
• https://www.mozilla.org/security/advisories/mfsa2026-88/

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