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.

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References

Other 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/

Frequently Asked Questions

What is CVE-2026-84640? +

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.

How do I check if I'm vulnerable to CVE-2026-84640? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.