Vulnerability record · CVE-2026-84642 · published 1 September 2026

CVE-2026-84642: Mozilla thunderbird inefficient regular expression (redos) vulnerability

Mozilla · Thunderbird

The values of the mail.allowed_attachment_hostnames advanced config setting were used in a regular expression without escaping. For some possible valid hostnames, this could allow certain unintended hostnames to also match and serve remote attachments. This vulnerability was fixed in Thunderbird 155 and Thunderbird 153.2.

7.5 CVSS 3.1 High EPSS 0.42% · top 66.5% CWE-1333 · Inefficient regular expression (ReDoS)

7.5CVSS 3.1 base score

0.42%EPSS exploitation probability, 30 days

NoNot in CISA KEV

1Affected product versions listed by NVD

3References

3 Sep 2026Last modified by NVD

Description

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

Thunderbird Mozilla

References

Link	Tags
https://bugzilla.mozilla.org/show_bug.cgi?id=2060462	Permissions Required
https://www.mozilla.org/security/advisories/mfsa2026-86/	Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-88/	Vendor Advisory

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Same products first, then exploited flaws of the same weakness class.

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Source: NIST National Vulnerability Database (record CVE-2026-84642), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.