Regex Injection via mail.allowed_attachment_hostnames in Thunderbird <155
CVE-2026-84642 Published on September 1, 2026
Allowed UNC hostnames for attachments interpreted as a regular expression
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.
Products Associated with CVE-2026-84642
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Affected Versions
Mozilla Thunderbird:- Version 153.2, <= 153.* is unaffected.
- Version 155, <= * is unaffected.