CVE-2026-84642 [HIGH] – The values of the mail.allowed_attachment_hostnames advanced config setting were used in a

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.