What’s Vulnerable › Vulnerabilities

CVE-2026-84642: Allowed UNC hostnames for attachments interpreted as a regular expression

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% 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-1333

Description

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.

Affected products

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

How to fix

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

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=2060462
• https://www.mozilla.org/security/advisories/mfsa2026-86/
• https://www.mozilla.org/security/advisories/mfsa2026-88/

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