CVE-2026-84642 Allowed UNC hostnames for attachments interpreted as a regular expression
Attackers can bypass attachment host restrictions by exploiting unescaped regex patterns in configuration, allowing unauthorized remote attachment retrieval.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-84642?
CVE-2026-84642 is a high-severity server-side request forgery vulnerability in Mozilla Thunderbird, published on 1 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-1333
Which versions are affected by CVE-2026-84642?
It affects 1 product.
Products include Mozilla Thunderbird.
How do I fix CVE-2026-84642?
Upgrade to a fixed version.
Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.
Is CVE-2026-84642 being exploited?
There is no known exploitation of CVE-2026-84642 in the wild and no public exploit code.
Other vulnerabilities in Mozilla Thunderbird
Sources
Also known as GHSA-5392-q9gh-3cc8, UBUNTU-CVE-2026-84642.