CVE-2026-84642
HIGHDescription
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.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Exploit Intelligence
Low risk: more likely to be exploited than 17% of all known CVEs.
References
Related Vulnerabilities
Other CWE-1333 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2023-3364 | High | 7.5 | 44% | - | Fix |
| CVE-2024-8124 | High | 7.5 | 40% | - | Fix |
| CVE-2024-25126 | Medium | 5.3 | 35% | - | Fix |
| CVE-2024-2651 | Medium | 6.5 | 33% | - | Fix |
| CVE-2021-32837 | High | 7.5 | 29% | - | Fix |
| CVE-2024-2829 | High | 7.5 | 26% | - | Fix |
Common questions
How do I fix CVE-2026-84642?
Upgrade Mozilla Thunderbird (en-US) to 155.0 or later.
Is CVE-2026-84642 being actively exploited?
Not that we know of. CVE-2026-84642 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.25% is the estimated probability that it will be exploited in the next 30 days. That is higher than 17% of all scored CVEs.
How severe is CVE-2026-84642?
CVE-2026-84642 has a CVSS v3 base score of 7.5, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-84642 affect?
Published advisories record a fix for Mozilla Thunderbird (en-US) (Windows application). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-09-03.