CVE-2026-84642
Allowed UNC hostnames for attachments interpreted as a regular expression
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.
INFO
Published Date :
Sept. 1, 2026, 10:17 p.m.
Last Modified :
Sept. 3, 2026, 7 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | HIGH | 134c704f-9b21-4f2e-91b3-4a467353bcc0 |
Solution
- Update Thunderbird to version 155 or later.
- Verify attachment hostname validation settings.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-84642.
| URL | Resource |
|---|---|
| https://bugzilla.mozilla.org/show_bug.cgi?id=2060462 | Permissions Required |
| https://www.mozilla.org/security/advisories/mfsa2026-86/ | Vendor Advisory |
| https://www.mozilla.org/security/advisories/mfsa2026-88/ | Vendor Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-84642 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-84642
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-84642 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-84642 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Sep. 03, 2026
Action Type Old Value New Value Added CPE Configuration OR *cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:* versions up to (excluding) 153.2.0 *cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:* versions from (including) 154.0 up to (excluding) 155.0 Added Reference Type Mozilla Corporation: https://bugzilla.mozilla.org/show_bug.cgi?id=2060462 Types: Permissions Required Added Reference Type Mozilla Corporation: https://www.mozilla.org/security/advisories/mfsa2026-86/ Types: Vendor Advisory Added Reference Type Mozilla Corporation: https://www.mozilla.org/security/advisories/mfsa2026-88/ Types: Vendor Advisory -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Sep. 02, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N Added CWE CWE-1333 Added SSVC {'id': 'CVE-2026-84642', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'yes'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-09-02T19:14:05.007405Z'} -
New CVE Received by [email protected]
Sep. 01, 2026
Action Type Old Value New Value Added 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. Added Affected New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/84xxx/CVE-2026-84642.json">CVE-2026-84642</a> Added Reference https://bugzilla.mozilla.org/show_bug.cgi?id=2060462 Added Reference https://www.mozilla.org/security/advisories/mfsa2026-86/ Added Reference https://www.mozilla.org/security/advisories/mfsa2026-88/