What is CVE-2026-57963?

CVE-2026-57963 is a medium-severity vulnerability in Mozilla Thunderbird, classified under Cross-site Scripting. CVSS score: 6.5/10. Published 2026-07-01.

How severe is CVE-2026-57963?

Medium severity. CVSS v3 base score is 6.5 out of 10.

XSS in Mozilla Thunderbird

CVE-2026-57963

An attacker who can send HTML chat messages (via Matrix or XMPP) can inject arbitrary styled content, phishing links, and CSS that manipulates the chat UI. This vulnerability was fixed in Thunderbird 152.0.1 and Thunderbird 140.12.1.

Vulnerability class: XSS (Cross-Site Scripting)

Published 2026-07-01 · last modified 2026-07-06.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.003 (20.9th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 21st percentile — 20.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.002 · newest: 0.003 · change: +0.001

Affected products

Mozilla Thunderbird — versions 140.12.1, 152.0.1

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

References

bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
www.mozilla.org/security/advisories/mfsa2026-62/
www.mozilla.org/security/advisories/mfsa2026-63/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-64/ (Vendor Advisory)

Frequently asked questions

What is CVE-2026-57963?: CVE-2026-57963 is a medium-severity vulnerability in Mozilla Thunderbird, classified under Cross-site Scripting. CVSS score: 6.5/10. Published 2026-07-01.
How severe is CVE-2026-57963?: Medium severity. CVSS v3 base score is 6.5 out of 10.