CVE-2026-57963 Chat UI manipulation by injection
Attackers can inject malicious HTML and CSS into chat messages to manipulate the UI and facilitate phishing attacks in Thunderbird.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-57963?
CVE-2026-57963 is a medium-severity cross-site scripting vulnerability in Mozilla Thunderbird, published on 1 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Weakness: CWE-79
Which versions are affected by CVE-2026-57963?
It affects 1 product.
Products include Mozilla Thunderbird.
How do I fix CVE-2026-57963?
Upgrade to a fixed version.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-57963 being exploited?
There is no known exploitation of CVE-2026-57963 in the wild and no public exploit code.
Other vulnerabilities in Mozilla Thunderbird
Sources
Also known as DEBIAN-CVE-2026-57963, GHSA-396p-jm6v-v2p3, UBUNTU-CVE-2026-57963.