CVE-2026-21393 Cross-site scripting in Movable Type (Cloud Edition)
Attackers can inject malicious scripts via comments, leading to arbitrary code execution in logged-in users' browsers.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- None listed
What is CVE-2026-21393?
CVE-2026-21393 is a medium-severity cross-site scripting vulnerability in Six Apart Ltd. Movable Type (Cloud Edition), published on 4 Feb 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Official description
Movable Type contains a stored cross-site scripting vulnerability in Edit Comment. If crafted input is stored by an attacker, arbitrary script may be executed on a logged-in user's web browser. Note that Movable Type 7 series and 8.4 series, which are End-of-Life (EOL), are affected by the vulnerability as well.
Weakness: CWE-79
Which versions are affected by CVE-2026-21393?
How do I fix CVE-2026-21393?
No fix has been published for CVE-2026-21393 yet. Limit exposure of the affected software until one is available.
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-2026-21393 being exploited?
There is no known exploitation of CVE-2026-21393 in the wild and no public exploit code.
Sources
Also known as GHSA-343f-q34x-rjhr.