CVE-2026-35539 Roundcube Webmail: Insufficient HTML attachment sanitization in preview mode
Attackers can execute arbitrary scripts in victim browsers by sending malicious HTML attachments that are improperly sanitized during preview.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-35539?
CVE-2026-35539 is a medium-severity cross-site scripting vulnerability in Roundcube Webmail, published on 3 Apr 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
An issue was discovered in Roundcube Webmail before 1.5.14 and 1.6.14. XSS exists because of insufficient HTML attachment sanitization in preview mode. A victim must preview a text/html attachment.
Weakness: CWE-79
Which versions are affected by CVE-2026-35539?
2 open source packages ship an affected version, including roundcube/roundcubemail. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
roundcube/roundcubemail Packagist | From 1.7-beta | 1.7-rc5 |
roundcube/roundcubemail Packagist | From 1.7-beta | 1.7-rc5 |
Products include Roundcube Webmail.
How do I fix CVE-2026-35539?
Upgrade roundcube/roundcubemail to 1.7-rc5 or later.
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-2026-35539 being exploited?
There is no known exploitation of CVE-2026-35539 in the wild and no public exploit code.
Other vulnerabilities in Roundcube Webmail
Sources
Also known as DEBIAN-CVE-2026-35539, GHSA-x4q5-8j5g-hpjc, UBUNTU-CVE-2026-35539.