CVE-2026-93630: Zimbra Collaboration Suite Email HTML Sanitizer Bypass via Hostname Canonicalization
A hostname canonicalization vulnerability in the email HTML sanitizers of Zimbra Collaboration Suite (ZCS) allows an unauthenticated sender to bypass the restriction on forms that target the Zimbra host. DefangFilter and the OWASP ActionAttributePolicy compare the form-action hostname with the request hostname before browser URL canonicalization. A percent-encoded hostname can therefore survive sanitization while the browser submits the form to the normal Zimbra origin with the recipient's authentication cookie. This permits authenticated form requests from a crafted email after the recipient opens it in Classic and submits the form. The issue is present in ZCS 10.1.16 and 10.1.21. It does not bypass an endpoint's CSRF-token checks; a state-changing attack additionally requires an endpoint that accepts the resulting request without that protection.
Affected Software
Event History
Frequently Asked Questions
Who is exposed to this issue?
Recipients who open a crafted email in the Zimbra Classic interface are exposed. An unauthenticated sender can supply the email, but exploitation requires the recipient to submit the embedded form while authenticated to Zimbra.
What conditions are required for a successful state-changing attack?
The attacker needs a recipient to open the crafted message in Classic and submit the form. The targeted Zimbra endpoint must also accept the resulting request without enforcing a CSRF token; this issue does not bypass endpoint CSRF-token validation.
Which versions are identified as affected?
The issue is present in Zimbra Collaboration Suite 10.1.16 and 10.1.21.