CVE-2026-93630: Zimbra Collaboration Suite Email HTML Sanitizer Bypass via Hostname Canonicalization

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-23 · updated 2026-10-28
Weakness: CWE-180

Description

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 products

Zimbra Collaboration: before 10.1.21 — fixed in 10.1.21

How to fix

Upgrade to a fixed version: Zimbra Collaboration 10.1.21. Check the vendor advisory for mitigations.

References

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