Zimbra Collaboration Suite Email HTML Sanitizer Bypass via Hostname Canonicalization

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.

Scores

EPSS (exploitation in 30 days)

0.29% · top 78% of all CVEs (7 Nov 2026)

CISA SSVC

Exploitation: none · Automatable: no · Technical impact: partial

The list uses the assigner's score when there is one, then CISA's, then NVD's: Medium 4.3.

Weakness

• CWE-180 Incorrect behavior order: validate before canonicalize (opens in a new tab)All about CWE-180 and its CVEs

Affected products

• Zimbra · Zimbra Collaboration Suite

  • affected < 10.1.21

References (2)

• https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories (opens in a new tab)vendor-advisory
• https://www.rapid7.com/blog/post/ve-breaking-zimbra-identity-boundaries-part-1/ (opens in a new tab)related

Sources

The same CVE in the primary databases and aggregators, to check against.

• CVE.org record (opens in a new tab)
• CVE List JSON (GitHub) (opens in a new tab)
• NVD (opens in a new tab)
• FIRST EPSS (opens in a new tab)
• OSV.dev (opens in a new tab)
• GitHub Advisories (opens in a new tab)
• CIRCL Vulnerability-Lookup (opens in a new tab)

Badge for a README or a ticket

[![CVE-2026-93630](https://trackcve.app/badge/CVE-2026-93630.svg)](https://trackcve.app/cve/CVE-2026-93630)

Record last updated 28 Oct 2026, 19:51 UTC.