Zimbra Collaboration Suite RestFilter Request Attribute Override via Unauthenticated URL Parameters

Summary

A request attribute override vulnerability in Zimbra Collaboration Suite (ZCS) allows unauthenticated requests to /h/* to populate servlet attributes intended for trusted internal dispatch. RestFilter copies URL parameters specifying an authentication token, target account, target item and dispatch state into the request attributes consumed by the Classic Web Client. Combined with CVE-2026-10632 on ZCS 10.1.16, this allows an attacker to supply their own valid token and document identifiers and execute JavaScript in another user's signed-in browser session. ZCS 10.1.17 blocks that XSS chain by changing the document renderer, but retains the request attribute override.

Remediation

1. Inventory every Synacor Zimbra Collaboration Suite deployment, including staging.
2. No fixed version appears in the public sources yet. Check the references below and the vendor directly.

References

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