An issue was discovered in Zimbra Collaboration (ZCS) 9.0 and 10.0 and 10.1. A Reflected Cross-Site Scripting (XSS) vulnerability exists in the /h/rest endpoint, allowing authenticated attackers to inject and execute arbitrary JavaScript in a victim's session. Exploitation requires a valid auth token and involves a crafted URL with manipulated query parameters that triggers XSS when accessed by a victim.