CVE-2016-3999 in Zimbra Collaboration
Summary
by MITRE
Multiple cross-site scripting (XSS) vulnerabilities in Zimbra Collaboration before 8.7.0 allow remote attackers to inject arbitrary web script or HTML via unspecified vectors, aka bugs 104552 and 104703.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 05/13/2026
The vulnerability identified as CVE-2016-3999 represents a critical cross-site scripting flaw affecting Zimbra Collaboration software versions prior to 8.7.0. This vulnerability manifests through multiple attack vectors that enable remote adversaries to inject malicious web scripts or HTML content into the targeted system. The issue was categorized under bugs 104552 and 104703, indicating that it stems from insufficient input validation and output encoding mechanisms within the Zimbra web interface components. These vulnerabilities expose organizations using Zimbra Collaboration to significant security risks as they allow attackers to execute malicious code in the context of authenticated users' browsers.
The technical nature of this vulnerability aligns with CWE-79, which specifically addresses cross-site scripting flaws in web applications. The flaw occurs when the application fails to properly sanitize user-supplied input before rendering it in web pages, creating opportunities for attackers to inject malicious scripts. In Zimbra's case, the unspecified vectors suggest that multiple entry points within the web interface could be exploited, including email content processing, user interface elements, and administrative functions. This broad attack surface increases the likelihood of successful exploitation and makes the vulnerability particularly dangerous for enterprise email systems.
From an operational perspective, the impact of CVE-2016-3999 extends beyond simple data theft or defacement. Attackers could leverage these XSS vulnerabilities to establish persistent access to user accounts, steal session cookies, redirect users to malicious sites, or perform actions on behalf of authenticated users. The remote nature of the attack means that threat actors do not require physical access to the network or insider knowledge of the organization's internal structure. This vulnerability directly maps to several ATT&CK techniques including T1059.001 for command and scripting interpreter and T1566.001 for spearphishing with links, as attackers could craft malicious emails containing the XSS payloads to compromise user systems. Organizations utilizing Zimbra Collaboration would face potential data breaches, unauthorized access to sensitive communications, and possible lateral movement within their network infrastructure.
The recommended mitigations for this vulnerability center around immediate patching of the Zimbra Collaboration software to version 8.7.0 or later, which contains the necessary fixes for the identified XSS flaws. Organizations should also implement comprehensive input validation and output encoding mechanisms throughout their web applications, following secure coding practices that align with OWASP Top Ten recommendations. Network segmentation and web application firewalls can provide additional layers of protection while patches are being deployed. Security monitoring should be enhanced to detect suspicious user activities and potential exploitation attempts. Regular security assessments and penetration testing should be conducted to identify similar vulnerabilities in other applications and systems within the organization's infrastructure.