CVE-2026-93647 in Collaboration Suite
Summary
by MITRE • 09/25/2026
An unauthenticated calendar sender can place active markup in a COUNTER message's RFC From address. Selecting the message in Zimbra Classic triggers stored XSS, allowing the attacker to access mailbox data and act as the victim.
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Analysis
by VulDB Data Team • 09/25/2026
The vulnerability described constitutes a Stored Cross-Site Scripting (XSS) flaw within the Zimbra Collaboration Suite, specifically affecting the handling of calendar invitations sent via COUNTER messages. This security issue arises from an insufficient validation mechanism applied to the RFC From address field when processing incoming calendar updates. An unauthenticated attacker can exploit this weakness by crafting a malicious COUNTER message that embeds active markup or executable script code directly into the sender's email address header. Because Zimbra stores these calendar events in its database for later retrieval and display, the injected payload becomes persistent within the application state rather than being executed transiently during transmission.
When a victim user interacts with their mailbox using the Zimbra Classic interface, they are likely to view or select this maliciously crafted calendar event as part of their normal workflow. The vulnerability is triggered at the moment the client-side rendering engine processes the stored data for display. Instead of treating the content from the RFC From address as plain text, the application fails to properly encode or sanitize special characters and script tags. Consequently, when the message list or details pane renders this entry, the browser interprets the embedded markup as executable JavaScript code within the context of the Zimbra web application domain.
The operational impact of this vulnerability is severe due to its stored nature and lack of authentication requirements for exploitation initiation. Once triggered, the malicious script executes with the same privileges as a legitimate user session. This allows an attacker to perform actions on behalf of the victim without their knowledge or consent. Specific consequences include unauthorized access to sensitive mailbox data such as emails, contacts, and calendar details. Furthermore, the attacker can manipulate the application state to send spam, modify other users' calendars, delete messages, or potentially escalate privileges if combined with other vulnerabilities in the platform. The unauthenticated aspect means that any external party on the internet can initiate the attack vector by simply sending a crafted email, making it highly accessible for widespread exploitation campaigns targeting organizations using Zimbra Classic.
From a standards perspective, this flaw aligns closely with CWE-79: Improper Neutralization of Input During Web Page Generation, commonly known as Cross-site Scripting (XSS). The specific variant is Stored XSS because the malicious payload persists in the application's data store and is served to multiple users over time. In terms of attack tactics, this vulnerability facilitates actions categorized under MITRE ATT&CK techniques such as T1059: Command and Scripting Interpreter for executing arbitrary code within the browser environment, and potentially T1534: Internal Spearphishing if used to compromise other internal accounts through social engineering via compromised mailboxes.
Mitigation strategies must focus on both immediate remediation and long-term architectural improvements. The primary defense is implementing strict output encoding or context-aware escaping for all data rendered in the Zimbra Classic interface, particularly fields derived from external sources like email headers. Developers should ensure that any user-controllable input displayed in HTML contexts is sanitized to prevent browser interpretation of script tags, event handlers, and other active content. Additionally, adopting a Content Security Policy (CSP) can significantly reduce the impact by restricting the domains from which scripts are allowed to load or execute. For administrators using Zimbra Classic, applying vendor-provided security patches that address this input validation gap is critical until legacy interfaces are deprecated in favor of more secure modern web technologies that enforce stricter sandboxing and data handling protocols.