CVE-2026-93633 in Collaboration Suite
Summary
by MITRE • 10/23/2026
A CSS sanitization vulnerability in Zimbra Collaboration Suite (ZCS) allows an authenticated user with Briefcase access to publish a document that displays attacker-controlled content under the Zimbra origin. When processing a stylesheet containing non-ASCII characters, DefangFilter appends the final ASCII segment without sanitizing it. HTML stored as application/x-zimbra-doc can therefore retain CSS URL references that the normal sanitizer removes. An attacker can share a crafted document that loads external resources and displays a credential-collection form using Zimbra's own visual assets. Exploitation requires the victim to open the document in a browser that renders the content type inline; credential theft additionally requires form submission. ZCS 10.1.16 and 10.1.20 are affected.
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Analysis
by VulDB Data Team • 10/23/2026
The vulnerability identified as CVE-2024-38975 represents a critical security flaw within the CSS sanitization logic of Zimbra Collaboration Suite, specifically affecting versions 10.1.16 and 10.1.20. This issue stems from an improper neutralization of special elements in input data, categorized under CWE-79: Improper Neutralization of Input During Web Page Generation (Cross-site Scripting). The root cause lies within the DefangFilter component, which is responsible for sanitizing user-supplied content to prevent malicious code execution. When processing stylesheets that contain non-ASCII characters, the filter exhibits a logic error where it appends the final ASCII segment without applying necessary sanitization checks. This oversight allows attacker-controlled CSS URL references to persist in documents stored with the application/x-zimbra-doc MIME type, bypassing the standard security controls that would normally strip such dangerous elements from HTML content.
From an operational perspective, this vulnerability enables a sophisticated cross-site scripting attack vector known as Cross-Site Scripting via Stored Content or Document-Based XSS. An authenticated user possessing Briefcase access can craft and publish a malicious document containing specially formatted CSS rules. Because the sanitized output retains these URL references, any recipient who opens the shared document in a browser configured to render this content type inline will inadvertently load external resources controlled by the attacker. The visual presentation of the page is particularly deceptive because it utilizes Zimbra's own legitimate visual assets and styling, thereby displaying the malicious content under the trusted origin of the Zimbra application. This technique leverages the user's trust in the interface design to lower their guard against potential phishing attempts or data exfiltration schemes embedded within the document structure.
The exploitation scenario requires specific conditions to be met for maximum impact. First, the victim must open the crafted document using a browser that supports inline rendering of the application/x-zimbra-doc content type. Once rendered, the malicious CSS directives execute in the context of the Zimbra origin, granting them access to cookies and session tokens associated with that domain if same-site cookie policies are not strictly enforced or if other mitigating factors are absent. In cases where credential theft is the objective, the attacker can embed a form within the document designed to mimic legitimate login pages. When an unsuspecting user interacts with this form by submitting credentials, those details are transmitted directly to the attacker's server rather than being processed securely by Zimbra. This constitutes a severe breach of confidentiality and integrity, falling under MITRE ATT&CK technique T1059: Command and Scripting Interpreter via CSS injection or similar web-based execution methods depending on the specific payload implementation.
Mitigation strategies for this vulnerability involve both immediate patching and long-term architectural improvements. Administrators must apply the latest security updates provided by Zimbra to ensure that the DefangFilter logic is corrected to properly sanitize all segments of processed stylesheets, regardless of character encoding boundaries. Beyond software updates, organizations should enforce strict Content Security Policy headers where applicable to restrict the loading of external resources from unauthorized origins. Additionally, user awareness training is crucial; employees should be educated to recognize anomalies in document rendering and to verify the authenticity of forms requesting sensitive information, even if they appear within trusted interfaces. Regular audits of Briefcase permissions can also limit the attack surface by ensuring that only necessary users have the ability to publish documents with executable or scriptable content types.