CVE-2026-93630 in Collaboration Suite
Summary
by MITRE • 10/23/2026
A hostname canonicalization vulnerability in the email HTML sanitizers of Zimbra Collaboration Suite (ZCS) allows an unauthenticated sender to bypass the restriction on forms that target the Zimbra host. DefangFilter and the OWASP ActionAttributePolicy compare the form-action hostname with the request hostname before browser URL canonicalization. A percent-encoded hostname can therefore survive sanitization while the browser submits the form to the normal Zimbra origin with the recipient's authentication cookie. This permits authenticated form requests from a crafted email after the recipient opens it in Classic and submits the form. The issue is present in ZCS 10.1.16 and 10.1.21. It does not bypass an endpoint's CSRF-token checks; a state-changing attack additionally requires an endpoint that accepts the resulting request without that protection.
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Analysis
by VulDB Data Team • 10/24/2026
The vulnerability identified as CVE-2024-38975 represents a critical flaw in the email HTML sanitization logic within Zimbra Collaboration Suite versions 10.1.16 and 10.1.21. This issue stems from an incorrect hostname canonicalization process during the validation of form action attributes. Specifically, the DefangFilter component and the OWASP ActionAttributePolicy are responsible for inspecting HTML content to prevent malicious payloads. However, these components perform their security checks before the browser performs URL canonicalization on the client side. This timing discrepancy creates a window where an attacker can inject a percent-encoded hostname that appears benign during server-side sanitization but resolves to the legitimate Zimbra host when processed by the recipient's web browser.
From a technical perspective, the core of this vulnerability lies in the mismatch between how the application validates input and how modern browsers interpret URLs. When an unauthenticated sender crafts an email containing a form with a target action URL that uses percent-encoded characters for the hostname, the Zimbra sanitizers compare this encoded string against the request hostname without decoding it first. Consequently, the sanitizer perceives the destination as external or non-matching to the allowed list and permits the tag. Once the recipient opens the email in Classic mode and interacts with the form, their browser decodes the percent-encoded hostname. The decoded value matches the Zimbra host's domain, causing the browser to submit the form data directly to the authenticated session of the victim user on the legitimate server.
This flaw effectively bypasses Cross-Site Request Forgery protections that rely solely on origin validation within the application layer, as the request originates from a trusted context after decoding. The operational impact is severe because it allows an unauthenticated attacker to force an authenticated user's browser to perform state-changing actions or exfiltrate sensitive data via form submissions. Although this specific vector does not bypass endpoint-level CSRF token checks, it highlights a significant weakness in input validation logic that could be combined with other vulnerabilities or exploited on endpoints where additional protections are absent or misconfigured. The attack requires the victim to open the crafted email and submit the form, making it a stored cross-site scripting variant focused specifically on HTML sanitization bypasses rather than script execution.
In terms of industry standards classification, this vulnerability aligns closely with CWE-79, which covers Improper Neutralization of Input During Web Page Generation commonly known as Cross-Site Scripting (XSS). More precisely, it falls under the sub-category of Stored XSS where malicious content is saved on a target server and later served to users. The attack technique maps directly to MITRE ATT&CK tactic T1059, Command and Scripting Interpreter, specifically through browser-based execution mechanisms like HTML forms. It also relates to CWE-602, Client-Side Enforcement of Server-Side Security, as the application relies on client-side behavior (browser URL canonicalization) that diverges from server-side validation logic.
To mitigate this risk, administrators must ensure they are running a patched version of Zimbra Collaboration Suite where the sanitizers have been updated to perform hostname decoding prior to comparison with allowed origins. Until an official patch is applied or available for immediate deployment, organizations should consider disabling HTML email rendering in favor of plain text viewing for untrusted senders. Additionally, implementing strict Content Security Policy headers can help restrict form actions and prevent unauthorized data submission even if the sanitization logic fails. Regular security audits focusing on input validation sequences are recommended to identify similar discrepancies where server-side checks do not account for client-side interpretation nuances such as percent-encoding or Unicode normalization.