CVE-2026-18430 in HumHub
Summary
by MITRE • 08/19/2026
HumHub 1.18.4 contains a stored cross-site scripting vulnerability in the comment-deletion notification flow. A Space administrator can delete another user's comment, choose to notify the original author, and place HTML/JavaScript in the deletion reason.
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Analysis
by VulDB Data Team • 08/19/2026
The identified security flaw resides within HumHub version 1.18.4, specifically targeting the administrative workflow for managing content within community spaces. This vulnerability is classified as a Stored Cross-Site Scripting issue, which represents a severe category of web application insecurity where malicious scripts are permanently stored on the target server and subsequently delivered to users without further interaction from the attacker. In this specific context, the attack vector involves the comment deletion notification mechanism. When a Space administrator exercises their privilege to remove a comment authored by another user, they have the option to send a notification to that original author explaining why the content was removed. The critical technical failure lies in the insufficient sanitization of the input field designated for this deletion reason. Because the application fails to properly validate or encode special HTML and JavaScript characters before storing them in the database, an administrator can inject arbitrary script code directly into the system's persistent storage layer rather than just a transient session variable.
From a technical perspective, this flaw aligns with CWE-79, which describes Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting. The vulnerability exploits the trust relationship between the application and its administrators. Since Space administrators are considered trusted entities within the HumHub ecosystem, their inputs are processed without rigorous security checks that would otherwise be applied to untrusted user input. When a notification is generated for the comment's author, the stored malicious payload is rendered directly in the browser context of the victim. This results in the execution of the injected script as if it were part of the legitimate web page content, granting the attacker full control over the DOM environment experienced by the targeted user.
The operational impact of this vulnerability is significant and extends beyond simple defacement or alert boxes. An adversary with Space administrator privileges can leverage this stored XSS to perform session hijacking by stealing authentication cookies or tokens transmitted via HTTP headers. This could allow the attacker to impersonate the victim, gaining unauthorized access to their account and potentially escalating privileges if the victim holds higher-level roles within other spaces. Furthermore, the malicious script can be used to conduct phishing attacks against the victim's contacts, harvest sensitive data entered into forms on the platform, or redirect the user to external malicious sites designed to exploit browser vulnerabilities. The persistent nature of stored XSS means that every time a new notification is generated for any action involving this compromised deletion reason, the payload will execute, potentially affecting multiple users if the logic allows repeated notifications or if the script contains self-propagating mechanisms.
This incident maps directly to several techniques within the MITRE ATT&CK framework, particularly T1059 which covers Command and Scripting Interpreter execution via web browsers, and more specifically T1189 Drive-by Client-side Web Traffic Hijacking when used for redirection or credential harvesting. The vulnerability also reflects weaknesses in input validation processes described under CWE-20 Improper Input Validation. To mitigate this risk, immediate patching to a version of HumHub that addresses this specific flaw is required. In the interim, administrators should be educated on the risks associated with entering untrusted data into administrative fields, although the primary responsibility lies with the software vendor to implement strict output encoding and input validation routines for all user-supplied content before it is rendered in HTML contexts. Security headers such as Content-Security-Policy can also serve as a secondary defense layer by restricting the sources from which scripts are allowed to execute, thereby mitigating the impact of any successfully injected payloads even if input sanitization fails.