CVE-2026-18526 in HumHub
Summary
by MITRE • 08/19/2026
HumHub Community Edition 1.18.4 and 1.18.4-pl1 contain a stored Cross-Site Scripting (XSS) vulnerability in the oEmbed confirmation rendering workflow.
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Analysis
by VulDB Data Team • 08/19/2026
The identified security flaw resides within HumHub Community Edition versions 1.18.4 and 1.18.4-pl1, specifically targeting the oEmbed confirmation rendering workflow. This vulnerability is classified as a stored Cross-Site Scripting issue, which represents a significant risk to web application integrity because malicious scripts are persisted on the target server rather than being executed solely within the context of a single user session or request. The oEmbed protocol allows websites to display embedded content such as videos, images, and other media when URLs are posted in text fields. In this specific implementation, the system processes these URL inputs to generate preview cards that appear directly on community pages, forums, or profile feeds without requiring further interaction from other users once rendered.
The technical root cause of this vulnerability lies in insufficient input validation and output encoding during the processing of oEmbed data before it is inserted into the HTML DOM. When a user submits a URL intended for embedding, the application retrieves metadata associated with that link to construct a visual preview component. However, the rendering engine fails to properly sanitize specific attributes or content fields returned by external providers or manipulated through crafted payloads. This allows an attacker to inject executable JavaScript code directly into the stored data structure of the platform. Because this is a stored vulnerability, the malicious script becomes part of the page content itself, ensuring that every user who views the affected post will have their browser execute the injected code automatically upon loading the page.
From an operational perspective, the impact of this flaw extends beyond simple defacement or visual disruption. An attacker can leverage this vector to perform session hijacking by stealing authentication cookies and transmitting them to a remote server under their control. This could lead to unauthorized access to user accounts with varying privilege levels within the community platform. Furthermore, attackers may use the script execution context to conduct phishing attacks against other users by dynamically altering page content to mimic legitimate login forms or warning dialogs. The persistence of this payload means that remediation requires not only patching the software but also actively purging any maliciously injected entries from the database to prevent continued exposure to all community members who view those specific posts.
This vulnerability aligns with CWE-79, which describes Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting. The attack methodology corresponds to MITRE ATT&CK technique T1059, specifically the execution of scripts via web browsers, and falls under the broader category of Client-Side Code Injection. To mitigate this risk effectively, administrators must upgrade HumHub Community Edition to a version that includes patches for this specific rendering logic flaw. It is critical to ensure that all instances within an organization are updated simultaneously to prevent exploitation across different nodes in a clustered environment. Additionally, implementing Content Security Policy headers can provide a layer of defense by restricting the sources from which scripts are allowed to execute, thereby limiting the impact even if similar vulnerabilities exist in other components. Regular security audits and code reviews focusing on output encoding practices for dynamic content generation should be conducted to prevent recurrence of such injection flaws.