CVE-2026-18756 in Community Edition
Summary
by MITRE • 08/19/2026
HumHub Community Edition 1.18.4 contains a reflected cross-site scripting vulnerability in the Space membership-request workflow. An attacker can place attacker-controlled button configuration in the options query-string parameter of space/membership/request-membership-form, lure an authenticated non-member into submitting the legitimate membership request form, and cause the server to return JavaScript containing attacker-controlled code.
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Analysis
by VulDB Data Team • 08/19/2026
The vulnerability identified in HumHub Community Edition version 1.18.4 represents a significant security flaw within its social networking infrastructure, specifically targeting the space membership-request workflow. This reflected cross-site scripting (XSS) issue arises from insufficient input validation and sanitization mechanisms when processing user-supplied data for query-string parameters. The core technical failure lies in how the application handles the options parameter associated with the space/membership/request-membership-form endpoint. Instead of strictly validating or encoding this input, the system incorporates attacker-controlled content directly into the server's response without adequate protection against script execution. This architectural oversight allows malicious actors to inject executable JavaScript code that is subsequently rendered by the victim's web browser as part of a legitimate-looking page load.
The operational impact of this vulnerability is severe due to its reliance on social engineering tactics combined with technical exploitation. An attacker can craft a deceptive URL containing specially crafted button configuration data within the options query-string parameter. By distributing this link through phishing emails, instant messages, or other communication channels, the adversary lures an authenticated user who does not yet belong to the target space into clicking the link and submitting what appears to be a standard membership request form. Because the victim is already authenticated with valid session credentials, their browser will execute the injected JavaScript in the context of the HumHub application domain. This enables the attacker to perform actions on behalf of the user without their knowledge or consent, potentially leading to account takeover, data exfiltration, or further propagation within the community platform.
From a classification perspective, this vulnerability aligns with CWE-79, which describes Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting. Specifically, it falls under the reflected variant where malicious scripts are returned immediately by the web application in response to a user's request rather than being stored for later execution. In terms of offensive security frameworks such as MITRE ATT&CK, this technique corresponds to T1059 Command and Control via Application Layer Protocol or more specifically aspects related to Client-Side Scripting Injection used for initial access and credential harvesting. The exploitation chain leverages the trust relationship between the user's browser and the application server, bypassing same-origin policy protections because the malicious payload originates from a trusted domain after being reflected by the server itself.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. For administrators using HumHub Community Edition 1.18.4, applying the latest security patch released by the development team is the primary corrective action to resolve this specific flaw in input handling logic. In environments where updates cannot be immediately applied, web application firewalls can be configured to detect and block requests containing suspicious patterns associated with script injection within query parameters. Furthermore, developers should implement strict output encoding for all dynamic content rendered in HTML contexts and enforce Content Security Policy headers that restrict the execution of inline scripts unless explicitly whitelisted. Regular security audits focusing on input validation across all form submission endpoints will help prevent similar vulnerabilities from being introduced during future development cycles.