Stored XSS in HumHub Community Edition

Executive Summary

HumHub Community Edition 1.18.4 and 1.18.4-pl1 have a stored Cross-Site Scripting (XSS) vulnerability in the oEmbed confirmation rendering workflow. A low-privilege authenticated Space member can craft a malicious rich-text oEmbed link with a script tag in the Post[message] field. When another user views the post, HumHub incorrectly processes the malformed URL as a supported oEmbed provider, injecting attacker-controlled script tags. The issue arises from improper URL validation and lack of output encoding.

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Detection Guidance

To detect this vulnerability, inspect HumHub posts containing oEmbed links for malicious script tags in the Post[message] field. Check for malformed URLs that include substrings like youtube.com but contain script tags. Review server logs for unusual oEmbed confirmation prompts or CSP nonce bypass attempts.

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Impact Analysis

An attacker can execute arbitrary JavaScript in the victim's browser session, allowing them to perform actions on behalf of the victim such as reading sensitive data. The injected script bypasses Content Security Policy (CSP) restrictions due to improper reconstruction with a valid CSP nonce.

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Compliance Impact

This vulnerability could lead to unauthorized access to sensitive user data, violating GDPR's data protection principles and HIPAA's security requirements for protected health information. Organizations using affected HumHub versions may face compliance breaches due to potential data exposure.

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Mitigation Strategies

Upgrade HumHub to version 1.18.5 or later immediately. If upgrading is not possible, apply the patch from pull request 8373 which adds Html::encode for oEmbed URLs. Disable oEmbed functionality temporarily if not critical. Monitor posts for suspicious content and review user permissions to limit Space member capabilities.

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