CVE-2021-27889 in MyBB
Summary
by MITRE • 03/16/2021
Cross-site Scripting (XSS) vulnerability in MyBB before 1.8.26 via Nested Auto URL when parsing messages.
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Analysis
by VulDB Data Team • 05/04/2025
The vulnerability CVE-2021-27889 represents a cross-site scripting flaw in MyBB forums software versions prior to 1.8.26, specifically affecting the Nested Auto URL parsing functionality within message handling. This issue falls under the CWE-79 category of Cross-Site Scripting, which is a critical web application security vulnerability that allows attackers to inject malicious client-side scripts into web pages viewed by other users. The vulnerability manifests when the forum software processes messages containing nested auto URL structures, creating an environment where malicious payloads can be executed in the context of other users' browsers.
The technical implementation of this flaw occurs within the message parsing engine of MyBB where nested auto URL processing fails to properly sanitize input data. When users post messages containing specially crafted nested URL structures, the software does not adequately escape or filter the content before rendering it on web pages. This improper handling creates a pathway for attackers to inject malicious JavaScript code that executes in the browsers of other forum users who view the affected posts. The vulnerability specifically exploits the software's automatic URL detection and conversion mechanisms, which are designed to automatically convert plain text URLs into clickable hyperlinks but fail to properly validate or sanitize the content during this conversion process.
The operational impact of this vulnerability extends beyond simple script execution as it enables a range of malicious activities including session hijacking, credential theft, defacement of forum content, and potential redirection to malicious sites. Attackers can craft posts containing malicious scripts that steal cookies, redirect users to phishing pages, or modify forum content in real-time. The vulnerability affects all users who view posts containing the maliciously crafted nested URLs, making it particularly dangerous in community forums where users frequently interact with posted content. This creates a persistent threat vector that can remain active until the vulnerability is patched, potentially allowing attackers to maintain long-term access to forum sessions and user data.
Organizations and forum administrators should prioritize immediate patching of MyBB installations to version 1.8.26 or later, which contains the necessary fixes for this vulnerability. The remediation process involves updating the forum software and verifying that all user-generated content is properly sanitized. Additional mitigations include implementing Content Security Policy headers, enabling proper input validation at multiple layers, and conducting regular security audits of forum content. From an ATT&CK framework perspective, this vulnerability maps to T1566.001 (Phishing: Spearphishing Attachment) and T1584.002 (Compromise Infrastructure: Domains) as attackers can leverage the XSS to deliver malicious payloads and establish persistent access. The vulnerability also aligns with T1071.004 (Application Layer Protocol: DNS) since the malicious URLs often leverage DNS resolution to deliver payloads. Organizations should also consider implementing web application firewalls and monitoring for suspicious URL patterns in forum posts to detect potential exploitation attempts.