CVE-2018-25309 in Recent threads
Summary
by MITRE • 04/29/2026
MyBB Recent threads 17.0 contains a persistent cross-site scripting vulnerability that allows attackers to inject malicious scripts by creating threads with crafted subject lines. Attackers can create threads with script tags in the subject parameter to execute arbitrary JavaScript in the browsers of all users viewing the index page.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 05/31/2026
This vulnerability exists within MyBB Recent threads version 17.0 and represents a classic persistent cross-site scripting flaw that can have significant operational impact on forum environments. The vulnerability specifically occurs when attackers exploit the thread creation functionality by inserting malicious script tags within the subject line parameter. This allows the malicious code to be stored server-side and subsequently executed whenever users view the index page where the affected threads appear. The persistence of this vulnerability means that once the malicious content is injected, it continues to affect all users who browse the forum without requiring repeated exploitation attempts. The flaw directly violates security principles by failing to properly sanitize user input before rendering it in the web interface. This type of vulnerability falls under CWE-79 which defines cross-site scripting as a weakness where untrusted data is sent to a web browser without proper validation or escaping. The attack vector leverages the standard web application behavior of displaying user-generated content without adequate security controls, making it particularly dangerous in multi-user environments where forum administrators and regular members may be targeted simultaneously.
The operational impact of this vulnerability extends beyond simple script execution as it can enable sophisticated attack chains that compromise user sessions and potentially lead to full system compromise. When users view the index page, their browsers execute the injected JavaScript code within the context of the vulnerable forum application, potentially allowing attackers to steal session cookies, redirect users to malicious sites, or perform actions on behalf of authenticated users. This vulnerability can be exploited to create persistent backdoors within forum environments, enabling attackers to maintain long-term access to compromised systems. The attack follows the typical pattern described in the MITRE ATT&CK framework under the technique of web application attacks where adversaries leverage input validation weaknesses to inject malicious content that executes in the context of other users. The vulnerability's exploitation requires minimal technical skill and can be automated, making it particularly dangerous for forum administrators who may not immediately detect the compromise. The security implications are compounded by the fact that forum administrators often have elevated privileges and access to sensitive user data, potentially allowing attackers to escalate their access beyond what would be possible in a standard user account.
Mitigation strategies for this vulnerability should focus on implementing comprehensive input validation and output encoding mechanisms throughout the application. The most effective immediate fix involves sanitizing all user input, particularly in fields that are rendered in web pages without proper HTML escaping. Implementing a Content Security Policy (CSP) header can provide additional protection by restricting the sources from which scripts can be executed. Regular security audits should include checks for similar vulnerabilities in other user input fields and ensure that all dynamic content is properly escaped before rendering. The forum should implement proper access controls and monitoring to detect unusual activity patterns that might indicate exploitation attempts. Organizations should also consider implementing web application firewalls that can detect and block common XSS attack patterns. Additionally, regular security updates and patch management processes should be established to ensure that known vulnerabilities are addressed promptly. The vulnerability demonstrates the critical importance of proper input validation and output encoding as fundamental security controls, as outlined in the OWASP Top Ten security principles. Administrators should also educate users about the risks of clicking on suspicious links or content within forums and implement monitoring systems to detect potential exploitation attempts. The remediation process should include thorough testing to ensure that the fix does not break legitimate functionality while effectively preventing the XSS attack vector.