CVE-2025-4126 is a stored Cross-Site Scripting (XSS) vulnerability affecting the EG-Series WordPress plugin developed by emmanuelg. The vulnerability exists in all versions up to and including 2.1.1, specifically within the plugin's [series] shortcode implementation. The root cause is insufficient input sanitization and output escaping in the shortcode_title function, which processes user-supplied attributes, notably the titletag attribute. Authenticated attackers with contributor-level privileges or higher on WordPress sites that have the Classic Editor plugin activated can exploit this flaw by injecting arbitrary JavaScript code into the titletag attribute. This malicious script is then stored persistently and executed in the browsers of any users who visit the affected page, enabling potential session hijacking, privilege escalation, or other malicious activities. The vulnerability is classified under CWE-80 (Improper Neutralization of Script-Related HTML Tags in a Web Page), indicating a failure to properly sanitize HTML or script content. The CVSS v3.1 base score is 6.4 (medium severity), reflecting that the attack vector is network-based, requires low attack complexity, and privileges (contributor or higher), but does not require user interaction. The scope is changed, meaning the vulnerability can affect components beyond the initially vulnerable plugin, impacting confidentiality and integrity but not availability. No known exploits are currently reported in the wild, and no official patches have been linked yet. However, the presence of this vulnerability in a popular CMS plugin used in WordPress sites poses a significant risk if left unmitigated.

For European organizations, this vulnerability presents a moderate risk primarily to websites and web applications running WordPress with the EG-Series plugin and the Classic Editor enabled. Exploitation could lead to unauthorized script execution, enabling attackers to steal session cookies, perform actions on behalf of legitimate users, or deliver further malware payloads. This can compromise user data confidentiality and integrity, damage organizational reputation, and potentially violate GDPR requirements concerning data protection and breach notification. Organizations in sectors with high web presence such as e-commerce, media, education, and government services are particularly vulnerable. Given that contributor-level access is required, insider threats or compromised accounts could be leveraged to exploit this vulnerability. The stored nature of the XSS means that once injected, the malicious script affects all visitors to the compromised page, amplifying the impact. Although availability is not directly affected, the indirect consequences such as loss of customer trust and regulatory penalties can be severe.

To mitigate this vulnerability, European organizations should first verify if their WordPress installations use the EG-Series plugin up to version 2.1.1 and have the Classic Editor plugin activated. Immediate steps include: 1) Restrict contributor-level privileges strictly to trusted users and audit existing user roles for unnecessary permissions. 2) Temporarily disable or remove the EG-Series plugin until a vendor patch is available. 3) Implement Web Application Firewall (WAF) rules to detect and block suspicious input patterns targeting the titletag attribute in the shortcode. 4) Employ Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts on affected sites. 5) Monitor logs for unusual activity related to shortcode usage or script injections. 6) Educate site administrators about the risks of stored XSS and the importance of input validation. Once the vendor releases a patch, promptly apply it and verify that input sanitization and output escaping are correctly implemented. Additionally, consider migrating to the WordPress block editor (Gutenberg) if feasible, as the vulnerability requires the Classic Editor plugin to be active.