CVE-2025-49290 is a high-severity reflected Cross-Site Scripting (XSS) vulnerability affecting the Off-Canvas Sidebars & Menus (Slidebars) plugin developed by Jory Hogeveen, up to version 0.5.8.4. The vulnerability arises due to improper neutralization of user-supplied input during web page generation, categorized under CWE-79. Specifically, the plugin fails to adequately sanitize or encode input that is reflected back in the HTTP response, allowing an attacker to inject malicious scripts. When a victim interacts with a crafted URL or input, the malicious script executes in the context of the victim’s browser, potentially leading to session hijacking, credential theft, or unauthorized actions performed on behalf of the user. The CVSS v3.1 base score is 7.1, indicating a high severity, with the vector AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:L. This means the attack can be launched remotely over the network with low attack complexity, requires no privileges but does require user interaction (clicking a malicious link), and impacts confidentiality, integrity, and availability to a limited extent. The scope is changed, indicating the vulnerability affects components beyond the vulnerable module itself. No known exploits are currently reported in the wild, and no patches have been linked yet. The vulnerability was published on June 27, 2025, and reserved earlier that month. The plugin is typically used to implement off-canvas sidebars and menus in web applications, often in content management systems or custom websites.

For European organizations, this vulnerability poses a significant risk especially to websites and web applications that utilize the Slidebars plugin for navigation menus. Successful exploitation can lead to theft of session cookies, enabling attackers to impersonate legitimate users, including administrators, which can result in unauthorized access to sensitive data or administrative functions. Additionally, injected scripts can be used to deliver malware or redirect users to phishing sites, damaging organizational reputation and potentially causing regulatory compliance issues under GDPR due to data breaches. The reflected XSS nature means attacks require user interaction, often via phishing emails or malicious links, which can be effective in targeted spear-phishing campaigns. Organizations in sectors with high web presence such as e-commerce, government portals, and media outlets are particularly vulnerable. The impact extends to loss of user trust, potential financial losses, and legal consequences if personal data is compromised.

European organizations should immediately audit their web applications to identify usage of the Slidebars plugin, particularly versions up to 0.5.8.4. If found, they should prioritize upgrading to a patched version once available or apply any official vendor-provided fixes. In the absence of patches, organizations can implement web application firewall (WAF) rules to detect and block typical reflected XSS payloads targeting the vulnerable parameters. Input validation and output encoding should be enforced at the application level to sanitize user inputs before rendering. Security teams should conduct penetration testing focusing on reflected XSS vectors in affected applications. Additionally, educating users about the risks of clicking suspicious links can reduce successful exploitation. Monitoring web logs for unusual query strings or repeated suspicious requests can help detect exploitation attempts early. Finally, adopting Content Security Policy (CSP) headers can mitigate the impact by restricting the execution of unauthorized scripts.