CVE-2026-2735 Overview
A Stored Cross-Site Scripting (XSS) vulnerability has been identified in Alkacon's OpenCms v18.0. This vulnerability occurs when user input is not properly validated when sending a POST request to /blog/new-article/org.opencms.ugc.CmsUgcEditService.gwt using the text parameter. Attackers can inject malicious scripts that are permanently stored on the target server and executed whenever users access the affected content.
Critical Impact
Authenticated attackers can inject persistent malicious scripts into OpenCms content, potentially leading to session hijacking, credential theft, or malicious redirects affecting any user who views the compromised content.
Affected Products
- Alkacon OpenCms v18.0
Discovery Timeline
- 2026-02-19 - CVE-2026-2735 published to NVD
- 2026-02-19 - Last updated in NVD database
Technical Details for CVE-2026-2735
Vulnerability Analysis
This Stored XSS vulnerability (CWE-79) exists in the user-generated content (UGC) functionality of OpenCms. The vulnerability is classified as network-accessible and requires low attack complexity, though it does require authenticated access and user interaction to exploit successfully. When a user submits content through the blog article creation endpoint, the text parameter is not properly sanitized before being stored in the database. This allows malicious JavaScript code to be persisted and later rendered in the browsers of other users who view the content.
The vulnerability affects the content management workflow where user input flows through the GWT (Google Web Toolkit) service endpoint without adequate input validation or output encoding.
Root Cause
The root cause of this vulnerability is improper input validation and output encoding in the OpenCms UGC module. The org.opencms.ugc.CmsUgcEditService.gwt service fails to sanitize the text parameter before storing content in the database, and subsequently fails to encode the output when rendering the stored content to users. This allows HTML and JavaScript to be interpreted by the browser rather than displayed as plain text.
Attack Vector
The attack requires an authenticated user to craft a malicious POST request to the vulnerable endpoint /blog/new-article/org.opencms.ugc.CmsUgcEditService.gwt. The attacker includes JavaScript code within the text parameter, which is then stored in the content management system. When any user navigates to view the blog article or content containing the injected payload, the malicious script executes in the context of their browser session.
The attack could be used to steal session cookies, redirect users to phishing sites, modify displayed content, or perform actions on behalf of the victim user. Because the malicious content is stored server-side, every user who views the affected content becomes a potential victim without requiring any additional attacker interaction.
Detection Methods for CVE-2026-2735
Indicators of Compromise
- Unusual script tags or JavaScript event handlers in blog article content stored in the OpenCms database
- HTTP POST requests to /blog/new-article/org.opencms.ugc.CmsUgcEditService.gwt containing suspicious payloads such as <script>, onerror=, onload=, or javascript: patterns
- User reports of unexpected browser behavior when viewing specific content pages
Detection Strategies
- Implement Web Application Firewall (WAF) rules to detect and block XSS payloads in POST requests to the vulnerable endpoint
- Deploy Content Security Policy (CSP) headers to restrict inline script execution and report policy violations
- Enable detailed logging for the UGC service endpoint and monitor for suspicious input patterns
Monitoring Recommendations
- Review stored content in the OpenCms database for signs of script injection
- Monitor web server access logs for repeated POST requests to the affected endpoint with encoded or obfuscated payloads
- Implement real-time alerting for CSP violation reports that may indicate XSS exploitation attempts
How to Mitigate CVE-2026-2735
Immediate Actions Required
- Restrict access to the blog article creation functionality to trusted users only until a patch is applied
- Implement input validation on the application or WAF level to filter XSS payloads before they reach the vulnerable endpoint
- Review existing content in the database for signs of injected scripts and sanitize any compromised entries
- Deploy Content Security Policy headers to limit the impact of any successful XSS attacks
Patch Information
No vendor patch information is currently available in the CVE data. Organizations should monitor Alkacon's official channels and the INCIBE Security Notice for updates regarding security fixes for this vulnerability.
Workarounds
- Implement server-side input sanitization to strip or encode HTML and JavaScript from user-submitted content in the text parameter
- Deploy a Web Application Firewall (WAF) with XSS detection rules to inspect and block malicious payloads
- Restrict access to the /blog/new-article/org.opencms.ugc.CmsUgcEditService.gwt endpoint through network segmentation or access control lists
- Enable strict Content Security Policy headers to prevent inline script execution
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
