CVE-2025-5591: Kentico Xperience Stored XSS Vulnerability

CVE-2025-5591 Overview

CVE-2025-5591 is a stored cross-site scripting (XSS) vulnerability in Kentico Xperience 13. The flaw resides in a form component that fails to sanitize user-supplied input before rendering it in application pages. An attacker can inject malicious JavaScript that executes in the browser of any user who views the affected page.

Successful exploitation allows session hijacking and impersonation of the victim within their existing security context. Because Kentico Xperience is a content management system, exploitation against administrative users can lead to broader compromise of the CMS environment. The vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation).

Critical Impact

Stored JavaScript payloads persist server-side and execute against every user who loads the affected form component, enabling session theft and privileged actions on behalf of victims.

Affected Products

• Kentico Xperience 13
• Deployments exposing the vulnerable form component to unauthenticated users
• Instances where administrators or editors access pages containing injected form data

Discovery Timeline

• 2026-01-05 - CVE-2025-5591 published to NVD
• 2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-5591

Vulnerability Analysis

The vulnerability is a stored (persistent) XSS flaw in a Kentico Xperience 13 form component. Data submitted through the form is stored server-side and later rendered without adequate output encoding. When another user views the page containing the stored payload, the browser interprets the injected script as legitimate content from the trusted origin.

Because the payload executes in the victim's authenticated context, the attacker inherits the victim's privileges within the CMS. This includes access to session cookies, anti-CSRF tokens accessible via the DOM, and the ability to issue authenticated requests on behalf of the victim. Attacks that target CMS administrators can pivot to content modification, user account creation, or configuration changes.

Exploitation requires user interaction: a victim must load the page hosting the malicious form data. Kentico Xperience is a widely deployed .NET CMS, which increases the population of exposed instances reachable over the internet.

Root Cause

The root cause is missing or insufficient input sanitization and output encoding in the vulnerable form component. Attacker-controlled data is written to the response body in an HTML or script context without being neutralized, allowing script tags and event handlers to render as executable code.

Attack Vector

The attack vector is network-based and requires no authentication. An attacker submits a crafted payload through a form exposed by the Kentico site. The payload is persisted and served to any user who subsequently accesses the affected page. Details of the affected component and payload construction are documented in The Missing Link Security Advisory.

Detection Methods for CVE-2025-5591

Indicators of Compromise

• Form submissions containing <script> tags, javascript: URIs, or DOM event handlers such as onerror and onload
• Stored form field values containing HTML entities decoded to executable JavaScript
• Unexpected outbound requests from browser sessions of authenticated CMS users to attacker-controlled domains
• New administrative accounts or content changes originating from legitimate user sessions

Detection Strategies

• Review Kentico form submission logs and database tables for entries containing HTML or script markup in fields expected to hold plain text
• Inspect web server logs for POST requests to form endpoints with payloads matching common XSS patterns
• Deploy Content Security Policy (CSP) reporting to surface script execution from unexpected sources
• Monitor session activity for anomalous administrative actions that follow a page view by the same user

Monitoring Recommendations

• Enable audit logging for CMS user actions, particularly account changes and configuration edits
• Alert on anomalous authenticated requests originating from user agents matching browser fingerprints of privileged accounts
• Track referrer chains that show privileged users landing on public form pages before executing sensitive actions

How to Mitigate CVE-2025-5591

Immediate Actions Required

• Apply the vendor-supplied hotfix for Kentico Xperience 13 as referenced in The Missing Link Security Advisory
• Audit existing form submission data for injected script content and purge malicious entries
• Rotate session tokens and credentials for any administrative accounts that accessed affected pages
• Restrict administrative access to the CMS from trusted networks where feasible

Patch Information

Refer to Kentico's official hotfix guidance for Xperience 13. The advisory published by The Missing Link provides remediation details and version information; administrators should upgrade to the fixed hotfix release identified by the vendor.

Workarounds

• Disable or remove the vulnerable form component from public-facing pages until the patch is applied
• Deploy a Web Application Firewall (WAF) rule to block form submissions containing script tags or JavaScript event handlers
• Enforce a strict Content Security Policy that disallows inline scripts and restricts script sources to trusted origins
• Segregate administrative browsing from public content browsing using separate browser profiles or sessions

# Example CSP header to mitigate reflected/stored XSS execution
Content-Security-Policy: default-src 'self'; script-src 'self'; object-src 'none'; base-uri 'self'; frame-ancestors 'none'