CVE-2026-23794: Apache Syncope XSS Vulnerability

CVE-2026-23794 Overview

A reflected Cross-Site Scripting (XSS) vulnerability has been identified in Apache Syncope's Enduser Login page. This security flaw allows attackers to inject malicious scripts through the login interface, potentially enabling credential theft when a legitimate user is tricked into clicking a crafted malicious link and subsequently logging in to the Syncope Enduser portal.

Apache Syncope is an open-source system for managing digital identities in enterprise environments. The vulnerability in its Enduser Login page represents a significant security concern for organizations relying on Syncope for identity management, as successful exploitation could lead to unauthorized access to user credentials and session information.

Critical Impact

An attacker can steal user credentials by tricking victims into clicking a malicious link and logging in to the compromised Syncope Enduser page. The vulnerability allows for credential harvesting through injected scripts executed in the context of authenticated user sessions.

Affected Products

• Apache Syncope versions 3.0 through 3.0.15
• Apache Syncope versions 4.0 through 4.0.3

Discovery Timeline

• 2026-02-03 - CVE CVE-2026-23794 published to NVD
• 2026-02-03 - Last updated in NVD database

Technical Details for CVE-2026-23794

Vulnerability Analysis

This vulnerability is classified as CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting (XSS). The reflected XSS flaw exists in the Enduser Login page of Apache Syncope, where user-supplied input is not properly sanitized before being reflected back in the HTTP response.

When a user clicks a malicious link containing crafted JavaScript payload and navigates to the Syncope Enduser Login page, the malicious script executes within the browser context of the victim. If the user proceeds to log in, the attacker's script can intercept the credentials being submitted, exfiltrate session tokens, or perform other malicious actions on behalf of the authenticated user.

The vulnerability requires user interaction—specifically, the victim must click a malicious link and complete the login process. However, the changed scope characteristic indicates that the vulnerability can affect resources beyond the vulnerable component, allowing credential theft to impact the broader identity management infrastructure.

Root Cause

The root cause of this vulnerability is insufficient input validation and output encoding in the Apache Syncope Enduser Login page. User-controllable input parameters are reflected in the HTML response without proper sanitization, allowing attackers to inject arbitrary JavaScript code that executes in the victim's browser.

The login page fails to implement adequate Content Security Policy (CSP) headers or properly encode special characters in user input, enabling the injection of script tags or event handlers that execute when the page renders.

Attack Vector

The attack requires network access and involves social engineering to trick users into clicking malicious links. The attacker constructs a URL to the Syncope Enduser Login page containing a malicious JavaScript payload embedded in a vulnerable parameter. When the victim clicks this link, the payload is reflected in the page response and executed by the browser.

The attack flow proceeds as follows: the attacker identifies a vulnerable input parameter in the login page, crafts a malicious URL containing JavaScript code, distributes this link via phishing emails or other social engineering methods, and waits for victims to click the link and authenticate. Upon login, the injected script can capture credentials, session cookies, or redirect the user to attacker-controlled infrastructure.

Detection Methods for CVE-2026-23794

Indicators of Compromise

• Suspicious URL parameters containing encoded JavaScript or HTML tags in requests to the Syncope Enduser Login page
• Unusual outbound network connections from client browsers after accessing the Syncope login page
• Web server logs showing requests with abnormally long query strings containing script-related keywords

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block XSS payloads in URL parameters targeting the Enduser Login endpoint
• Monitor HTTP access logs for requests containing encoded script tags such as <script>, javascript:, or event handlers like onerror
• Deploy browser-based security monitoring to detect unexpected JavaScript execution or DOM manipulation on login pages

Monitoring Recommendations

• Enable detailed logging for all authentication-related endpoints in Apache Syncope
• Configure alerts for unusual patterns in login page access, particularly requests with suspicious query string parameters
• Implement Content Security Policy (CSP) violation reporting to detect attempted script injection attacks

How to Mitigate CVE-2026-23794

Immediate Actions Required

• Upgrade Apache Syncope version 3.x installations to version 3.0.16 or later
• Upgrade Apache Syncope version 4.x installations to version 4.0.4 or later
• Review web server and application logs for any signs of exploitation attempts
• Notify users about potential phishing attempts targeting Syncope login pages

Patch Information

Apache has released patched versions that address this reflected XSS vulnerability. Users running affected versions are strongly recommended to upgrade immediately:

• Version 3.x users: Upgrade to Apache Syncope 3.0.16
• Version 4.x users: Upgrade to Apache Syncope 4.0.4

For additional details, refer to the Apache Security Mailing List Thread and the Openwall OSS-Security Discussion.

Workarounds

• Implement a Web Application Firewall (WAF) with XSS detection rules to filter malicious payloads before they reach the application
• Configure strict Content Security Policy (CSP) headers to prevent inline script execution on the login page
• Educate users about the risks of clicking suspicious links, especially those directing to authentication pages
• Consider restricting access to the Syncope Enduser portal to trusted network segments until patching is complete

# Example Apache configuration to add CSP headers as temporary mitigation
# Add to your Apache virtual host configuration for Syncope

<IfModule mod_headers.c>
    Header always set Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; frame-ancestors 'self';"
    Header always set X-Content-Type-Options "nosniff"
    Header always set X-XSS-Protection "1; mode=block"
</IfModule>