CVE-2026-24037: Horilla HRMS XSS Vulnerability

CVE-2026-24037 Overview

A Cross-Site Scripting (XSS) filter bypass vulnerability has been identified in Horilla, a free and open source Human Resource Management System (HRMS). In version 1.4.0, the has_xss() function attempts to block XSS attacks by matching input against a set of regex patterns. However, these regex patterns are incomplete and context-agnostic, making them trivial to bypass. Successful exploitation allows attackers to redirect users to malicious domains, execute external JavaScript, and steal CSRF tokens that can be leveraged to craft secondary CSRF attacks against administrators.

Critical Impact

Attackers can bypass XSS protections to redirect users to malicious sites, execute arbitrary JavaScript, and steal CSRF tokens to compromise administrator accounts.

Affected Products

• Horilla HRMS version 1.4.0
• Horilla HRMS versions prior to 1.5.0

Discovery Timeline

• 2026-01-22 - CVE CVE-2026-24037 published to NVD
• 2026-01-22 - Last updated in NVD database

Technical Details for CVE-2026-24037

Vulnerability Analysis

This vulnerability represents a classic case of insufficient input validation in web application security controls. The has_xss() function in Horilla HRMS was designed as a defensive measure against Cross-Site Scripting attacks, but its implementation relies on pattern matching with incomplete regular expressions.

The fundamental flaw lies in the context-agnostic nature of the regex patterns used. XSS attacks can manifest in numerous contexts within HTML documents—within attribute values, script tags, event handlers, URLs, and CSS—each requiring different encoding and escaping strategies. A regex-based approach that doesn't account for these different contexts will inevitably leave gaps that attackers can exploit.

When successfully exploited, this vulnerability enables three primary attack scenarios: user redirection to attacker-controlled domains for phishing or malware distribution, execution of external JavaScript for data exfiltration or session hijacking, and theft of CSRF tokens that can be weaponized for follow-on attacks targeting administrative functions.

Root Cause

The root cause is the reliance on incomplete, context-agnostic regex patterns in the has_xss() function for XSS prevention. This blocklist-based approach fails to account for the myriad ways XSS payloads can be encoded, obfuscated, or structured to evade pattern matching while still executing in the browser. Proper XSS prevention requires context-aware output encoding and Content Security Policy (CSP) implementation rather than input filtering alone.

Attack Vector

The attack is network-based and requires user interaction. An attacker with high privileges can craft malicious input that bypasses the regex-based XSS filter. When a victim user views content containing the payload, the malicious JavaScript executes in their browser context. The changed scope (S:C in CVSS) indicates that the vulnerability in Horilla can impact resources beyond its security scope, potentially affecting user browsers and enabling attacks on other web applications through stolen credentials or session tokens.

Since no verified code examples are available, the specific bypass techniques are documented in the GitHub Security Advisory GHSA-rqw5-fjm4-rgvm. Common bypass techniques for regex-based XSS filters include case manipulation, HTML entity encoding, Unicode encoding, and breaking up keywords with null bytes or comments.

Detection Methods for CVE-2026-24037

Indicators of Compromise

• Unusual JavaScript execution or redirect behaviors reported by users accessing the HRMS
• Web application logs showing requests with encoded or obfuscated script payloads
• Unexpected CSRF token exfiltration attempts to external domains
• Browser console errors indicating blocked or attempted XSS execution (if CSP is partially implemented)

Detection Strategies

• Deploy web application firewall (WAF) rules to detect common XSS bypass patterns and encoded payloads
• Monitor HTTP request logs for suspicious patterns in user-controllable input fields
• Implement Content Security Policy (CSP) with report-uri to receive notifications of policy violations
• Review application logs for anomalous redirect patterns or external JavaScript references

Monitoring Recommendations

• Enable detailed logging for all user input processing in the Horilla application
• Set up alerts for CSP violation reports indicating attempted script execution
• Monitor network traffic for connections to unknown external domains from user sessions
• Audit administrative actions for signs of CSRF-based attacks following token theft

How to Mitigate CVE-2026-24037

Immediate Actions Required

• Upgrade Horilla HRMS to version 1.5.0 or later immediately
• Review recent user input and stored content for potential XSS payloads
• Implement Content Security Policy headers as a defense-in-depth measure
• Audit logs for any signs of successful exploitation prior to patching

Patch Information

The vulnerability has been addressed in Horilla version 1.5.0. Organizations running version 1.4.0 or earlier should upgrade immediately. The fix is available via the official GitHub Release 1.5.0. For complete details on the vulnerability and remediation, refer to the GitHub Security Advisory GHSA-rqw5-fjm4-rgvm.

Workarounds

• Implement a strict Content Security Policy (CSP) header to prevent inline script execution and restrict script sources
• Deploy a web application firewall (WAF) with XSS protection rules as an additional layer of defense
• Restrict privileged account access to minimize the attack surface until patching is complete
• Consider temporarily disabling features that process user-generated content if immediate patching is not feasible

# Example Content Security Policy header configuration (Apache)
# Add to .htaccess or virtual host configuration
Header set Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; object-src 'none'; frame-ancestors 'self';"