CVE-2020-8115: Revive Adserver Reflected XSS Vulnerability

CVE-2020-8115 Overview

A reflected Cross-Site Scripting (XSS) vulnerability has been discovered in the publicly accessible afr.php delivery script of Revive Adserver versions 5.0.3 and earlier. The vulnerability was discovered by security researcher Jacopo Tediosi and reported through HackerOne. The query string sent to the www/delivery/afr.php script was printed back without proper escaping in a JavaScript context, allowing an attacker to execute arbitrary JavaScript code on the browser of the victim.

Critical Impact

While there are currently no known active exploits, on older versions prior to v3.2.2 (where the session identifier was not stored in an http-only cookie), attackers could potentially steal session identifiers and gain unauthorized access to the admin interface under specific circumstances.

Affected Products

• Revive Adserver versions 5.0.3 and earlier
• www/delivery/afr.php delivery script component
• All installations with publicly accessible afr.php endpoint

Discovery Timeline

• 2020-02-04 - CVE-2020-8115 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-8115

Vulnerability Analysis

This reflected XSS vulnerability exists in the afr.php delivery script of Revive Adserver. The script is publicly accessible and is used for ad delivery functionality. The core issue stems from improper output encoding when the script reflects user-supplied input back to the browser within a JavaScript context.

When a victim clicks a maliciously crafted link or visits an attacker-controlled page that redirects to the vulnerable endpoint, the injected JavaScript code executes in the context of the Revive Adserver domain. This allows attackers to perform actions on behalf of the victim, steal sensitive information, or redirect users to malicious sites.

The vulnerability's impact varies based on the Revive Adserver version deployed. Since version 3.2.2, the session identifier is stored in an http-only cookie, which prevents JavaScript-based session theft. However, on older installations, this vulnerability could enable session hijacking and unauthorized access to the administrative interface.

Root Cause

The root cause of CVE-2020-8115 is improper output encoding in the afr.php script. The query string parameters passed to the script are echoed back to the client without proper sanitization or escaping for the JavaScript context in which they appear. This violates secure coding practices for output encoding, specifically CWE-79 (Improper Neutralization of Input During Web Page Generation).

Attack Vector

The attack vector for this vulnerability is network-based and requires user interaction. An attacker would need to craft a malicious URL containing JavaScript payload in the query string and trick a victim into clicking the link. The attack could be delivered through:

• Phishing emails with malicious links
• Compromised or malicious websites redirecting to the vulnerable endpoint
• Social engineering tactics through messaging platforms
• Malicious advertisements pointing to the crafted URL

The vulnerability affects the confidentiality and integrity of the victim's browser session, potentially enabling data theft, session hijacking (on older versions), or unauthorized actions performed on behalf of the authenticated user.

Detection Methods for CVE-2020-8115

Indicators of Compromise

• Unusual HTTP requests to www/delivery/afr.php containing JavaScript code or encoded script payloads in query parameters
• Web server logs showing access to afr.php with suspicious query strings containing characters like <script>, javascript:, or URL-encoded equivalents
• Unexpected network traffic from ad server domains to external destinations following access to the afr.php endpoint
• Reports from users about unexpected browser behavior or redirects when interacting with advertisements

Detection Strategies

• Configure Web Application Firewall (WAF) rules to detect and block XSS patterns in requests to delivery scripts
• Implement log monitoring for requests containing script tags, event handlers, or JavaScript protocol handlers in query strings
• Deploy Content Security Policy (CSP) headers to mitigate the impact of XSS attacks by restricting script execution
• Utilize intrusion detection systems (IDS) with signatures for common XSS attack patterns targeting ad server infrastructure

Monitoring Recommendations

• Review web server access logs regularly for requests to /www/delivery/afr.php with abnormally long or suspicious query strings
• Set up alerting for high volumes of requests to delivery scripts from single IP addresses or unusual geographic locations
• Monitor for authentication anomalies following visits to ad delivery endpoints, which could indicate session hijacking attempts
• Implement real-time analysis of outbound requests from browser sessions interacting with the ad server

How to Mitigate CVE-2020-8115

Immediate Actions Required

• Upgrade Revive Adserver to a version newer than 5.0.3 that includes the security fix for this vulnerability
• Review web server logs for any evidence of exploitation attempts targeting the afr.php script
• Implement Web Application Firewall rules to filter XSS payloads in requests to delivery scripts as a temporary measure
• Ensure session cookies are configured with the HttpOnly flag to prevent JavaScript-based session theft

Patch Information

Revive Adserver has released a security patch addressing this vulnerability. Organizations should upgrade to the latest version of Revive Adserver as documented in the Revive Adserver Security Advisory. The vulnerability was reported through the HackerOne Bug Bounty Program and has been remediated by the vendor.

Workarounds

• Deploy a Web Application Firewall (WAF) with XSS filtering rules to inspect and sanitize requests to the afr.php endpoint
• Implement Content Security Policy headers with strict script-src directives to reduce the impact of successful XSS attacks
• Consider restricting access to the afr.php delivery script via IP whitelisting if business requirements permit
• Enable HttpOnly and Secure flags on all session cookies if not already configured (default since v3.2.2)

# Apache configuration to add XSS protection headers
<IfModule mod_headers.c>
    Header always set X-XSS-Protection "1; mode=block"
    Header always set X-Content-Type-Options "nosniff"
    Header always set Content-Security-Policy "default-src 'self'; script-src 'self'"
</IfModule>

# Nginx configuration for XSS protection headers
add_header X-XSS-Protection "1; mode=block" always;
add_header X-Content-Type-Options "nosniff" always;
add_header Content-Security-Policy "default-src 'self'; script-src 'self'" always;