Referrer Information Leakage

1. Executive Threat Overview

Navigating to third-party links from secure areas can leak sensitive tokens, IDs, or search queries contained in the URL via the HTTP Referer header.

In modern web infrastructure, referrer information leakage represents a critical failure mode in security controls. According to A05:2021 - Security Misconfiguration and documented under CWE-200: Exposure of Sensitive Information to an Unauthorized Actor, applications failing to enforce proper controls at this layer expose their attack surface to automated crawlers, botnets, and skilled penetration testers.

When evaluating target perimeters, threat actors systematically probe for this condition using fingerprinting engines like Nmap, Nuclei, and custom fuzzing scripts. If left unresolved, this weakness can be chained with auxiliary application vulnerabilities to achieve elevated privileges, exfiltrate sensitive data, or bypass downstream security filters.

2. Technical & Business Impact

3. Exploitation Mechanics & Attack Vectors

Threat actors identify and exploit this weakness through the following attack phases:

1. Phase 1 (Reconnaissance): The attacker performs automated reconnaissance against the target domain to identify server headers, software versions, and security configurations.
2. Phase 2 (Exploitation): Utilizing known attack vectors associated with CWE-200: Exposure of Sensitive Information to an Unauthorized Actor, the attacker crafts specific payloads or request patterns to trigger the vulnerability.
3. Phase 3 (Post-Exploitation & Impact): Upon successful validation, the attacker leverages the condition to compromise confidential records, manipulate user sessions, or pivot toward internal services.

4. Production Hardening & Remediation

To eliminate this vulnerability, apply the following hardened configurations to your edge web server or application codebase:

Nginx Web Server Configuration

# Recommended Nginx Hardening Configuration for Referrer Information Leakage:
add_header Referrer-Policy "strict-origin-when-cross-origin" always;

Apache 2.4 / .htaccess

# Apache 2.4 Configuration for Referrer Information Leakage:
Header always set Referrer-Policy "strict-origin-when-cross-origin"

Application Code Implementation

Configure a strict Referrer-Policy header (e.g. Referrer-Policy: strict-origin-when-cross-origin) to protect private URL structures.

5. Verification & Testing Commands

Run the following terminal command to audit and verify that your production fixes are actively enforced:

curl -sI https://example.com | grep -i 'leak'

6. Frequently Asked Questions