HSTS Not Applied to Subdomains

1. Executive Threat Overview

Enabling HSTS without the includeSubDomains directive leaves subdomains unprotected and vulnerable to protocol downgrade attacks.

In modern web infrastructure, hsts not applied to subdomains represents a critical failure mode in security controls. According to A05:2021 - Security Misconfiguration and documented under CWE-319: Cleartext Transmission of Sensitive Information, 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-319: Cleartext Transmission of Sensitive Information, 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 HSTS Not Applied to Subdomains:
add_header Strict-Transport-Security "max-age=31536000; includeSubDomains; preload" always;

Apache 2.4 / .htaccess

# Apache 2.4 Configuration for HSTS Not Applied to Subdomains:
Header always set Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"

Application Code Implementation

Append includeSubDomains to your Strict-Transport-Security header configuration to cover all subdomains: Strict-Transport-Security: max-age=31536000; includeSubDomains

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 'subdomains'

6. Frequently Asked Questions