Server Side Template Injection with Mutual Tls
How Server Side Template Injection Manifests in Mutual Tls
Server Side Template Injection (SSTI) in Mutual Tls environments occurs when untrusted data flows through template rendering systems that have access to Mutual Tls client certificates or authentication contexts. The unique challenge in Mutual Tls scenarios is that the template engine often needs to display certificate metadata, client authentication status, or dynamically generate content based on Mutual Tls properties.
A common vulnerability pattern emerges when developers use Mutual Tls client certificate information directly in template variables without proper sanitization. Consider this Express.js example with the express-mutual-tls middleware:
Mutual Tls-Specific Detection
Detecting SSTI in Mutual Tls environments requires specialized scanning that understands both template injection patterns and Mutual Tls certificate structures. middleBrick's approach combines black-box scanning with template-specific payloads targeting Mutual Tls certificate fields.
The detection process begins by identifying Mutual Tls endpoints through certificate exchange attempts. middleBrick sends crafted client certificates with suspicious subject and issuer fields containing template injection payloads:
Mutual Tls-Specific Remediation
Remediating SSTI in Mutual Tls environments requires a defense-in-depth approach that combines proper certificate validation, template engine hardening, and input sanitization. The first line of defense is strict certificate validation before any template processing occurs.
Using the node-forge library for certificate validation:
Frequently Asked Questions
How can I test my Mutual Tls endpoints for Server Side Template Injection?
Use middleBrick's CLI tool to scan your Mutual Tls endpoints. The scanner automatically attempts template injection through certificate fields and analyzes responses for template evaluation. Run middlebrick scan https://your-api.com --mtls to target Mutual Tls endpoints specifically. The tool tests multiple injection patterns and provides detailed findings about any successful template injection attempts.What's the difference between SSTI in Mutual Tls vs regular web applications?
SSTI in Mutual Tls environments is more complex because certificate fields contain structured data that template engines often try to render dynamically. Unlike regular web applications where user input is typically simple strings, Mutual Tls certificate subjects and issuers contain hierarchical data with special characters. This makes template injection harder to detect and requires specialized validation of certificate fields before template rendering. Additionally, Mutual Tls error pages often display certificate information, creating more attack surface for SSTI.