Server-Side Template Injection (SSTI): A Practical Overview
SSTI is what happens when the server merges your input into a template language that was never meant to be a playground. Jinja, Twig, Freemarker—they're powerful and dangerous in the wrong place.
Testers focus on finding controllable template contexts, not on dropping public polyglot strings into production.
SSTI findings need engine context—remediation for one template language doesn't translate cleanly to another, and triage knows the difference.
Common entry points
Email preview features, PDF generators, CMS themes, error pages with reflected names, and 'custom notification' fields admins love.
Framework docs list dangerous constructs—devs should avoid them; you verify they did.
Authorized validation
Identify parameters reflected in rendered output. Work in staging with benign arithmetic or echo proofs when approved—escalation depth per ROE.
Separate client-side template bugs from server-side
Check sandboxing and engine defaults
Map to CWE-94 in reports
Remediation
Never pass user input to template evaluation. Use logic-less templates or strict context separation.
SSTI severity tracks execution depth—document what you proved under authorization.
Don't run public SSTI exploit chains against production without explicit approval.
When staging proves SSTI, coordinate fix validation with devs before public disclosure timelines on bounties.
Small habits compound—what feels like overhead early becomes speed when deadlines hit.
Label template engine confidence explicitly—misidentification sends devs down wrong fix paths.
Sandbox awareness
Modern engines sandbox differently—impact statements should reflect what you proved in staging, not generic RCE claims.
Check background jobs rendering templates async—those paths skip WAF rules tuned for HTTP requests.
Engine-specific notes
Name the template engine in reports when confirmed—remediation differs widely.
Background PDF generation is a frequent SSTI sink—test export features slowly.
Recommend disabling debug template modes in production builds.
Worth reading next
RCE and File Inclusion Risks: A High-Level Map for Testers
Web Hacking for Pentesters: Cross-Site Scripting (XSS)
Penetration Testing: Web Application Checklist (WAPT)
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