The hook
Server-Side Template Injection is sneakier than its cousins. SQLi takes you to the database, XSS takes you to the user's browser — SSTI usually takes you straight to a shell on the application server. It happens when developers reach for a template engine's flexibility (let users add a name to the email subject) and let user input cross the line from data to code.
How it works
Server-side template injection appears when user-controlled text is evaluated by a server template engine instead of being rendered as plain data. In severe cases, that can expose sensitive data or reach server-side execution paths.
The blast radius
Most template engines, by design, can call into the host language. That means SSTI typically grades up to remote code execution: arbitrary shell commands, file reads, lateral movement into adjacent services, or just a reverse shell. Even on engines with sandboxes (Handlebars, Mustache without helpers) the attacker can usually exfiltrate context variables that include secrets and session data.
// what fixvibe checks
What FixVibe checks
FixVibe checks this class with verified-domain active testing that is bounded, non-destructive, and evidence-driven. Public reports describe the affected surface and remediation. For check-specific questions about exact detection heuristics, active payload details, or source-code rule patterns, contact support@fixvibe.app.
Ironclad defenses
Don't concatenate user input into templates. Pass user data as template variables, never as template source. If you must compose templates dynamically, use a sandboxed engine (Liquid is well-suited; Handlebars without runtime helpers is reasonable) and feed the user data as the data context only. As a second layer, run the rendering in a least-privileged process — separate Linux user, no shell, no network egress beyond what it needs. SSTI is one of the few classes where 'secure by construction' really applies — separate code from data and the entire family of bugs disappears.