For this challenge, we get presented with a simple website. The website consists of a single page with a form where we can select the eyes (radio buttons) and enter a name for our gingerbread. Once we submit the form, the server returns a page with an ASCII art gingerbread and a speech bubble containing our name.
Doing some basic information gathering and looking at the response headers of the server, we can see that it's implemented in python (server: Werkzeug/1.0.1 Python/2.7.16 - even using an outdated and insecure python version). However, this instantly reminded me of a Flask / Jinja template injection attack. Using a payload like {{7+7}} we can confirm that this is actually the case as the expression gets evaluated on the server and the response contains the output of the evaluation (14). This means that we actually conducted a successful SSTI (server side template injection) attack.
Now we can just follow the standard template injection attack path to get a remote file inclusion (RCE is not really needed, as we already now the location of the flag). By accessing the __subclasses__ array of the object class, we can get a reference to the file class. After trying a couple of payloads, we can see that the file class is the 40th subclass of object:
{{[].__class__.__base__.__subclasses__()[40] }}
Hello, mighty <type 'file'>
Now we can simply instantiate an object of the file class and read the contents of /flag.txt using the following payload: {{[].__class__.__base__.__subclasses__().pop(40)('/flag.txt').read() }}
Flag: HV20{SST1_N0t_0NLY_H1Ts_UB3R!!!}