Patched code sample

import jinja2

def get_fixed_manifest(user_provided_kernel_env_value: str) -> str:
    """
    Demonstrates the fix for the SSTI vulnerability by treating user input
    as data, not as executable template code.
    """
    # The manifest template contains a standard Jinja2 placeholder.
    # It is a static string and is not constructed from user input.
    kubernetes_manifest_template = """
apiVersion: v1
kind: Pod
spec:
  containers:
  - name: kernel
    env:
    - name: KERNEL_USER_VAR
      value: "{{ kernel_value }}"
"""

    # Initialize the Jinja2 environment.
    # The template is parsed once from the static string.
    env = jinja2.Environment()
    template = env.from_string(kubernetes_manifest_template)

    # THE FIX:
    # The user-controlled value is passed into the `render` function as context.
    # Jinja2 will safely substitute the `{{ kernel_value }}` placeholder
    # with the literal string content of the user input. It will NOT
    # parse and execute the user input as a nested template.
    rendered_manifest = template.render(kernel_value=user_provided_kernel_env_value)

    return rendered_manifest

# Example of a malicious payload a user might provide.
# This attempts to execute the 'whoami' command via SSTI.
malicious_payload = "{{ ''.__class__.__mro__[1].__subclasses__()[133]('whoami', shell=True, stdout=-1).stdout.read().decode() }}"

# Generate the manifest using the fixed, safe method.
safe_output = get_fixed_manifest(malicious_payload)

# When printed, `safe_output` will show the malicious payload as a literal
# string in the 'value' field, proving it was not executed.
#
# Expected 'value' field in the output:
# value: "{{ ''.__class__.__mro__[1].__subclasses__()[133]('whoami', shell=True, stdout=-1).stdout.read().decode() }}"
#
# The vulnerability existed because the user input was incorporated into the
# template string *before* rendering, rather than being passed as data *during*
# rendering.