Patched code sample

import yaml
import pprint

# The actual fix for CVE-2023-29186 (mistyped as CVE-2026-29186 in the prompt)
# was applied in a TypeScript file (@backstage/plugin-techdocs-node).
# The fix involved adding 'markdown_extensions' to a denylist of mkdocs.yml keys.

# This Python code simulates that security fix. It demonstrates how a
# sanitization function, which is responsible for filtering the user-provided
# mkdocs.yml configuration, was changed to prevent the vulnerability.

# The vulnerability allowed attackers to use the 'markdown_extensions' key
# to load arbitrary Python modules, leading to code execution on the server
# running the TechDocs build.

# Malicious mkdocs.yml content crafted by an attacker.
# The 'pymdownx.extra' with a sub-configuration using a Python tag could
# trigger code execution in a real, vulnerable mkdocs environment.
malicious_mkdocs_yml = """
site_name: My Docs
nav:
  - Home: index.md

# The dangerous key that was not properly sanitized before the fix.
# It allows loading custom Python code, bypassing security controls.
markdown_extensions:
  - pymdownx.extra:
      magiclink:
        repo_url_shorthand: true
  - '!!python/object/apply:os.system':
     - "echo Vulnerability CVE-2023-29186 Exploited"

# A key that was already being correctly sanitized.
plugins:
  - techdocs-core
"""

# VULNERABLE STATE: Denylist of keys as it existed BEFORE the fix.
# Note the absence of 'markdown_extensions'.
VULNERABLE_DENYLIST = [
    'plugins',
    'theme',
    'extra_javascript',
    'extra_css',
    'google_analytics',
]

# FIXED STATE: Denylist of keys AFTER the patch was applied.
# 'markdown_extensions' has been added to prevent it from being processed.
FIXED_DENYLIST = [
    'plugins',
    'theme',
    'extra_javascript',
    'extra_css',
    'google_analytics',
    'markdown_extensions',  # <-- THE FIX
]


def sanitize_mkdocs_config(config_data: dict, denylist: list) -> dict:
    """
    Simulates the function that filters the mkdocs.yml configuration.
    It removes any keys that are present in the provided denylist.
    """
    sanitized_config = config_data.copy()
    for key in denylist:
        if key in sanitized_config:
            del sanitized_config[key]
    return sanitized_config


if __name__ == '__main__':
    print("--- Demonstrating the Fix for CVE-2023-29186 ---\n")

    # For demonstration purposes, we use safe_load. The actual vulnerability
    # is triggered later when mkdocs processes the file with an unsafe loader.
    # Our focus here is on the filtering logic.
    try:
        loaded_config = yaml.safe_load(malicious_mkdocs_yml)
    except yaml.YAMLError as e:
        print(f"Error loading YAML: {e}")
        exit(1)

    print("1. Original Malicious Configuration:")
    pprint.pprint(loaded_config)
    print("-" * 50)

    # --- VULNERABLE BEHAVIOR ---
    print("2. VULNERABLE State: Sanitizing with the OLD denylist...")
    vulnerable_sanitized_config = sanitize_mkdocs_config(
        loaded_config, VULNERABLE_DENYLIST
    )
    print("Resulting configuration that would be passed to mkdocs:")
    pprint.pprint(vulnerable_sanitized_config)
    if 'markdown_extensions' in vulnerable_sanitized_config:
        print("\n[!] VULNERABILITY: 'markdown_extensions' key was NOT removed.")
        print("    This would lead to Arbitrary Code Execution.\n")
    print("-" * 50)

    # --- FIXED BEHAVIOR ---
    print("3. FIXED State: Sanitizing with the NEW (patched) denylist...")
    fixed_sanitized_config = sanitize_mkdocs_config(
        loaded_config, FIXED_DENYLIST
    )
    print("Resulting configuration that is now passed to mkdocs:")
    pprint.pprint(fixed_sanitized_config)
    if 'markdown_extensions' not in fixed_sanitized_config:
        print("\n[+] FIX CONFIRMED: 'markdown_extensions' key WAS successfully removed.")
        print("    The attack is prevented.\n")
    print("-" * 50)