Patched code sample

import os
import sys

def get_validated_path(base_dir: str, user_path: str) -> str:
    """
    Prevents path traversal attacks by ensuring a user-provided path is
    safely contained within a trusted base directory.

    This function represents the core logic used to fix the path traversal
    vulnerability (similar to CVE-2024-24123) where fields in a
    'bentofile.yaml' could be manipulated to exfiltrate arbitrary files.
    It works by resolving both the base directory and the user-provided
    path to their absolute, canonical forms and then verifying that the base
    directory is a common parent of the resulting path.

    Args:
        base_dir: The absolute path to the trusted root directory (e.g., the
                  BentoML project build context).
        user_path: The untrusted path provided by a user or configuration file
                   (e.g., from a 'description' or 'docker.setup_script' field).

    Returns:
        The resolved, absolute path if it is safe and within the base_dir.

    Raises:
        ValueError: If the path is determined to be unsafe (i.e., it
                    traverses outside the `base_dir`).
    """
    # 1. Resolve the trusted base directory to its real, absolute path to
    #    prevent symbolic link manipulation and normalize its representation.
    resolved_base_dir = os.path.realpath(base_dir)

    # 2. Join the base directory with the user path and resolve it to its
    #    real, absolute path. This collapses any '..' components.
    candidate_path = os.path.join(resolved_base_dir, user_path)
    resolved_candidate_path = os.path.realpath(candidate_path)

    # 3. Use `os.path.commonpath` to find the longest common sub-path.
    #    If the user path is safe, the common path will be the base directory.
    #    This is a robust method for detecting traversal attacks.

    # On Windows, drive letters can differ in case. For the commonpath check to
    # work reliably, we normalize the case before comparison.
    if sys.platform == "win32":
        common_prefix = os.path.commonpath(
            [resolved_base_dir.lower(), resolved_candidate_path.lower()]
        )
        if common_prefix != resolved_base_dir.lower():
            raise ValueError(
                f"Path traversal detected. The path '{user_path}' resolves to "
                f"'{resolved_candidate_path}', which is outside the "
                f"allowed directory '{resolved_base_dir}'."
            )
    else:
        # On POSIX systems, the comparison is case-sensitive and straightforward.
        common_prefix = os.path.commonpath([resolved_base_dir, resolved_candidate_path])
        if common_prefix != resolved_base_dir:
            raise ValueError(
                f"Path traversal detected. The path '{user_path}' resolves to "
                f"'{resolved_candidate_path}', which is outside the "
                f"allowed directory '{resolved_base_dir}'."
            )

    return resolved_candidate_path