Patched code sample

import sys

# The CVE-2026-27628 is a fictional CVE number. The following code demonstrates
# a plausible fix for the described vulnerability type: an infinite loop
# caused by a crafted file with circular references. This is a common issue
# in parsers for complex formats like PDF.

# In a PDF, objects can have parent-child relationships. A malicious PDF
# could define a circular reference, e.g., Object A is the parent of Object B,
# and Object B is the parent of Object A. A naive parser traversing this
# structure would loop infinitely.

# This simulates a simplified PDF object structure.
# Object 1's parent is Object 2.
# Object 2's parent is Object 1, creating a circular reference.
malicious_pdf_objects = {
    # In a real PDF, these would be object IDs and dictionaries.
    1: {"Type": "/Page", "Parent": 2},
    2: {"Type": "/Pages", "Parent": 1},
    3: {"Type": "/Catalog", "Parent": None}, # A valid root object
}

# --- VULNERABLE (CONCEPTUAL) CODE ---
# A vulnerable function would look something like this.
# It does not track visited objects and would enter an infinite loop.
#
# def find_document_root_vulnerable(start_obj_id, all_objects):
#     current_obj_id = start_obj_id
#     while 'Parent' in all_objects.get(current_obj_id, {}):
#         current_obj_id = all_objects[current_obj_id]['Parent']
#     return all_objects.get(current_obj_id)
#
# Calling find_document_root_vulnerable(1, malicious_pdf_objects)
# would loop forever: 1 -> 2 -> 1 -> 2 -> ...


# --- FIXED CODE ---
# The fix involves tracking the objects that have already been visited during
# the traversal up the parent tree. If an object is encountered a second time,
# a circular reference is detected, and the operation is aborted.

def find_document_root_fixed(start_obj_id, all_objects):
    """
    Traverses up the parent chain to find the document root.

    This fixed version tracks visited object IDs to prevent an infinite loop
    caused by circular references in a malicious file.
    """
    if start_obj_id not in all_objects:
        raise ValueError(f"Starting object {start_obj_id} not found.")

    visited_ids = {start_obj_id}
    current_obj_id = start_obj_id

    # As a secondary defense (defense-in-depth), we can add a hard limit
    # on the number of traversals to prevent excessively long (but not
    # infinite) chains from causing a denial of service.
    MAX_TRAVERSAL_DEPTH = 1000  # A sane limit for PDF parent chains
    depth = 0

    # The main traversal loop
    while 'Parent' in all_objects.get(current_obj_id, {}) and all_objects[current_obj_id]['Parent'] is not None:
        if depth >= MAX_TRAVERSAL_DEPTH:
            raise RecursionError(
                "Maximum parent traversal depth exceeded, possible DoS attack."
            )

        parent_id = all_objects[current_obj_id]['Parent']

        # THE CORE OF THE FIX: Check if we have already visited this parent.
        if parent_id in visited_ids:
            raise RecursionError(
                f"Circular reference detected: Object {parent_id} was "
                "encountered again in the parent chain. Aborting."
            )

        # If not, add it to our set of visited objects and continue.
        visited_ids.add(parent_id)
        current_obj_id = parent_id
        depth += 1

    return all_objects.get(current_obj_id)


if __name__ == "__main__":
    print("Demonstrating the fix for a circular reference vulnerability.")
    print("-" * 60)

    # We will attempt to process the malicious structure.
    # The vulnerable code would hang here. The fixed code will raise an error.
    start_node = 1
    print(f"Attempting to find root starting from malicious object {start_node}...")

    try:
        find_document_root_fixed(start_node, malicious_pdf_objects)
    except RecursionError as e:
        print("\nSUCCESS: The fix worked as expected.")
        print(f"Caught expected error: {e}")
        # Exit with a success code because the vulnerability was correctly mitigated.
        sys.exit(0)
    except Exception as e:
        print(f"\nFAILURE: An unexpected error occurred: {e}")
        sys.exit(1)

    print("\nFAILURE: The code did not detect the infinite loop.")
    sys.exit(1)