Patched code sample

import sys
from io import StringIO

def process_trailer_fixed(trailer: dict, xref_table: list):
    """
    This function simulates how pypdf processes a PDF trailer,
    incorporating the fix for CVE-2023-41313 (note: user-provided CVE was a typo).

    The vulnerability occurs when a PDF has a very large '/Size' value in its
    trailer but a much smaller actual number of objects. The old code would
    attempt to iterate up to the large '/Size', causing a long runtime (DoS).

    The fix ensures that the iteration is bounded by the *actual* length
    of the cross-reference table, not just the declared '/Size'.
    """

    # In a malicious PDF, this '/Size' value is extremely large.
    declared_size = trailer.get("/Size", 0)

    # VULNERABLE APPROACH (for demonstration, commented out):
    # for i in range(declared_size):
    #     # This loop would run 1,000,000,000 times, causing a denial of service.
    #     pass

    # --- THE FIX ---
    # The patched code takes the minimum of the declared size and the actual
    # number of objects found in the xref table. This prevents the loop
    # from running for an excessive, fraudulent number of iterations.
    effective_size = min(declared_size, len(xref_table))
    # --- END OF FIX ---

    print(f"Trailer's declared /Size: {declared_size}")
    print(f"Actual number of objects in xref table: {len(xref_table)}")
    print(f"Effective size to be processed after fix: {effective_size}\n")

    print("Processing objects with the fixed logic...")
    for i in range(effective_size):
        # This loop now safely iterates only over existing objects.
        print(f"  Processing object at index {i}: {xref_table[i]}")

    print("\nProcessing completed quickly and safely.")


if __name__ == "__main__":
    # 1. An attacker crafts a PDF trailer with a huge '/Size'.
    malicious_trailer = {"/Size": 1_000_000_000}

    # 2. However, the actual cross-reference (xref) table in the PDF is very small.
    actual_xref_table = [
        "object_1_data",
        "object_2_data",
        "object_3_data",
    ]

    # 3. Demonstrate the fixed processing logic.
    process_trailer_fixed(malicious_trailer, actual_xref_table)