Vulnerable code sample

import sys
import time

class VulnerablePdfParser:
    """
    This class is a conceptual representation of a PDF parser that is
    vulnerable to a DoS attack via a crafted cross-reference stream.
    It does not use the pypdf library but mimics the flawed logic
    described in the CVE.
    """

    def _read_xref_stream(self, stream_dict):
        """
        Simulates the vulnerable function that processes a cross-reference stream.
        """
        try:
            size = stream_dict["/Size"]
            w = stream_dict["/W"]
        except KeyError:
            print("Stream missing /Size or /W keys.")
            return

        # The vulnerability is the lack of validation before processing.
        # If /W contains all zeros, the code still attempts to loop 'size' times.
        if sum(w) == 0 and size > 0:
            print(f"Vulnerable logic triggered. Preparing to loop {size} times...", file=sys.stderr)
            
            # This loop causes a Denial of Service (DoS) by consuming CPU
            # for a very long time.
            for i in range(size):
                # In a real implementation, this might read 0 bytes, but the
                # loop itself is the resource exhaustion vector.
                pass
            
            print("Loop finished.", file=sys.stderr) # This line is reached after a long delay.

    def parse(self, data):
        print(f"Starting to parse at {time.ctime()}...")
        start_time = time.time()
        self._read_xref_stream(data)
        end_time = time.time()
        print(f"Parsing finished. Total time: {end_time - start_time:.2f} seconds.")


if __name__ == "__main__":
    # A dictionary simulating the properties of a malicious cross-reference stream.
    # Note: A very large 'Size' (e.g., 2**30) would cause a MemoryError on range().
    # We use a smaller number that still effectively demonstrates the long runtime.
    malicious_stream_data = {
        "/Type": "/XRef",
        "/Size": 50_000_000,   # A large number of objects to cause a long loop.
        "/W": [0, 0, 0],      # The key part of the vulnerability.
    }

    parser = VulnerablePdfParser()

    print("Demonstrating the vulnerability. The program will now hang for several seconds.")
    # The following call will trigger the vulnerable logic and take a long time to complete.
    parser.parse(malicious_stream_data)