Patched code sample

import io

def _safe_scan_for_end_of_inline_image(stream: io.BytesIO) -> None:
    """
    A conceptual representation of the fix for CVE-2023-39936 in pypdf.

    The original vulnerability was an infinite loop when a PDF's inline image
    data was not terminated by an 'EI' (End Image) marker. The code would
    continuously try to read from a stream that had already ended.

    The fix ensures the loop terminates by checking the current stream
    position against its total length, preventing an infinite loop.

    Args:
        stream: A stream object simulating PDF content. For this example,
                it is assumed the stream reader is positioned right after
                the 'BI' (Begin Image) marker.
    """
    # Before entering the loop, determine the absolute end of the stream.
    current_pos = stream.tell()
    stream.seek(0, io.SEEK_END)
    stream_length = stream.tell()
    stream.seek(current_pos)  # Restore original position

    data_scanned = b""
    end_marker = b"EI"

    # The vulnerable code would loop infinitely here if 'EI' was missing.
    while end_marker not in data_scanned:
        # THE FIX: Check if the stream's end has been reached.
        # This prevents an infinite loop on a malformed stream.
        if stream.tell() >= stream_length:
            # End of stream reached, but 'EI' marker was not found.
            # Stop scanning to prevent an infinite loop.
            break

        # Continue reading data to find the end marker.
        # In a real implementation, this would read smaller chunks.
        data_scanned += stream.read(20)

    # The function now safely exits even if the 'EI' marker is never found.
    # To demonstrate, we can check the result.
    if end_marker in data_scanned:
        print("Safely found 'EI' marker.")
    else:
        print("Safely terminated scanning: 'EI' marker was not found.")


if __name__ == '__main__':
    # This simulates a malformed PDF stream with a Begin Image ('BI')
    # but no corresponding End Image ('EI'). The vulnerable code would
    # loop forever on this stream.
    malformed_stream = io.BytesIO(b"\x00\x01\x02malformed image data that never ends")

    print("Processing a malformed stream that would cause an infinite loop...")
    _safe_scan_for_end_of_inline_image(malformed_stream)

    # This simulates a well-formed stream for comparison.
    well_formed_stream = io.BytesIO(b"\x00\x01\x02 some image data EI more data")
    print("\nProcessing a well-formed stream...")
    _safe_scan_for_end_of_inline_image(well_formed_stream)