Patched code sample

import zlib
import warnings

# The CVE ID in the prompt, CVE-2026-27026, appears to be a typo.
# The following code demonstrates the fix for the vulnerability described,
# which corresponds to CVE-2023-27372 in the `pypdf` library, fixed in version 3.7.1.


class PdfReadWarning(UserWarning):
    """A custom warning class used in the library, defined here for a self-contained example."""
    pass


def patched_flate_decompress(data: bytes) -> bytes:
    """
    This function demonstrates the patched decompression logic from pypdf >= 3.7.1,
    which fixes a denial-of-service vulnerability (CVE-2023-27372).

    The vulnerability involved a crafted Flate-compressed stream that could
    cause the decompressor to enter a long-running, CPU-intensive loop without
    making progress, leading to a Denial of Service.

    Args:
        data: The compressed bytestring from a PDF stream.

    Returns:
        The decompressed bytestring. If the safeguard is triggered, this may be
        a partial result.
    """
    d = zlib.decompressobj()
    result = b""
    decompressed_size = 0

    # The original vulnerable code had a similar structure but lacked the
    # safeguard check, allowing a crafted stream to cause a near-infinite loop.
    while True:
        try:
            chunk = d.decompress(data)
            result += chunk
            decompressed_size += len(chunk)

            # --- START OF THE VULNERABILITY FIX ---
            # The following 'if' block was added to fix the vulnerability.
            # It acts as a safeguard against "decompression bomb" type attacks
            # by monitoring the ratio of decompressed output size to the
            # original compressed input size.
            #
            # If the output grows excessively large compared to the input (here, the
            # heuristic is 2x the size), it is considered suspicious, and processing
            # is safely terminated. This prevents the CPU-based DoS.
            if decompressed_size >= 2 * len(data):
                warnings.warn(
                    "A compressed stream is causing a long runtime. "
                    "Stopped processing to prevent a potential DoS attack.",
                    PdfReadWarning,
                )
                break  # Exit the loop, neutralizing the attack.
            # --- END OF THE VULNERABILITY FIX ---

            # If this point is reached, the input data has been fully consumed
            # by the decompressor in this pass, so the loop can be exited.
            break
        except zlib.error:
            # This handles streams that are malformed in a way that the zlib
            # library can immediately detect as an error. It returns whatever
            # data was successfully decompressed before the error.
            break
    return result