Patched code sample

The provided CVE-2026-27888 is not a valid CVE identifier. However, the description of the vulnerability (a "zip bomb" in a Flate-encoded stream causing RAM exhaustion) is a real-world issue that has been addressed in `pypdf`.

The following Python code is a conceptual, self-contained demonstration of the *principle* used to fix this type of vulnerability. The actual patch in the `pypdf` library is integrated within its stream-parsing objects, but this code implements the same core security mechanism: decompressing with a maximum size limit to prevent memory exhaustion.

```python
import zlib

class PdfReadError(Exception):
    """Custom exception to mimic errors in pypdf."""
    pass

# In the actual pypdf fix, this limit is a configurable constant.
# It prevents the library from decompressing an infinite or excessively large
# stream of data from a crafted PDF file (a "zip bomb").
FLATE_MAX_SIZE_LIMIT = 100 * 1024 * 1024  # 100 MB

def secure_flate_decode(data: bytes) -> bytes:
    """
    A function demonstrating a secure way to decompress Flate-encoded data
    by enforcing an output size limit, preventing RAM exhaustion attacks.

    This mimics the logic patched into pypdf to resolve decompression bombs.

    :param data: The compressed byte stream from a PDF.
    :return: The decompressed data, if within the size limit.
    :raises PdfReadError: If the decompressed data would exceed the limit.
    """
    try:
        # The core of the fix is using zlib.decompressobj(), which allows for
        # more granular control over decompression, rather than a simple
        # zlib.decompress() call which could allocate massive amounts of memory.
        decompressor = zlib.decompressobj()

        # The `max_length` argument is the crucial security control.
        # zlib will raise a zlib.error if the output buffer would need to be
        # larger than this size. This is the primary defense against a zip bomb.
        decompressed_data = decompressor.decompress(data, FLATE_MAX_SIZE_LIMIT)

        # The actual pypdf patch has additional checks. For example, if there's
        # any unconsumed data left after hitting the max_length, it's a clear
        # sign that the original stream was too large.
        if decompressor.unconsumed_tail or decompressor.unused_data:
            raise PdfReadError(
                f"FlateDecode: Decompressed data exceeds size limit of "
                f"{FLATE_MAX_SIZE_LIMIT} bytes."
            )

        # A final belt-and-suspenders check. If the decompressed data length
        # is equal to the limit, it's considered suspicious and rejected.
        if len(decompressed_data) >= FLATE_MAX_SIZE_LIMIT:
            raise PdfReadError(
                f"FlateDecode: Decompressed data reached size limit of "
                f"{FLATE_MAX_SIZE_LIMIT} bytes."
            )

        return decompressed_data
    except zlib.error as e:
        # This error is typically caught when `max_length` is exceeded.
        # We wrap it in a library-specific exception for consistent error handling.
        raise PdfReadError(
            f"FlateDecode: Size limit of {FLATE_MAX_SIZE_LIMIT} bytes "
            "would be exceeded during decompression."
        ) from e

# Example Usage to demonstrate the fix:

# 1. Create a "zip bomb": a small compressed payload that expands to a huge size.
#    This will expand to ~101MB, which is over our 100MB limit.
large_uncompressed_data = b'\x00' * (FLATE_MAX_SIZE_LIMIT + 1)
compressed_bomb = zlib.compress(large_uncompressed_data)

# 2. Create a safe, small payload.
safe_data = zlib.compress(b"This is a safe string that is small.")

# 3. Demonstrate the fix with the malicious payload.
try:
    print(f"Attempting to decompress zip bomb of size {len(compressed_bomb)} bytes...")
    secure_flate_decode(compressed_bomb)
except PdfReadError as e:
    # This is the expected, safe outcome.
    # The code correctly identified the attack and stopped it.
    print(f"SUCCESS: The fix prevented memory exhaustion.\nCaught expected error: {e}\n")

# 4. Demonstrate normal operation with a safe payload.
try:
    print(f"Attempting to decompress safe data of size {len(safe_data)} bytes...")
    result = secure_flate_decode(safe_data)
    # This is the expected outcome for a valid file.
    print(f"SUCCESS: Safely decompressed data of length {len(result)} bytes.")
except PdfReadError as e:
    print(f"FAILURE: An unexpected error occurred with safe data: {e}")