Vulnerable code sample

import sys

# This code represents the vulnerable logic present in pypdf before version 6.1.3.
# The vulnerability (CVE-2025-62708, a conceptual ID for this example) lies in the
# LZWDecode filter, which can be exploited by a crafted PDF to cause a
# "decompression bomb". This means a small amount of compressed data expands
# to an extremely large amount of data, consuming excessive memory.

# A real-world LZW bomb would use clever dictionary manipulation to achieve
# exponential expansion. For this demonstration, we simulate the *result*
*of such an attack: an unbounded allocation of memory during decoding.

def LZWDecode_vulnerable(data):
    """
    A simplified and conceptual representation of the vulnerable LZWDecode function.
    It simulates the behavior of decoding a malicious stream that leads to
    massive memory allocation, without any checks on the output size.
    """
    # In a real exploit, the content of `data` would instruct the decoder
    # on how to create the bomb. Here, we hardcode the bomb's effect for clarity.
    # The vulnerability is that the library trusts the compressed stream's content.

    # This simulates the decoder being instructed to generate a multi-gigabyte output.
    # A value of 2 attempts to allocate 2GB of memory.
    gigabytes_to_allocate = 2
    target_output_size = gigabytes_to_allocate * (1024**3)

    # The vulnerable operation: creating a large data buffer without any safety limits.
    # In a real decoder, this would happen in a loop that appends decoded data.
    # This direct allocation represents the final outcome of that process.
    try:
        # We use a bytearray for a more realistic memory allocation pattern.
        # A real decoder would append byte by byte or chunk by chunk.
        # This loop simulates that incremental, yet unbounded, growth.
        decompressed_data = bytearray()
        i = 0
        while i < target_output_size:
            decompressed_data.append(ord('A'))
            i += 1
        return bytes(decompressed_data)
    except MemoryError:
        # This is the expected outcome on most systems: the process runs out of memory.
        sys.stderr.write("MemoryError: Failed to allocate memory, as expected from the vulnerability.\n")
        # In a real server or application, this could lead to a crash or DoS.
        raise


# --- Triggering the vulnerability ---

# This represents a small, malicious stream from a crafted PDF file.
# Its actual content is irrelevant in this simulation, as we have hardcoded
# the bomb's effect in the function above. In a real attack, these bytes
# would contain the LZW instructions to create the bomb.
malicious_lzw_compressed_data = b'\x80\x0b\xef\x01\x95...'

# This is the call that would happen inside pypdf when parsing a page
# content stream with the /LZWDecode filter.
# WARNING: Executing this line will attempt to allocate a large amount of
# memory (2 GB in this example) and will likely cause the program to crash
# or become unresponsive.
print("Attempting to decode a malicious LZW stream. This may cause a MemoryError.")
vulnerable_output = LZWDecode_vulnerable(malicious_lzw_compressed_data)

# This line will likely not be reached.
print(f"Vulnerability failed to trigger; allocated {len(vulnerable_output)} bytes.")