Patched code sample

import sys

# The vulnerability in older aiohttp versions (prior to 3.9.2, related to
# CVE-2024-23334, which matches the user's description) was the lack of a limit
# on the total size of headers, allowing a malicious client to cause
# unbounded memory consumption on the server.
#
# The fix, introduced in aiohttp 3.9.2, was to add a limit for the total
# size of all headers (defaulting to 32KiB).
#
# This code provides a simplified, self-contained demonstration of the *logic*
# used in the fix. It is not the literal aiohttp source code but an
# example that implements the same protective principle.

class HeaderSizeExceededError(Exception):
    """Custom exception for when header size limits are violated."""
    def __init__(self, message, limit, actual_size):
        super().__init__(f"{message}. Limit: {limit}, Actual: {actual_size}")
        self.limit = limit
        self.actual_size = actual_size

class PatchedHttpRequestParser:
    """
    A simplified parser demonstrating the fix for excessive header size.
    It enforces a total size limit on headers.
    """

    def __init__(self, max_headers_size=8192):
        """
        Initializes the parser with a limit on the total size of headers.

        Args:
            max_headers_size (int): The maximum allowed total size of headers in bytes.
                                    The aiohttp default is 32768. We use 8192
                                    for easier demonstration.
        """
        self._max_headers_size = max_headers_size
        self._headers_size = 0
        self._headers = []
        self._reading_headers = True

    def get_headers(self):
        return self._headers

    def feed_data(self, data: bytes):
        """
        Feeds data to the parser and enforces the header size limit.
        """
        # In a real parser, this would be a more complex state machine.
        # Here, we simulate line-by-line processing for clarity.
        lines = data.split(b'\r\n')

        for line in lines:
            # An empty line signifies the end of headers
            if not line and self._reading_headers:
                self._reading_headers = False
                print("End of headers detected.")
                continue

            if self._reading_headers:
                # The size of each line (including CRLF) contributes to the total.
                line_size = len(line) + 2
                
                # FIX: Check if the new line would exceed the total headers size limit.
                if self._headers_size + line_size > self._max_headers_size:
                    raise HeaderSizeExceededError(
                        "Total headers size exceeds the limit",
                        limit=self._max_headers_size,
                        actual_size=self._headers_size + line_size,
                    )

                # If the check passes, update the total size and store the header.
                self._headers_size += line_size
                
                # Simple parsing for demonstration
                if b':' in line:
                    key, value = line.split(b':', 1)
                    self._headers.append((key.strip(), value.strip()))
                
                print(f"Processed header line. Total size so far: {self._headers_size} bytes")
                
            else:
                # In a real implementation, this would handle the request body.
                print(f"Ignoring body chunk: {line[:30]}...")
                break


def demonstrate_fix():
    """
    Runs a demonstration showing how the patched parser prevents a memory
    exhaustion attack from oversized headers.
    """
    print("--- Demonstration of Header Size Limit Fix ---")

    # 1. Define a request with headers that are too large.
    # We create 100 headers, each about 100 bytes long, to exceed the 8KB limit.
    large_headers_list = [f"X-Custom-Header-{i}: {'x'*80}".encode('ascii') for i in range(100)]
    request_with_large_headers = (
        b"GET / HTTP/1.1\r\n" +
        b"Host: example.com\r\n" +
        b'\r\n'.join(large_headers_list) +
        b"\r\n\r\n" +
        b"This is the body, which will not be processed."
    )
    
    print(f"Attempting to parse a request with total header size > {len(request_with_large_headers)} bytes.")
    print(f"Parser is configured with max_headers_size = 8192 bytes.\n")
    
    # 2. Instantiate the parser with the protective limit.
    parser = PatchedHttpRequestParser(max_headers_size=8192)

    # 3. Attempt to feed the oversized headers to the parser.
    try:
        parser.feed_data(request_with_large_headers)
        # This part of the code should not be reached.
        print("\n[VULNERABILITY] FAILED: Parser accepted oversized headers without error.")
        print(f"Total headers parsed: {len(parser.get_headers())}")
    except HeaderSizeExceededError as e:
        # The exception demonstrates that the fix is working correctly.
        print("\n[SUCCESS] The fix was demonstrated successfully.")
        print(f"Caught expected exception: {e}")
        print("The parser correctly rejected the request before consuming excessive memory.")

if __name__ == "__main__":
    demonstrate_fix()