Twisted is an event-based framework for internet applications, supporting Python 3.6+. Prior to 26.10.0rc1, twisted.names.dns.DNSDatagramProtocol accepts a UDP DNS response for a pending query when its 16-bit transaction ID matches but does not verify that the response came from the queried host and port. An attacker able to deliver a packet to the resolver's listening port with the correct transaction ID can inject a forged answer. The standard twisted.names.client.Resolver uses a new random source port for each query, increasing off-path attack complexity, while applications that reuse a DNSDatagramProtocol instance expose only the transaction-ID search space after the first query. A successful forgery can redirect hostname resolution and enable mail interception, server-side request forgery, or man-in-the-middle positioning. This issue is fixed in version 26.10.0rc1.

Affected: PyPI: Twisted

### Summary

`twisted.names.dns.DNSDatagramProtocol`, the UDP transport used by twisted's DNS client (`twisted.names.client.Resolver`, `createResolver`, and anything built on it, including MX lookups in `twisted.mail`), accepts a DNS response for a pending query solely on the basis of the 16-bit transaction ID matching. It never checks that the response actually originated from the server the query was sent to. Any host that can deliver a UDP packet to the resolver's listening port with the correct transaction ID can inject a forged answer, independent of the real server's address. This is a materially weaker defense than standard DNS resolver practice (transaction ID plus source-address validation, typically also source-port rotation), and enables off-path DNS response spoofing / cache poisoning.

### Details

`DNSDatagramProtocol.datagramReceived` (`src/twisted/names/dns.py`, line 3350) is:

```python def datagramReceived(self, data, addr): """ Read a datagram, extract the message in it and trigger the associated Deferred. """ m = Message() try: m.fromStr(data) except EOFError: ... except ValueError as ex: ... except BaseException: ...

if m.id in self.liveMessages: d, canceller = self.liveMessages[m.id] del self.liveMessages[m.id] canceller.cancel() try: d.callback(m) except BaseException: log.err() else: if m.id not in self.r…

Affected: Ubuntu:Pro:14.04:LTS: twisted, Ubuntu:Pro:16.04:LTS: twisted, Ubuntu:Pro:18.04:LTS: twisted, Ubuntu:20.04:LTS: twisted, Ubuntu:22.04:LTS: twisted, Ubuntu:24.04:LTS: twisted, Ubuntu:26.10: twisted, Ubuntu:26.04:LTS: twisted

Twisted is an event-based framework for internet applications, supporting Python 3.6+. Prior to 26.10.0rc1, twisted.names.dns.DNSDatagramProtocol accepts a UDP DNS response for a pending query when its 16-bit transaction ID matches but does not verify that the response came from the queried host and port. An attacker able to deliver a packet to the resolver's listening port with the correct transaction ID can inject a forged answer. The standard twisted.names.client.Resolver uses a new random source port for each query, increasing off-path attack complexity, while applications that reuse a DNSDatagramProtocol instance expose only the transaction-ID search space after the first query. A successful forgery can redirect hostname resolution and enable mail interception, server-side request forgery, or man-in-the-middle positioning. This issue is fixed in version 26.10.0rc1.