Predictable DNS Transaction IDs Enable Cache Poisoning in Built-in Resolver

Generation of Predictable Numbers or Identifiers vulnerability in Erlang/OTP kernel (inet_res, inet_db modules) allows DNS Cache Poisoning.

The built-in DNS resolver (inet_res) uses a sequential, process-global 16-bit transaction ID for UDP queries and does not implement source port randomization. Response validation relies almost entirely on this ID, making DNS cache poisoning practical for an attacker who can observe one query or predict the next ID. This conflicts with RFC 5452 recommendations for mitigating forged DNS answers.

inet_res is intended for use in trusted network environments and with trusted recursive resolvers. Earlier documentation did not clearly state this deployment assumption, which could lead users to deploy the resolver in environments where spoofed DNS responses are possible.

This vulnerability is associated with program files lib/kernel/src/inet_db.erl and lib/kernel/src/inet_res.erl.

This issue affects OTP from OTP 17.0 before OTP 28.4.2, OTP 27.3.4.10 and OTP 26.2.5.19, corresponding to kernel from 3.0 before 10.6.2, 10.2.7.4 and 9.2.4.11.

Install the Erlang nodes in a trusted network shielded from DNS reply spoofing by firewalls, and configure the inet_res resolver to only talk to trusted recursive name servers within that network.

The application must use inet_res for DNS resolution, either by configuring the lookup method to include dns in the kernel inet configuration, or by calling inet_res functions directly. The default Erlang/OTP configuration uses native OS resolution and is not affected.