Phoenix: Long-poll NDJSON body splitting causes large memory allocation

Summary

An unauthenticated denial-of-service vulnerability in Phoenix's long-poll transport allows a remote client to allocate a large amount of memory with a HTTP request. A handful of concurrent requests can be sufficient to let the node run out of memory.

See also https://cna.erlef.org/cves/CVE-2026-32689.html.

Details

The unoptimised code path exists on the application/x-ndjson POST handling in the LongPoll transport. The endpoint requires only a session token, which any client can obtain by issuing a GET to the same URL with a matching Origin header, so exploitation is unauthenticated.

Impact

Anyone who runs a LiveView app with a public Longpoll socket or uses a Phoenix.Socket with longpoll option. Longpoll has been enabled for newly generated Phoenix projects since Phoenix 1.7.11.