Long-poll NDJSON body splitting causes unbounded memory allocation in Phoenix

Summary

Allocation of Resources Without Limits or Throttling vulnerability in phoenixframework phoenix allows a denial of service via the long-poll transport's NDJSON body handling. In 'Elixir.Phoenix.Transports.LongPoll':publish/4, when a POST request is received with Content-Type: application/x-ndjson, the request body is split on newline characters using String.split/2 with no limit on the number of resulting segments. An attacker can send a body consisting entirely of newline bytes, causing a 1:1 amplification into a list of empty binaries — a 1 MB body produces approximately one million list elements, an 8 MB body approximately 8.4 million. Each element is then walked by Enum.map, materializing another list of the same size. This exhausts BEAM memory and schedulers, crashing the node and terminating all active sessions. A session token required to reach the vulnerable endpoint is freely obtainable by any client via an unauthenticated GET request to the same URL with a matching Origin header, making this attack effectively unauthenticated. This issue affects phoenix: from 1.7.0 before 1.7.22 and 1.8.6.

Timeline

1. May 5, 2026, 3:17 PM UTCCVE published
2. May 5, 2026, 6:37 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. May 7, 2026, 4:25 AM UTCRecord last updated

Affected versions

• phoenix

  From 1.7.0 · fixed in 1.7.22; From 1.8.0 · fixed in 1.8.6

• phoenixframework/phoenix

  Version range not reported

References 5