CVE-2026-56811
HighCVSS 7.5Exploitation Probability (EPSS)
Low risk30th percentile - higher than 30% of all known CVEs
Summary
A vulnerability in the Phoenix.Socket module of the Phoenix framework allows an unauthenticated attacker to perform a DoS attack by sending a large number of phx_join messages over a single WebSocket or LongPoll connection. This exhausts the BEAM virtual machine process limit, blocking legitimate traffic across the entire node.
Risk Assessment
The organization is at risk of complete service disruption because a single attacker can cause a denial of service at the BEAM node level without authentication.
Recommendation
Immediately update the Phoenix framework to version 1.5.15, 1.6.17, 1.7.24, or 1.8.9 (depending on the branch used) and configure the :max_channels_per_transport option to the default value of 100.
Original NVD description (English source)
Allocation of Resources Without Limits or Throttling vulnerability in phoenixframework phoenix (Phoenix.Socket module) allows an unauthenticated attacker to cause a denial of service against any endpoint that mounts a Phoenix socket with a reachable channel transport (WebSocket or LongPoll). This vulnerability is associated with program files lib/phoenix/socket.ex and program routine 'Elixir.Phoenix.Socket':handle_in/4. Phoenix transports do not limit the number of channels that a single transport process may join. Every phx_join message a client sends over one connection starts a persistent channel process, and the socket process accepts an unbounded number of them. A single unauthenticated client can therefore open one WebSocket or LongPoll connection and stream a large number of phx_join messages, spawning hundreds of thousands of channel processes over that one connection and eventually reaching the BEAM maximum process limit. Once the process table is exhausted the virtual machine can no longer start new processes, denying service to legitimate traffic across the whole node. Because the amplification happens inside a single connection, network-layer connection caps and rate limiting do not mitigate it. The fix adds a :max_channels_per_transport option (default 100) that bounds the number of channels a single transport process can join, forcing abusive clients to open many connections instead, where external load balancers and reverse proxies can throttle them. This issue affects phoenix: from 0.11.0 before 1.5.15, from 1.6.0-rc.0 before 1.6.17, from 1.7.0-rc.0 before 1.7.24, and from 1.8.0-rc.0 before 1.8.9.