phoenixframework phoenix prior 1.5.15/1.6.17/1.7.24/1.8.9 Socket lib/phoenix/socket.ex handle_in/4 allocation of resources

Summaryinfo

A vulnerability was found in phoenixframework phoenix. It has been rated as problematic. This impacts the function handle_in/4 of the file lib/phoenix/socket.ex of the component Socket Module. The manipulation leads to allocation of resources. This vulnerability is traded as CVE-2026-56811. It is possible to initiate the attack remotely. There is no exploit available.

Detailsinfo

A vulnerability was found in phoenixframework phoenix. It has been rated as problematic. This issue affects the function handle_in/4 of the file lib/phoenix/socket.ex of the component Socket Module. The manipulation with an unknown input leads to a allocation of resources vulnerability. Using CWE to declare the problem leads to CWE-770. The product allocates a reusable resource or group of resources on behalf of an actor without imposing any restrictions on the size or number of resources that can be allocated, in violation of the intended security policy for that actor. Impacted is availability. The summary by CVE is:

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, 1.6.17, 1.7.24, and 1.8.9.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-56811 since 06/23/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details are known, but no exploit is available. MITRE ATT&CK project uses the attack technique T1499 for this issue.

Upgrading to version 1.5.15, 1.6.17, 1.7.24 or 1.8.9 eliminates this vulnerability.

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• phoenixframework

Name

• phoenix

Website

• Product: https://github.com/phoenixframework/phoenix/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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