CVE-2026-56811 Phoenix transports do not limit channel joins per connection, enabling process-exhaustion denial of service

What is CVE-2026-56811?

CVE-2026-56811 is a high-severity denial of service vulnerability in phoenixframework phoenix, published on 7 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

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.

Weakness: CWE-770

Which versions are affected by CVE-2026-56811?

9 open source packages ship an affected version, including phoenix. It affects 1 product.

Products include Phoenixframework Phoenix.

How do I fix CVE-2026-56811?

Upgrade phoenix to a fixed version for your release line: 1.5.15, 1.6.17, 1.7.24 or 1.8.9.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-56811 being exploited?

There is no known exploitation of CVE-2026-56811 in the wild and no public exploit code.

Other vulnerabilities in phoenixframework phoenix

• CVE-2022-42975 Phoenix before 1.6.14 mishandles check_origin wildcarding High 7.5
• CVE-2026-56812 Phoenix: Presence keys colliding with `Object.prototype` members break existence checks Medium 6.3
• CVE-2017-1000163 Phoenix Arbitrary URL Redirect Medium 6.1

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as EEF-CVE-2026-56811, GHSA-6983-jfq8-485w.