Denial of Service in Phoenix Framework via Unbounded Channel Joins

Executive Summary

This vulnerability exists in the Phoenix framework's Phoenix.Socket module, where there is no limit on the number of channels a single transport process (such as a WebSocket or LongPoll connection) can join.

An unauthenticated attacker can exploit this by opening one connection and sending a large number of channel join requests (phx_join messages), spawning hundreds of thousands of channel processes.

This unbounded creation of processes exhausts the Erlang VM's process table, preventing new processes from starting and causing a denial of service (DoS) for legitimate users.

Network-layer protections like connection caps or rate limiting do not mitigate this because the attack happens within a single connection.

The vulnerability affects Phoenix versions from 0.11.0 before 1.5.15, 1.6.0-rc.0 before 1.6.17, 1.7.0-rc.0 before 1.7.24, and 1.8.0-rc.0 before 1.8.9.

The fix introduces a configuration option :max_channels_per_transport (default 100) to limit how many channels a single transport process can join, forcing abusive clients to open multiple connections where external throttling can be applied.

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Impact Analysis

This vulnerability can cause a denial of service (DoS) on your Phoenix-based application by exhausting server resources.

An attacker can spawn hundreds of thousands of channel processes over a single connection, reaching the maximum process limit of the Erlang VM.

Once the process table is exhausted, the virtual machine cannot start new processes, which disrupts legitimate traffic and causes the application or system to crash or become unresponsive.

Because the attack happens within a single connection, typical network-layer protections like connection caps or rate limiting are ineffective.

This impacts system availability and can lead to downtime or degraded service for users.

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Detection Guidance

This vulnerability involves an unauthenticated client opening a single WebSocket or LongPoll connection and sending a large number of phx_join messages, spawning many channel processes until the BEAM process limit is reached. Detection involves monitoring for unusually high numbers of channel join requests or an excessive number of channel processes spawned from a single connection.

Since the attack happens within a single connection and network-layer connection caps or rate limiting do not mitigate it, detection should focus on application-level logs and metrics.

You can look for log entries containing warnings about "too many channels joined," which are generated when the limit is exceeded in patched versions.

Suggested commands or approaches include:

• Check Phoenix application logs for warnings or errors related to channel joins exceeding limits.
• Use BEAM VM tools (such as observer or :erlang.system_info(:process_count)) to monitor the number of processes and detect spikes.
• Monitor WebSocket or LongPoll connections for abnormal numbers of phx_join messages per connection.
• If using a reverse proxy or load balancer, enable detailed logging to identify connections with excessive channel join attempts.

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Mitigation Strategies

The primary mitigation is to upgrade Phoenix to a patched version that includes the :max_channels_per_transport option, which limits the number of channels a single transport process can join (default 100). This prevents unbounded channel creation and resource exhaustion.

If upgrading immediately is not possible, consider the following workarounds:

• Use a reverse proxy or load balancer to limit the number of channel-join frames per connection.
• Disable unnecessary transports (e.g., LongPoll) if not required by your application.
• Configure external rate limiting on multiple connections, since the fix forces abusive clients to open many connections.

Lowering the BEAM process limit is not recommended as it does not mitigate the issue and may worsen it.

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Compliance Impact

This vulnerability primarily causes a denial of service (DoS) by exhausting server resources through unbounded channel creation in the Phoenix framework. It impacts system availability by allowing an unauthenticated attacker to disrupt legitimate traffic.

While the CVE description and resources do not explicitly mention compliance with standards such as GDPR or HIPAA, denial of service attacks can indirectly affect compliance by causing service outages, which may violate availability requirements in these regulations.

However, there is no direct information provided about how this vulnerability impacts data confidentiality, integrity, or specific regulatory compliance obligations.

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