SSH daemon allocates unbounded idle session channels, bypassing max_channels

Allocation of Resources Without Limits or Throttling vulnerability in Erlang/OTP ssh allows an authenticated remote attacker to exhaust node memory by repeatedly opening session channels that are never assigned a handler.

The "session" clause of ssh_connection:handle_msg/4 checks only minimal_remote_max_packet_size before calling setup_session/5, which unconditionally builds a #channel{} record and stores it in the ETS channel cache. The max_channels daemon option is consulted only by ssh_channel_sup:max_num_channels_not_exceeded/2, which counts supervisor children, so a channel that never gets a shell, exec, or subsystem handler is invisible to the limit and setting the option to a finite value does not mitigate the attack. RFC 4254 section 5.1 permits many session channels per connection, and each record costs only a few hundred bytes, so a single authenticated connection can accumulate channels until the node runs out of memory and the emulator terminates, affecting every application on it. No file contents, credentials, or write access are obtainable.

This issue affects OTP from OTP 18.1.2 before OTP 27.3.4.18, OTP 28.5.0.7, and OTP 29.1.1, corresponding to ssh from 4.1.1 before 5.2.11.13, 5.5.2.6, and 6.0.6. Whether OTP before OTP 18.1.2, corresponding to ssh before 4.1.1, is affected is unknown.

No option bounds the number of idle session channels, so the following reduce exposure without eliminating the issue.

• Restrict network access to the SSH daemon to trusted clients. Exploitation requires a successfully authenticated session, so limiting who can authenticate is the most effective control available.
• Set the max_sessions daemon option to a finite value to bound the number of concurrent connections. This limits how many connections can mount the attack in parallel, though a single connection still suffices to exhaust memory.
• Apply operating-system or firewall connection-rate and connection-count limits per source address.
• Monitor node memory externally and restart the emulator when it exceeds a threshold.

Setting max_channels is not a workaround: idle session channels bypass that check entirely.