EEF-CVE-2026-70399

httpd does not enforce the documented default max_clients connection limit

## Summary Allocation of Resources Without Limits or Throttling vulnerability in Erlang/OTP inets httpd allows an unauthenticated remote attacker to cause denial of service by opening and holding open a large number of connections. The `max_clients` option is documented to default to `150`, and the inets hardening guide presents that limit as the first layer of denial-of-service defence, but a server that does not set it explicitly accepts an unlimited number of simultaneous connections. Establishing the connections is sufficient; no valid request and no authentication are required. The accept gate in `httpd_manager:handle_new_connection/4` reads the option with `httpd_util:lookup/2`, which returns `undefined` when the key is absent, rather than the three-argument form carrying the `150` default that the neighbouring `get_ustate/2` uses. Erlang term ordering places every integer before every atom, so the `Count =< Max` guard holds for any connection count and the server never returns `{reject, busy}`. Each accepted connection occupies a worker process and a socket for as long as it is held, driving the node towards process, memory and file descriptor exhaustion. Servers that set `max_clients` explicitly are unaffected, because a configured value is applied as intended. This issue affects OTP from OTP 17.0 before OTP 27.3.4.17, from OTP 28.0 before OTP 28.5.0.6, and from OTP 29.0 before OTP 29.0.6, corresponding to inets from 5.10 before 9.3.2.7, from 9.4 before 9.6.2.3, and from 9.7 before 9.7.2. ## Workarounds * Set `max_clients` explicitly in the httpd configuration, for example `{max_clients, 150}`. An explicitly configured value is read by the same accept gate and applied as intended, so this restores the documented limit without upgrading. * Limit concurrent connections in front of httpd, for example with reverse proxy or firewall connection-count and connection-rate rules. * Restrict access to the server to trusted clients where the deployment allows it. ## Configurations Only servers that do not set `max_clients` in the httpd configuration are affected. A server that sets the option to any integer value applies the limit as intended.