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CVE-2026-70399 – Unauthenticated Denial of Service – Erlang/OTP inets httpd Module

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Summary

CVE-2026-70399 is a resource-exhaustion (CWE-770: Allocation of Resources Without Limits or Throttling) vulnerability in the inets httpd HTTP server component of Erlang/OTP. Due to a coding defect in the connection-acceptance logic, servers that do not explicitly configure the max_clients option silently accept an unlimited number of simultaneous connections instead of enforcing the documented default limit of 150. An unauthenticated remote attacker can exploit this by opening a large number of connections, causing denial of service through exhaustion of server processes, memory, and file descriptors. The issue carries a CVSS v4.0 base score of 8.7 (High).

Technical details

  • Root cause: The connection accept gate in httpd_manager:handle_new_connection/4 retrieves the max_clients setting using httpd_util:lookup/2 (a two-argument lookup with no default), which returns the atom undefined when the option is not explicitly configured, rather than falling back to the documented default of 150.
  • Trigger condition: Because Erlang’s standard term ordering places all integers before all atoms, the comparison Count =< Max evaluates as true for any connection count when Max is undefined. As a result, the server’s {reject, busy} safeguard never triggers, and the connection limit is effectively bypassed on any httpd instance that has not explicitly set max_clients in its configuration.
  • Attack vector: Network-based, no authentication or valid HTTP request required — simply opening and holding open a large number of TCP connections to the httpd listener is sufficient.
  • Impact: High impact to availability only (no confidentiality or integrity impact); sustained connection flooding can exhaust worker processes, memory, and file descriptors, causing the server to become unresponsive or crash.

Affected software

  • Erlang/OTP: versions 17.0–27.3.4.16, 28.0–28.5.0.5, and 29.0–29.0.5
  • inets module (bundled with OTP): versions 5.10–9.3.2.6, 9.4–9.6.2.2, and 9.7–9.7.1
  • Only deployments of the inets httpd server that do not explicitly configure max_clients are vulnerable; servers with an explicit max_clients setting are not affected.

Severity

  • CVSS v4.0 Base Score: 8.7 (High)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade to the patched Erlang/OTP releases: OTP 27.3.4.17, 28.5.0.6, or 29.0.6 (corresponding inets versions 9.3.2.7, 9.6.2.3, or 9.7.2, respectively).
  • If immediate patching is not possible:
    • Explicitly set {max_clients, 150} (or a value appropriate to your capacity) in the httpd server configuration to restore the intended connection-limiting safeguard.
    • Place a reverse proxy or load balancer in front of the httpd service and enforce concurrent-connection limits or rate limiting at that layer.
    • Restrict network access to the httpd listener to trusted clients/networks via firewall rules where feasible.

References

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