DTLS Listener Crash via TOCTOU Race Condition in Erlang/OTP ssl

Executive Summary

CVE-2026-55950 is a time-of-check time-of-use (TOCTOU) race condition vulnerability in the Erlang/OTP ssl library's dtls_packet_demux module. This module handles routing of incoming UDP datagrams to the correct DTLS connection handler on a server listener.

The vulnerability occurs when a DTLS client rapidly reconnects from the same source IP address and port, sending multiple ClientHello messages in quick succession. This triggers a race condition in the internal key-value store of the demux process, causing it to crash with a {key_exists, {old, Client}} error.

Because the dtls_packet_demux gen_server process is shared among all DTLS sessions on that listener, its crash terminates all active DTLS sessions, not just the attacker's. The attack requires no authentication or completed handshake and can be repeated indefinitely to cause persistent denial of service.

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Impact Analysis

This vulnerability can cause a denial of service (DoS) condition on DTLS servers using Erlang/OTP. An unauthenticated remote attacker can crash the shared dtls_packet_demux process by sending multiple rapid ClientHello messages from the same source IP and port.

The crash of this shared process immediately terminates all active DTLS sessions on the listener, disrupting service for all connected clients, not just the attacker.

Because the attack requires no credentials or completed handshake, it can be executed from untrusted networks and repeated indefinitely, resulting in persistent denial of service and loss of availability.

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Detection Guidance

This vulnerability involves a race condition in the Erlang/OTP DTLS server's dtls_packet_demux gen_server process that crashes when multiple ClientHello messages are sent rapidly from the same source IP and port. Detection would involve monitoring for repeated crashes or restarts of the dtls_packet_demux process or observing denial of service symptoms affecting all active DTLS sessions on a listener.

Since the vulnerability is triggered by rapid reconnections sending multiple ClientHello messages from the same source address and port, network monitoring tools could be used to detect unusual bursts of UDP datagrams containing ClientHello messages to the DTLS server.

However, the provided resources do not include specific detection commands or scripts.

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Mitigation Strategies

The primary mitigation is to upgrade Erlang/OTP and the ssl library to the patched versions that fix this vulnerability. The issue is resolved in OTP versions 29.0.3, 28.5.0.3, and 27.3.4.14, and corresponding ssl versions 11.7.3, 11.6.0.3, and 11.2.12.10.

Applying the security fix involves updating the dtls_packet_demux.erl file where the function call was changed from kv_insert to kv_enter to properly handle the race condition.

Until the upgrade can be applied, consider monitoring and limiting rapid repeated DTLS ClientHello messages from the same source IP and port to reduce the risk of triggering the denial of service.

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Compliance Impact

The vulnerability causes a denial of service (DoS) by crashing all active DTLS sessions on a listener, which leads to loss of availability of the affected service.

Loss of availability can impact compliance with standards and regulations such as GDPR and HIPAA, which require maintaining availability and integrity of systems that process sensitive or personal data.

However, the provided information does not explicitly describe direct effects on data confidentiality or integrity, nor does it specify compliance impacts with these regulations.

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