Weakness Type (CWE)

CWE-367 — CWE-367 Time-of-check Time-of-use (TOCTOU) Race Condition

CAPEC

CVSS 4.0 Score

8.7

HIGH

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

Vulnerability description

Time-of-check Time-of-use (TOCTOU) race condition vulnerability in Erlang/OTP ssl (dtls_packet_demux module) allows an unauthenticated remote attacker to crash all active DTLS sessions on a listener.

A DTLS server listener uses a single shared dtls_packet_demux gen_server process to route incoming UDP datagrams to the correct connection handler. When a DTLS client reconnects rapidly from the same source address and port (sending multiple ClientHello messages in quick succession), a race condition in the demux's internal gb_trees key-value store causes a {key_exists, {old, Client}} crash, terminating the demux process. Because the demux is shared across all DTLS associations on that listener, its crash immediately kills every active DTLS session, not just the attacker's.

The attack is pre-authentication: the attacker only needs to send UDP datagrams containing valid ClientHello messages from the same source IP and port before the intermediate DOWN monitor message is processed by the gen_server. No credentials, no completed handshake, and no special configuration are required, and the crash can be repeated indefinitely to create a persistent denial of service for all clients of that listener.

This vulnerability is associated with program file lib/ssl/src/dtls_packet_demux.erl.

This issue affects OTP from OTP 25.3 before 29.0.3, 28.5.0.3, and 27.3.4.14 corresponding to ssl from 10.9 before 11.7.3, 11.6.0.3, and 11.2.12.10.

Affected

pkg:otp/ssl

Module Source File Routine
dtls_packet_demux src/dtls_packet_demux.erl dtls_packet_demux:handle_call/3
dtls_packet_demux:handle_info/2
dtls_packet_demux:new_connection/2
Status Type Version Changes / Fixed in
affected otp ⓘ 10.9
  • unaffected at 11.7.3
  • unaffected at 11.6.0.3
  • unaffected at 11.2.12.10

pkg:github/erlang/otp

Module Source File Routine
dtls_packet_demux lib/ssl/src/dtls_packet_demux.erl dtls_packet_demux:handle_call/3
dtls_packet_demux:handle_info/2
dtls_packet_demux:new_connection/2
Status Type Version Changes / Fixed in
affected otp ⓘ 25.3
affected git ⓘ 44dcb4c3d9 < e44d2bf01c

Configurations

The application must accept incoming DTLS connections via ssl:listen/2 with a UDP-based transport. TLS-only deployments are not affected.

References

Credits

  • Finder: Lukas Backström
  • Remediation developer: Ingela Anderton Andin
  • Remediation reviewer: Dan Gudmundsson

CVE record as JSON:  GET /cves/CVE-2026-55950.json
OSV record as JSON:  GET /osv/EEF-CVE-2026-55950.json