# CVE-2026-55950: DTLS listener crash via race condition in dtls_packet_demux causes denial of service for all sessions

Status: published. Published: 2026-07-02. Updated: 2026-11-11.

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

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: EEF) — https://github.com/erlang/otp/commit/e44d2bf01c4473ef2ea7f09e3523cf96de6e4a04

## Affected products and versions

- Erlang / OTP (otp): ≥ 25.3; < * (affected, otp); unaffected at 27.3.4.14; unaffected at 28.5.0.3; unaffected at 29.0.3. Source: CNA: EEF
- Erlang / OTP (ssl): ≥ 10.9; < * (affected, otp); unaffected at 11.2.12.10; unaffected at 11.6.0.3; unaffected at 11.7.3. Source: CNA: EEF
- Erlang / OTP (erlang/otp): ≥ 44dcb4c3d900777493ce2a6129f451aa475811f9; < e44d2bf01c4473ef2ea7f09e3523cf96de6e4a04 (affected, git). Source: CNA: EEF

## Severity and prioritization

- CVSS 4.0: 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; source: CNA: EEF

- CISA KEV: No listing in collected sources

- EPSS: 0.38% as of 2026-11-10; percentile 32.22%

- SSVC: exploitation none; automatable yes; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-367

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/erlang/otp/security/advisories/GHSA-hwfc-5hf4-gvr3 (CNA: EEF; vendor-advisory, related)

- https://cna.erlef.org/cves/CVE-2026-55950.html (CNA: EEF; related)

- https://osv.dev/vulnerability/EEF-CVE-2026-55950 (CNA: EEF; related)

- https://www.erlang.org/doc/system/versions.html#order-of-versions (CNA: EEF; x_version-scheme)

- https://github.com/erlang/otp/commit/44dcb4c3d900777493ce2a6129f451aa475811f9 (CNA: EEF; related)

- https://github.com/erlang/otp/commit/e44d2bf01c4473ef2ea7f09e3523cf96de6e4a04 (CNA: EEF; patch)

## Provenance

- CNA: EEF: https://www.cve.org/CVERecord?id=CVE-2026-55950; retrieved 2026-11-11; source updated 2026-11-11

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-55950; retrieved 2026-11-11; source updated Not reported

Revision: c5e2c88c08978c34e500747300d7f5366e17ced2e64b3bfbe71a0681fb5823af

Canonical: https://y2.dev/cve/CVE-2026-55950

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