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Vulnerability · Published Jul 27, 2026 · Assigned by EEF

CVE-2026-55953TLS 1.2 and DTLS client accepts unoffered anonymous cipher suite, bypassing server authentication

Critical 9.1

In plain English

A critical-severity security flaw in OTP from Erlang. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 14% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 4.0)9.1Critical
Exploit odds (30 days)0.2%top 85.7% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official description

The Erlang/OTP ssl TLS 1.2 (and earlier) and DTLS client does not verify that the cipher suite selected by the server in ServerHello was among the suites offered by the client in ClientHello. The client-side tls_handshake:hello/5 handler validates the negotiated protocol version and the downgrade sentinel but hands the server-chosen suite directly to ssl_handshake:handle_server_hello_extensions/9, which installs it without a membership check. The TLS 1.3 client path performs this check (per RFC 8446), so it is not affected. An on-path attacker between the client and the intended server can respond with a ServerHello selecting an anonymous key exchange suite such as TLS_DH_anon_* or TLS_ECDH_anon_* that the client never offered. Anonymous suites do not require the server to present a certificate, so the entire verify_peer and cacerts configuration is bypassed: the attacker completes the handshake with its own ephemeral parameters, no certificate is validated, no hostname is checked, and ssl:connect returns {ok, Socket}. All subsequent application traffic is readable and modifiable by the attacker. This issue affects OTP from OTP 17.0 before OTP 27.3.4.15, from OTP 28.0 before OTP 28.5.0.4, and from OTP 29.0 before OTP 29.0.4, corresponding to ssl from 5.3.4 before 11.2.12.11, from 11.3 before 11.6.0.4, and from 11.7 before 11.7.4. Whether OTP before OTP 17.0, corresponding to ssl before 5.3.4, is affected is unknown.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Debian:12erlangno fix listedDEBIAN-CVE-2026-55953
Debian:13erlang1:27.3.4.1+dfsg-1+deb13u3DEBIAN-CVE-2026-55953
Debian:14erlang1:29.0.4+dfsg-1DEBIAN-CVE-2026-55953
Ubuntu:22.04:LTSerlang1:24.2.1+dfsg-1ubuntu0.7UBUNTU-CVE-2026-55953
Ubuntu:22.04:LTSerlang1:24.2.1+dfsg-1ubuntu0.7USN-8827-1
Ubuntu:24.04:LTSerlang1:25.3.2.8+dfsg-1ubuntu4.7USN-8827-1
Ubuntu:24.04:LTSerlang1:25.3.2.8+dfsg-1ubuntu4.7UBUNTU-CVE-2026-55953
Ubuntu:26.04:LTSerlang1:27.3.4.6+dfsg-1ubuntu0.1USN-8827-1
Ubuntu:26.04:LTSerlang1:27.3.4.6+dfsg-1ubuntu0.1UBUNTU-CVE-2026-55953

Affected products (as reported)

VendorProductVersions
Erlang OTP 17.0 – before 27.3.4.1528.0 – before 28.5.0.429.0 – before 29.0.4
Erlang OTP 5.3.4 – before 11.2.12.1111.3 – before 11.6.0.411.7 – before 11.7.4
Erlang OTP 84adefa331c4159d432d22840663c38f155cd4c1 – before *

Weakness: CWE-757 · Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

References