CVE-2026-55953

Ein entfernter, anonymer Angreifer kann mehrere Schwachstellen in Erlang/OTP ausnutzen, um einen Denial of Service Angriff durchzuführen, beliebigen Code auszuführen, Sicherheitsmaßnahmen zu umgehen sowie Daten zu manipulieren oder offenzulegen.

## Summary 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. ## Workarounds - Restrict client connections to TLS 1.3 only by setting `versions` to `['tlsv1.3']` in the client's `ssl` options. The TLS 1.3 client path performs the required cipher suite membership check and is not affected. ## Configurations The vulnerability only affects TLS clients using TLS 1.2 (or earlier) or DTLS. TLS 1.3 client connections perform the cipher suite membership check and are not affected. Servers are not affected. Exploitation additionally requires an on-path (adversary-in-the-middle) attacker between the client and the intended server.