Weakness Type (CWE)

CWE-757 — CWE-757 Selection of Less-Secure Algorithm During Negotiation ('Algorithm Downgrade')

CAPEC

CVSS 4.0 Score

9.1

CRITICAL

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

Vulnerability 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 29.0.4, OTP 28.5.0.4 and OTP 27.3.4.15, corresponding to ssl from 5.3.4 before 11.7.4, 11.6.0.4 and 11.2.12.11.

Affected

pkg:otp/ssl

Module Source File Routine
tls_handshake src/tls_handshake.erl tls_handshake:hello/5
ssl_handshake src/ssl_handshake.erl ssl_handshake:handle_server_hello_extensions/9
ssl ssl:connect/2
ssl:connect/3
ssl:connect/4
Status Type Version Changes / Fixed in
affected otp ⓘ 5.3.4
  • unaffected at 11.7.4
  • unaffected at 11.6.0.4
  • unaffected at 11.2.12.11

pkg:github/erlang/otp

Module Source File Routine
tls_handshake lib/ssl/src/tls_handshake.erl tls_handshake:hello/5
ssl_handshake lib/ssl/src/ssl_handshake.erl ssl_handshake:handle_server_hello_extensions/9
ssl ssl:connect/2
ssl:connect/3
ssl:connect/4
Status Type Version Changes / Fixed in
affected otp ⓘ 17.0
affected git ⓘ 84adefa331

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.

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.

References

Credits

  • Finder: Jonatan Männchen / EEF
  • Remediation developer: Ingela Anderton Andin
  • Remediation reviewer: Jakub Witczak

CVE record as JSON:  GET /cves/CVE-2026-55953.json
OSV record as JSON:  GET /osv/EEF-CVE-2026-55953.json