CVE-2026-55953 TLS 1.2 and DTLS client accepts unoffered anonymous cipher suite, bypassing server authentication
An on-path attacker can bypass server authentication in Erlang OTP TLS clients by forcing anonymous cipher suites, enabling man-in-the-middle attacks.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-55953?
CVE-2026-55953 is a critical vulnerability in Erlang OTP, published on 27 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Weakness: CWE-757
Which versions are affected by CVE-2026-55953?
It affects 2 products.
Products include Erlang Erlang\/Otp, Erlang Erlang\/Ssl.
How do I fix CVE-2026-55953?
Upgrade to a fixed version.
Is CVE-2026-55953 being exploited?
There is no known exploitation of CVE-2026-55953 in the wild and no public exploit code.
Other vulnerabilities in Erlang OTP
- CVE-2022-37026 Auth bypass in n/a
- CVE-2016-10253 Memory corruption in n/a
- CVE-2026-49759 Stack buffer overflow in SCTP error cause parsing in inet_drv allows remote VM crash
- CVE-2026-58227 TLS/DTLS denial of service via unbounded recursion on cross-signed peer certificate chain
- CVE-2026-59251 Denial of service via exponential certificate policy tree growth in path validation
Sources
Also known as AZL-94196, DEBIAN-CVE-2026-55953, EEF-CVE-2026-55953, GHSA-c6cw-pr89-w882, UBUNTU-CVE-2026-55953.