CVE-2026-55953
Received Received - Intake

TLS Downgrade to Anonymous Cipher Suite in Erlang/OTP

Publication date: 2026-07-27

Last updated on: 2026-07-27

Assigner: EEF

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.

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Meta Information

Published
2026-07-27
Last Modified
2026-07-27
Generated
2026-07-27
AI Q&A
2026-07-27
EPSS Evaluated
N/A
NVD
CVE-2026-55953
EUVD
EUVD-2026-49323

Affected Vendors & Products

Showing 10 associated CPEs
Vendor Product Version / Range
erlang otp From 17.0 (inc) to 29.0.4 (exc)
erlang ssl From 5.3.4 (inc) to 11.7.4 (exc)
erlang ssl From 11.6.0.4 (inc)
erlang ssl From 11.2.12.11 (inc)
erlang otp to 29.0.4 (exc)
erlang otp 28.5.0.4
erlang otp 27.3.4.15
erlang ssl to 11.7.4 (exc)
erlang ssl 11.6.0.4
erlang ssl 11.2.12.11

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-757 A protocol or its implementation supports interaction between multiple actors and allows those actors to negotiate which algorithm should be used as a protection mechanism such as encryption or authentication, but it does not select the strongest algorithm that is available to both parties.

Attack-Flow Graph

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Executive Summary

This vulnerability is in the Erlang/OTP SSL/TLS implementation. It allows an on-path attacker to bypass certificate validation by forcing the client to use an anonymous cipher suite not offered in the initial handshake. The client does not verify if the server-selected cipher suite was among those offered, enabling the attacker to intercept and modify all traffic without detection.

Detection Guidance

To detect this vulnerability, check if your Erlang/OTP SSL/TLS client is using TLS versions prior to 1.3. Inspect the cipher suites offered by the client and compare them with those selected by the server during the handshake. Use tools like Wireshark to capture and analyze TLS handshake traffic for mismatches between ClientHello and ServerHello cipher suites.

Impact Analysis

An attacker could read, modify, or inject malicious data into your communications with the server. Sensitive information like passwords or personal data could be exposed. The attacker bypasses certificate checks, so no warnings would appear even if the server's identity is fake.

Compliance Impact

This vulnerability undermines data confidentiality and integrity, which are core requirements for GDPR and HIPAA. Unauthorized interception of data could lead to non-compliance, legal penalties, and loss of trust. Encrypted communications must ensure proper authentication to meet these standards.

Mitigation Strategies

Upgrade Erlang/OTP to a patched version: OTP 29.0.4 or later, OTP 28.5.0.4 or later, or OTP 27.3.4.15 or later. Ensure ssl versions 11.7.4, 11.6.0.4, or 11.2.12.11 are installed.

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