Description

Netty (io.netty:netty-handler) versions from 4.2.0.Final through 4.2.16.Final and versions through 4.1.136.Final disable TLS hostname verification on the SslProvider.OPENSSL client path when a plain (non-extended) X509TrustManager is used and Unsafe-based trust-manager wrapping is unavailable (Java 25+). In this configuration the OpenSSL client does not perform hostname verification, allowing a man-in-the-middle attacker to present a certificate issued for a different hostname that is accepted without validation. Fixed in 4.2.17.Final and 4.1.137.Final.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.5",
  "pubDate": "2026-08-22T13:16:39.687Z",
  "pubdate": "2026-08-22T13:16:39.687Z",
  "executiveSummary": "A hostname verification bypass vulnerability exists in Netty (io.netty:netty-handler) affecting versions from 4.2.0.Final through 4.2.16.Final and versions through 4.1.136.Final. The vulnerability arises on the SslProvider.OPENSSL client path when utilizing a plain, non-extended X509TrustManager in environments where Unsafe-based trust-manager wrapping is unavailable, specifically on Java 25 and later. Under these exact conditions, the OpenSSL client completely omits TLS hostname verification.\nThe primary impact of this flaw is the exposure of client communications to sophisticated man-in-the-middle (MitM) attacks. An adversary positioned on the network path between the client and the intended server can intercept traffic by presenting a valid X.509 certificate issued for an entirely arbitrary or different hostname. Because validation logic fails to confirm that the presented certificate matches the expected target host, the client willingly accepts the fraudulent certificate.\nThis vulnerability compromises the cryptographic integrity and confidentiality of TLS sessions established by affected Java applications, potentially exposing sensitive data, credentials, or session tokens to unauthorized interception and tampering without requiring prior authentication or elevated privileges.",
  "technicalDetails": "The root cause of the vulnerability resides in the implementation of the TLS handshake logic within Netty's SslProvider.OPENSSL client execution path. Specifically, when establishing secure connections under Java 25 or newer runtime environments, the mechanism relies on Unsafe-based trust-manager wrapping to enforce proper X.509 trust management and associated post-handshake checks, including hostname verification. When a plain, non-extended X509TrustManager is supplied by the application and Unsafe-based wrapping is unavailable, the internal fallback or initialization routine fails to correctly enforce hostname verification checks against the peer certificate.\nAffected components include io.netty:netty-handler across versions 4.2.0.Final through 4.2.16.Final and all versions up to 4.1.136.Final. The vulnerability is network-exposed, affecting any client-side implementation utilizing the Netty OpenSSL provider configuration under the specified Java runtime constraints. Exploitation requires no prior authentication or administrative privileges, provided the attacker possesses network positioning capable of intercepting, redirecting, or spoofing the target TCP connection.\nThe step-by-step attack flow proceeds as follows: First, a client application initiates a TLS connection to a legitimate remote server using the vulnerable Netty OpenSSL client configuration with a plain X509TrustManager on Java 25+. Second, a man-in-the-middle attacker intercepts the TLS handshake packets. Third, the attacker presents an SSL/TLS certificate issued by a trusted Certificate Authority (CA) but for a completely different, attacker-controlled domain name. Fourth, during the verification phase, the vulnerable Netty client validates the certificate path against trusted CAs but fails to execute the required hostname verification logic due to the missing Unsafe-based trust-manager wrapping context. Fifth, the client erroneously accepts the invalid certificate, completes the TLS handshake, and establishes a secure channel directly with the attacker.\nPost-exploitation impact includes full decryption, inspection, and potential modification of application-layer traffic traversing the compromised TLS session. Attackers can harvest sensitive payloads, execute session hijacking, or perform further protocol-specific attacks against the client application."
}