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How CVE-2026-50010 Works
This vulnerability in Netty’s TLS client arises from a silent failure in hostname verification when a custom `X509TrustManager` is used. In Netty, the default `SslContextBuilder` for a client automatically enables hostname verification by setting the `endpointIdentificationAlgorithm` to “HTTPS”. However, a security issue occurs when a developer supplies their own plain `X509TrustManager` via .trustManager(somePlainX509TrustManager), a common practice for custom certificate validation.
The root cause lies in how Netty handles this custom trust manager. Internally, `SimpleTrustManagerFactory` wraps the user-provided plain `X509TrustManager` in a class called X509TrustManagerWrapper. This wrapper extends `X509ExtendedTrustManager` but only partially implements its key method. The `checkServerTrusted` method that accepts three arguments (including the SSLEngine) is implemented to discard the `SSLEngine` and merely call the two-argument version of the delegate trust manager.
This creates a critical problem. Because the object is now of type X509ExtendedTrustManager, Netty’s own security wrappers, like `OpenSslX509TrustManagerWrapper` and SunJSSE’s internal AbstractTrustManagerWrapper, assume it already handles endpoint identification and will not wrap it again to add hostname verification. As a result, the default “HTTPS” endpoint identification algorithm is never applied.
The final consequence is severe: a Netty client built with a custom `X509TrustManager` performs no hostname verification at all. This allows a remote attacker to present a certificate for an unexpected hostname, effectively impersonating any legitimate server and enabling man-in-the-middle (MitM) attacks to intercept and manipulate traffic.
DailyCVE Form:
Platform: io.netty:netty-handler
Version: 4.1.0-4.1.134,4.2.0-4.2.14
Vulnerability : Hostname Verification Bypass
Severity: High
date: Jun 5, 2026
Prediction: Jun 12, 2026
What Undercode Say:
Vulnerable configuration detection:
Check for usage of custom TrustManager in source code grep -r "SslContextBuilder.forClient()" --include=".java" | grep "trustManager"
Proof-of-Concept (PoC) for verification:
Use openssl s_client to simulate a MitM attack openssl s_client -connect vulnerable-host.com:443 -servername malicious.com
Developer logs to check for missing validation:
Look for absence of CertificateException in app logs grep "CertificateException" /var/log/application.log
Exploit:
An attacker on the network can intercept TLS traffic between a vulnerable Netty client and a legitimate server. By presenting any valid certificate (e.g., self-signed or for a different domain), the client will accept it without checking if the certificate’s Subject Alternative Name (SAN) or Common Name (CN) matches the target hostname. This allows session hijacking and sensitive data exposure.
Protection:
- Immediate Update: Upgrade Netty to version `4.1.135.Final` or `4.2.15.Final` which contain the patch.
- Patch Details: In the patched versions, `X509TrustManagerWrapper` was updated to properly delegate hostname verification to the underlying trust manager.
- Mitigation: If upgrading is not possible, avoid using a custom `X509TrustManager` via
.trustManager(). Instead, use Netty’s default trust management or ensure any custom implementation is a fullX509ExtendedTrustManager.
Impact:
- Confidentiality Breach (High): An attacker can perform MitM attacks to decrypt and read all TLS traffic.
- Integrity Compromise: Data in transit can be altered.
- Authentication Bypass: The client cannot verify the server’s identity, enabling spoofing attacks.
- CVSS Score: 7.5 (High) (CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N)
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Sources:
Reported By: github.com
Extra Source Hub:
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