CVE-2026-56822 in Netty
Summary
by MITRE • 07/29/2026
Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.136.Final and 4.2.16.Final, the OcspServerCertificateValidator forwards the SslHandshakeCompletionEvent before the asynchronous OCSP validation completes. This allows the client's downstream handlers to send sensitive application data (e.g., HTTP requests) to a revoked server before the channel is closed by the OCSP check. n io.netty.handler.ssl.ocsp.OcspServerCertificateValidator#userEventTriggered, when an SslHandshakeCompletionEvent is received, the validator immediately calls ctx.fireUserEventTriggered(evt). It then initiates an asynchronous OCSP query using OcspClient.query. Because the handshake completion event is forwarded immediately, downstream handlers in the client's pipeline are notified that the TLS handshake is successful. They may then begin reading and processing incoming application data or sending outgoing data. If the OCSP response later indicates the server's certificate is REVOKED, the validator closes the channel, but by this time, the client may have already leaked sensitive data to a revoked server or processed malicious responses from it. This issue has been fixed in versions 4.1.136.Final and 4.2.16.Final.
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Analysis
by VulDB Data Team • 07/29/2026
The vulnerability described represents a critical race condition in Netty's SSL certificate validation mechanism that directly impacts the security of network applications relying on asynchronous event handling. This flaw exists in the OcspServerCertificateValidator component where the framework fails to properly synchronize the completion of OCSP validation with the notification of successful SSL handshakes. The issue stems from the immediate forwarding of SslHandshakeCompletionEvent before asynchronous OCSP validation completes, creating a temporal window where applications can operate under false security assumptions.
The technical implementation flaw occurs within the userEventTriggered method of OcspServerCertificateValidator at io.netty.handler.ssl.ocsp.OcspServerCertificateValidator#userEventTriggered. When an SslHandshakeCompletionEvent is received, the validator executes ctx.fireUserEventTriggered(evt) immediately without waiting for the asynchronous OCSP query initiated through OcspClient.query to complete. This design pattern violates fundamental security principles by allowing downstream handlers to proceed with normal application logic before certificate status validation is finalized. The asynchronous nature of the OCSP query creates a scenario where the certificate validation process can complete after the handshake notification has already been sent, leading to a dangerous state where applications operate with revoked certificates.
The operational impact of this vulnerability extends beyond simple certificate validation failure and represents a sophisticated attack vector that can lead to data exfiltration and man-in-the-middle attacks. When downstream handlers receive the SslHandshakeCompletionEvent notification, they assume the TLS connection is secure and proceed with sending sensitive application data such as HTTP requests to what they believe is a valid server. However, if the OCSP validation later determines the certificate has been revoked, the channel is closed but not before potentially compromising sensitive information that was already transmitted. This vulnerability specifically affects applications where network security is paramount, including financial services, healthcare systems, and enterprise applications handling confidential data.
This vulnerability maps directly to CWE-215, which addresses information exposure through improper error handling and validation timing issues in security-critical components. The flaw also aligns with ATT&CK technique T1071.004 for application layer protocol: DNS where attackers might exploit the window of opportunity between handshake completion and certificate revocation detection to establish covert communication channels. Organizations using affected Netty versions face significant risk of data leakage and security breaches, particularly in environments where certificate validation is critical for maintaining trust boundaries.
The recommended mitigation strategy involves upgrading to Netty versions 4.1.136.Final or 4.2.16.Final where the fix ensures proper synchronization between SSL handshake completion notification and OCSP validation completion. Additionally, security teams should implement monitoring for anomalous certificate validation patterns and consider implementing additional layers of security such as certificate pinning to provide defense-in-depth. Organizations should also conduct thorough security testing of their network applications to identify any custom handlers that might be vulnerable to similar race condition scenarios in their own code implementations, particularly those handling asynchronous SSL events and certificate validation workflows.