CVE-2026-56817 in Netty
Summary
by MITRE • 07/22/2026
Netty is a network application framework for development of protocol servers and clients. In versions 4.2.0.Final through 4.2.15.Final and 4.1.0.Final through 4.1.135.Final, any caller that can deliver bytes to a Netty channel pipeline containing `XmlDecoder` can send XML with a `DOCTYPE` declaration to an `AsyncXMLInputFactory` instantiated with no security configuration, leaving DTD and entity handling active depending on Aalto XML async parser behavior and creating conditional XML external entity risk. This issue is fixed in versions 4.1.136.Final and 4.2.16.Final.
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Analysis
by VulDB Data Team • 07/22/2026
The vulnerability in Netty's XmlDecoder component represents a critical security flaw that exposes applications to XML External Entity (XXE) attacks through improper configuration of the underlying AsyncXMLInputFactory parser. This issue affects versions ranging from 4.2.0.Final through 4.2.15.Final and 4.1.0.Final through 4.1.135.Final, creating a persistent risk across multiple release lines of the framework. The vulnerability stems from the default behavior of Aalto XML async parser which maintains DTD and entity handling enabled when instantiated without explicit security configuration, allowing malicious actors to exploit this misconfiguration.
The technical flaw manifests when an attacker can deliver crafted XML data through a Netty channel pipeline containing XmlDecoder. When such XML contains a DOCTYPE declaration, it triggers the unsafe parsing behavior of AsyncXMLInputFactory, potentially enabling attackers to access local files, perform server-side request forgery attacks, or cause denial of service through resource exhaustion. This vulnerability operates at the application layer and can be exploited remotely without authentication, making it particularly dangerous in networked environments where Netty applications process untrusted XML input from external sources.
The operational impact of this vulnerability extends beyond simple data exposure to include potential system compromise and business disruption. Applications using affected Netty versions become susceptible to various attack vectors including sensitive information disclosure through local file inclusion, service availability attacks via resource exhaustion, and potentially code execution depending on the target system configuration. The conditional nature of the risk means that exploitation success may depend on specific Aalto XML parser behaviors and system configurations, yet the potential for abuse remains significant across diverse deployment scenarios.
Security mitigations for this vulnerability require immediate upgrade to Netty versions 4.1.136.Final or 4.2.16.Final where the issue has been resolved through proper configuration of AsyncXMLInputFactory with appropriate security restrictions. Organizations should also implement additional protective measures including input validation, XML schema enforcement, and network segmentation to limit potential attack surfaces. This vulnerability aligns with CWE-611 (Improper Restriction of XML External Entity Reference) and maps to ATT&CK technique T1213.002 (Data from Information Repositories) as it enables unauthorized access to system resources through malformed XML processing. Network administrators should monitor for exploitation attempts through unusual XML processing patterns and implement proper logging to detect potential XXE attacks targeting affected Netty applications.