CVE-2019-20444 in Netty
Summary
by MITRE
HttpObjectDecoder.java in Netty before 4.1.44 allows an HTTP header that lacks a colon, which might be interpreted as a separate header with an incorrect syntax, or might be interpreted as an "invalid fold."
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Analysis
by VulDB Data Team • 07/02/2025
The vulnerability identified as CVE-2019-20444 affects Netty's HttpObjectDecoder.java component and represents a significant parsing flaw in HTTP header processing that can lead to various security implications. This issue exists in Netty versions prior to 4.1.44 and stems from the library's handling of malformed HTTP headers that lack the required colon separator between header names and values. The vulnerability arises when the HTTP parser encounters headers without colons, creating ambiguity in how these malformed headers are interpreted by the application. This parsing inconsistency can result in unexpected behavior during HTTP request processing, potentially allowing attackers to manipulate header interpretation and affect application logic.
The technical nature of this vulnerability falls under CWE-129, which addresses improper handling of malformed input, and specifically relates to improper validation of HTTP header syntax. When an HTTP header lacks a colon separator, the Netty library's HttpObjectDecoder.java implementation may interpret the header in unpredictable ways, either treating it as a separate header with incorrect syntax or as an "invalid fold" that could be processed differently than intended. This behavior creates potential attack vectors where malicious actors could craft HTTP requests with malformed headers to exploit the inconsistent parsing logic, potentially leading to header injection attacks or manipulation of application state. The vulnerability demonstrates a classic example of insufficient input validation in network protocol handling components.
The operational impact of CVE-2019-20444 extends beyond simple parsing errors and can affect applications that rely on Netty for HTTP processing, particularly those that perform header-based security checks or authentication logic. When applications process HTTP requests through vulnerable Netty versions, they may inadvertently accept or misinterpret headers that should be rejected as malformed, potentially allowing unauthorized access or bypass of security controls. The vulnerability can be exploited in scenarios where header values are used for access control decisions, authentication token validation, or request routing logic. Attackers could leverage this weakness to inject additional headers or manipulate existing header values, potentially leading to privilege escalation, data leakage, or service disruption. The impact is particularly concerning in web applications, API gateways, and reverse proxies that use Netty as their underlying HTTP processing framework.
Mitigation strategies for CVE-2019-20444 primarily involve upgrading to Netty version 4.1.44 or later, which contains the necessary fixes to properly handle malformed HTTP headers. Organizations should conduct thorough testing of their applications after applying the upgrade to ensure compatibility and verify that the fix resolves the parsing behavior without introducing regressions. Additionally, implementing proper input validation at the application level can provide an additional layer of defense by sanitizing HTTP headers before they reach the Netty processing layer. Network-level protections such as web application firewalls can also help detect and block malformed HTTP requests that exploit this vulnerability. Security teams should also monitor their systems for unusual header processing behavior and implement logging mechanisms to detect potential exploitation attempts. The vulnerability aligns with ATT&CK technique T1071.004 for application layer protocol manipulation and represents a common pattern of protocol parsing flaws that can be exploited for various attack vectors including injection attacks and authentication bypasses.