CVE-2026-19203 in Jetty
Summary
by MITRE • 09/08/2026
A client may issue specially crafted HTTP/1.1 chunked requests to a Jetty server that cause Jetty and an intermediary proxy to interpret different request boundaries, potentially resulting in HTTP request smuggling.
This is caused by Jetty accepting a lone LF character as a terminator in parts of chunked request parsing. Depending on the Jetty version and configured HTTP compliance mode, this may occur in chunk extensions, chunk data termination, or trailer termination.
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Analysis
by VulDB Data Team • 09/08/2026
The vulnerability described involves a critical flaw in the HTTP/1.1 chunked transfer coding implementation within the Jetty web server, which can lead to HTTP request smuggling attacks. This issue arises from an inconsistency in how Jetty parses the boundaries of chunks compared to other servers or intermediary proxies such as load balancers and reverse proxies. Specifically, Jetty incorrectly accepts a lone Line Feed (LF) character as a valid terminator for chunked data under certain conditions, whereas standard RFC 7230 compliance requires that each chunk be terminated by both a Carriage Return and a Line Feed (CRLF). This deviation from the expected behavior creates an ambiguity in request parsing where Jetty and downstream or upstream proxies may interpret the same stream of bytes differently.
In practical terms, this discrepancy allows an attacker to craft specially designed HTTP/1.1 requests that exploit these divergent interpretations. When such a crafted request passes through an intermediary proxy before reaching the Jetty server, the proxy might parse one set of boundaries while Jetty parses another. This misalignment can result in the smuggling of malicious payloads into subsequent legitimate requests or the injection of unauthorized commands into the application logic. The attack vector relies on the fact that different components in a request chain do not share identical parsing rules for chunked encoding, particularly regarding how they handle line endings within chunks and trailers.
The technical root cause lies in specific segments of Jetty's HTTP parser where it accepts LF-only terminators instead of requiring CRLF sequences. This behavior can manifest in multiple areas depending on the version of Jetty and its configured compliance mode. It may occur during the parsing of chunk extensions, which are optional metadata fields associated with each chunk, or within the termination of chunk data itself. Additionally, vulnerabilities exist in the handling of trailer headers, which appear at the end of a chunked message to provide additional information similar to HTTP response trailers. By exploiting these inconsistencies, an attacker can manipulate the state machine of either the proxy or Jetty, leading to unauthorized access, data leakage, or denial of service conditions depending on the specific context and configuration of the environment.
From a security classification perspective, this vulnerability aligns with CWE-440, which refers to HTTP Request Smuggling via inconsistent boundary parsing. It also maps to MITRE ATT&CK techniques related to injection attacks, specifically those involving web application proxies or gateways that fail to normalize input correctly before forwarding it to backend services. The impact of such a flaw is severe because it undermines the integrity of request routing and processing in modern microservices architectures where multiple layers of proxying are common. Attackers can bypass access controls, steal session cookies, or perform server-side request forgery by leveraging this parsing ambiguity.
Mitigation strategies primarily involve updating Jetty to versions that enforce strict RFC 7230 compliance for chunked transfer coding. Administrators should ensure that their deployment configurations do not allow legacy compatibility modes that relax these requirements unless absolutely necessary and understood in the context of security implications. Furthermore, organizations should implement Web Application Firewalls (WAFs) configured to detect anomalies in HTTP request structures, such as mixed line endings or unusual chunk sizes, which may indicate an attempt at request smuggling. Regular auditing of proxy configurations against backend server behaviors is also recommended to ensure consistent parsing logic across the entire infrastructure chain.