CVE-2026-73117 in Juneau
Summary
by MITRE • 11/05/2026
Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Response Splitting') vulnerability in Apache Juneau.
The Apache Juneau REST server passed response header values and redirect targets to the servlet container unchecked. An application that sets a response header or redirect target from request data could let a remote client inject header lines (where the servlet container does not itself reject CR/LF) or redirect users to an arbitrary external site. The framework itself does not build headers or redirects from request data; exposure depends on application code.
This issue affects Apache Juneau: through 9.2.0.
Users are recommended to upgrade to version 9.2.1, which fixes the issue. Header values are now sanitized by default; applications that build redirect targets from request data must also switch from sendRedirect() to the new sendSafeRedirect().
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Analysis
by VulDB Data Team • 11/05/2026
The vulnerability identified in Apache Juneau represents a classic instance of HTTP Response Splitting, technically classified under CWE-113 as Improper Neutralization of CRLF Sequences in HTTP Headers. This security flaw arises when an application processes untrusted input and incorporates it directly into the headers or redirect targets of an HTTP response without adequate validation or sanitization. In this specific context, the Apache Juneau REST server framework allows developers to pass user-supplied data from incoming requests directly into outgoing response headers or redirection URLs. Because the underlying servlet container may not strictly reject carriage return (CR) and line feed (LF) characters within header values, an attacker can inject these control sequences to terminate a legitimate HTTP header field prematurely. This capability enables the injection of arbitrary new lines, effectively allowing the construction of a completely fake second HTTP response that is sent back to the client along with the intended original response.
The operational impact of this vulnerability is significant and multifaceted. By successfully injecting CRLF characters, an attacker can manipulate the content of the subsequent injected headers or body. This often facilitates Cross-Site Scripting (XSS) attacks where malicious JavaScript code is embedded in a header such as Set-Cookie or Content-Type, which the browser then executes upon receiving the split response. Furthermore, if the vulnerability affects redirect targets, it enables open redirection attacks. An attacker can craft a URL that causes the server to issue an HTTP 3xx status code pointing to a malicious external domain under their control. This technique is frequently leveraged in phishing campaigns to deceive users into believing they are still on the legitimate site while being redirected to a fraudulent login page designed to harvest credentials or distribute malware. The severity of these impacts relies heavily on how the application utilizes user input, as the framework itself does not automatically build headers from request data; rather, it provides the mechanism that allows such misuse if developers do not implement proper safeguards.
From an offensive security perspective, this vulnerability aligns with several tactics in the MITRE ATT&CK framework. The injection of malicious content into HTTP responses corresponds to T1059 Command and Scripting Interpreter through XSS payloads embedded via header manipulation. Additionally, the ability to redirect users to arbitrary external sites maps directly to T1189 Drive-by Compromise or T1566 Phishing, depending on whether the goal is immediate code execution or social engineering. The root cause lies in a failure to enforce strict input validation and output encoding standards, which are fundamental principles of secure coding practices outlined by organizations such as OWASP. Specifically, this falls under the category of Injection flaws where special characters used for command structure are not properly escaped or rejected before being processed by downstream components like the servlet container.
To mitigate this vulnerability, immediate action is required to upgrade Apache Juneau to version 9.2.1 or later. This updated release addresses the core issue by implementing default sanitization of header values, thereby neutralizing CRLF sequences before they can be interpreted as structural delimiters within the HTTP protocol. For applications that construct redirect targets dynamically from user input, it is critical to migrate away from standard servlet methods like sendRedirect() and instead utilize the newly introduced sendSafeRedirect() method provided in the updated framework. This specialized function ensures that redirection URLs are validated against a whitelist of allowed domains or properly encoded to prevent host header manipulation. Beyond upgrading software, developers must adopt a defense-in-depth strategy by validating all user-supplied input at the entry point of their application and encoding output appropriately for its context. Regular security audits and static code analysis tools configured to detect potential injection points can further reduce the risk of such vulnerabilities persisting in production environments.