CVE-2021-41084 in http4s
Summary
by MITRE • 09/22/2021
http4s is an open source scala interface for HTTP. In affected versions http4s is vulnerable to response-splitting or request-splitting attacks when untrusted user input is used to create any of the following fields: Header names (`Header.name`å), Header values (`Header.value`), Status reason phrases (`Status.reason`), URI paths (`Uri.Path`), URI authority registered names (`URI.RegName`) (through 0.21). This issue has been resolved in versions 0.21.30, 0.22.5, 0.23.4, and 1.0.0-M27 perform the following. As a matter of practice http4s services and client applications should sanitize any user input in the aforementioned fields before returning a request or response to the backend. The carriage return, newline, and null characters are the most threatening.
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Analysis
by VulDB Data Team • 12/11/2025
CVE-2021-41084 represents a critical response-splitting vulnerability affecting the http4s scala HTTP interface library. This vulnerability stems from inadequate input validation in several key HTTP field components including header names and values, status reason phrases, URI paths, and URI authority registered names. The flaw allows attackers to inject malicious characters that can disrupt HTTP protocol communication by splitting responses or requests into unintended segments. The vulnerability is particularly dangerous because it can be exploited through multiple vectors within the HTTP message structure, making it a comprehensive security weakness that affects the core functionality of http4s applications.
The technical implementation of this vulnerability involves the improper handling of user-supplied input in HTTP field construction. When untrusted data is directly used to populate header names through Header.name, header values through Header.value, status reason phrases through Status.reason, URI paths through Uri.Path, or URI authority registered names through URI.RegName, the library fails to properly sanitize these inputs. The most dangerous characters in this context are carriage return, newline, and null characters which can be used to inject additional HTTP headers or manipulate response boundaries. This vulnerability maps directly to CWE-117, which addresses improper output neutralization for logs, and also aligns with ATT&CK technique T1071.004 for application layer protocol manipulation.
The operational impact of CVE-2021-41084 is significant for any http4s-based applications that process user input in HTTP fields. Attackers could potentially perform header injection attacks, manipulate HTTP responses, or create cross-site scripting opportunities by exploiting this vulnerability. The vulnerability affects multiple versions of the library including all versions through 0.21, making it a widespread concern for organizations using older http4s implementations. This issue can be leveraged for session hijacking, cache poisoning, or more sophisticated attack vectors that exploit the fundamental HTTP protocol structure.
The recommended mitigation strategy involves upgrading to patched versions of http4s including 0.21.30, 0.22.5, 0.23.4, and 1.0.0-M27 which contain proper input sanitization mechanisms. Organizations should implement comprehensive input validation practices that sanitize all user-supplied data before it is used in HTTP field construction. This includes implementing character filtering for carriage return, newline, and null characters in all HTTP field components. Additionally, developers should adopt defensive programming practices such as using parameterized inputs, implementing strict input validation at multiple layers, and ensuring that user data is properly escaped or encoded before being inserted into HTTP messages. The vulnerability highlights the importance of following secure coding practices and adhering to HTTP protocol specifications when handling user input in web applications.