Summary
Response Splitting from unsanitized headers
Workarounds
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.
Not all backends were affected: jetty-server, tomcat-server, armeria, and netty on the server; async-http-client, okhttp-client, armeria, and netty as clients.
References
- https://owasp.org/www-community/attacks/HTTP_Response_Splitting
- https://httpwg.org/http-core/draft-ietf-httpbis-semantics-latest.html#fields.values
For more information
If you have any questions or comments about this advisory:
- Open an issue in GitHub
- Contact us via the http4s security policy
Impact
Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside. Typical impact: access to internal metadata services, internal APIs, or cloud credentials.
CVE-2021-41084 has a CVSS score of 8.7 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.21.29, 0.22.5, 0.23.4); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Versions 0.21.29, 0.22.5, 0.23.4, and 1.0.0-M27 perform the following:
- If a status reasoon phrase is invalid, it is dropped. Rendering is optional per spec.
- If a header name is invalid in a request or response, the header is dropped. There is no way to generically sanitize a header without potentially shadowing a correct one.
- If a header value is invalid in a request or response, it is sanitized by replacing null (
\u0000), carriage return (\r), and newline (\n) with space ( - If a URI path or registered name is invalid in a request line, the client raises an
IllegalArgumentException. - If a URI registered name is invalid in a host header, the client raises an
IllegalArgumentException.
Frequently Asked Questions
- What is CVE-2021-41084? CVE-2021-41084 is a high-severity server-side request forgery (SSRF) vulnerability in org.http4s:http4s-client_2.12 (maven), affecting versions <= 0.21.28. It is fixed in 0.21.29, 0.22.5, 0.23.4. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
- How severe is CVE-2021-41084? CVE-2021-41084 has a CVSS score of 8.7 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which packages are affected by CVE-2021-41084?
org.http4s:http4s-client_2.12(maven) (versions <= 0.21.28)org.http4s:http4s-client_2.13(maven) (versions <= 0.21.28)org.http4s:http4s-client_3(maven) (versions <= 0.21.28)org.http4s:http4s-server_2.10(maven) (versions <= 0.21.28)org.http4s:http4s-server_2.11(maven) (versions <= 0.21.28)org.http4s:http4s-server_2.12(maven) (versions <= 0.21.28)org.http4s:http4s-server_2.13(maven) (versions <= 0.21.28)org.http4s:http4s-server_2.13.0-M5(maven) (versions <= 0.21.28)org.http4s:http4s-server_3(maven) (versions >= 0.22.0, <= 0.22.4)
- Is there a fix for CVE-2021-41084? Yes. CVE-2021-41084 is fixed in 0.21.29, 0.22.5, 0.23.4. Upgrade to this version or later.
- Is CVE-2021-41084 exploitable, and should I be worried? Whether CVE-2021-41084 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-41084 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-41084?
- Upgrade
org.http4s:http4s-client_2.12to 0.21.29 or later - Upgrade
org.http4s:http4s-client_2.12to 0.22.5 or later - Upgrade
org.http4s:http4s-client_2.12to 0.23.4 or later - Upgrade
org.http4s:http4s-client_2.13to 0.21.29 or later - Upgrade
org.http4s:http4s-client_2.13to 0.22.5 or later - Upgrade
org.http4s:http4s-client_2.13to 0.23.4 or later - Upgrade
org.http4s:http4s-client_3to 0.21.29 or later - Upgrade
org.http4s:http4s-client_3to 0.22.5 or later - Upgrade
org.http4s:http4s-client_3to 0.23.4 or later - Upgrade
org.http4s:http4s-server_2.12to 0.21.29 or later - Upgrade
org.http4s:http4s-server_2.12to 0.22.5 or later - Upgrade
org.http4s:http4s-server_2.12to 0.23.4 or later - Upgrade
org.http4s:http4s-server_2.13to 0.21.29 or later - Upgrade
org.http4s:http4s-server_2.13to 0.22.5 or later - Upgrade
org.http4s:http4s-server_2.13to 0.23.4 or later - Upgrade
org.http4s:http4s-server_3to 0.22.5 or later - Upgrade
org.http4s:http4s-server_3to 0.23.4 or later
- Upgrade