Response Splitting from unsanitized headers

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

8.7 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:N

EPSS

• CVE-2021-41084: 1.23% estimated exploitation activity in the next 30 days; percentile 69.08 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact 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.nameå Header values (Header.value) Status reason phrases (Status.reason) URI paths (Uri.Path) URI authority registered names (URI.RegName) (through 0.21) The following backends render invalid carriage return, newline, or null characters in an unsafe fashion. | | blaze-server | ember-server | blaze-client | ember-client | jetty-client | |:---------------|:-------------|:-------------|:-------------|--------------|--------------| | header names | ⚠ | ⚠ | ⚠ | ⚠ | ⚠ | | header values | ⚠ | ⚠ | ⚠ | ⚠ | | | status reasons | ⚠ | ⚠ | | | | | URI paths | | | ⚠ | ⚠ | | | URI regnames | | | ⚠ < 0.22 | ⚠ < 0.22 | | For example, given the following service: A clean author parameter returns a clean response: A malicious author parameter allows a user-agent to hijack the response from our server and return different content:…

Published by the source

22 Sep 2021, 19:18 UTC

Last modified by the source

11 Dec 2025, 21:12 UTC

Source last checked

26 Oct 2026, 16:43 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-5vcm-3xc3-w7x3

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# Patch Next advisory brief: CVE-2021-41084

Advisory: Response Splitting from unsanitized headers
IDs: CVE-2021-41084, GHSA-5vcm-3xc3-w7x3
Record: https://patchnext.com/advisories/cve-2021-41084 (revision 1, source checked 26 Oct 2026, 16:43 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Partial vendor fix; fixed versions by package: org.http4s:http4s-client_2.12 0.23.4, 0.22.5, 0.21.29; org.http4s:http4s-server_3 0.23.4, 0.22.5; org.http4s:http4s-server_2.13 0.23.4, 0.22.5, 0.21.29; org.http4s:http4s-client_3 0.23.4, 0.22.5, 0.21.29; org.http4s:http4s-client_2.13 0.23.4, 0.22.5, 0.21.29; org.http4s:http4s-server_2.12 0.23.4, 0.22.5, 0.21.29. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven org.http4s:http4s-server_2.11: affected <= 0.21.28
- Maven org.http4s:http4s-client_2.12: affected <= 0.21.28; >= 0.22.0, <= 0.22.4; >= 0.23.0, <= 0.23.3; fixed in 0.22.5, 0.21.29, 0.23.4
- Maven org.http4s:http4s-server_2.13.0-M5: affected <= 0.21.28
- Maven org.http4s:http4s-server_3: affected >= 0.22.0, <= 0.22.4; >= 0.23.0, <= 0.23.3; fixed in 0.22.5, 0.23.4
- Maven org.http4s:http4s-server_2.13: affected <= 0.21.28; >= 0.22.0, <= 0.22.4; >= 0.23.0, <= 0.23.3; fixed in 0.22.5, 0.21.29, 0.23.4
- Maven org.http4s:http4s-client_3: affected <= 0.21.28; >= 0.22.0, <= 0.22.4; >= 0.23.0, <= 0.23.3; fixed in 0.22.5, 0.21.29, 0.23.4
- Maven org.http4s:http4s-client_2.13: affected <= 0.21.28; >= 0.22.0, <= 0.22.4; >= 0.23.0, <= 0.23.3; fixed in 0.22.5, 0.21.29, 0.23.4
- Maven org.http4s:http4s-server_2.10: affected <= 0.21.28
- Maven org.http4s:http4s-server_2.12: affected <= 0.21.28; >= 0.22.0, <= 0.22.4; >= 0.23.0, <= 0.23.3; fixed in 0.22.5, 0.21.29, 0.23.4

## Suggested inspection (read-only)
- Maven org.http4s:http4s-server_2.11: check the installed version with your package manager.
- Maven org.http4s:http4s-client_2.12: check the installed version with your package manager.
- Maven org.http4s:http4s-server_2.13.0-M5: check the installed version with your package manager.
- Maven org.http4s:http4s-server_3: check the installed version with your package manager.
- Maven org.http4s:http4s-server_2.13: check the installed version with your package manager.
- Maven org.http4s:http4s-client_3: check the installed version with your package manager.
- Maven org.http4s:http4s-client_2.13: check the installed version with your package manager.
- Maven org.http4s:http4s-server_2.10: check the installed version with your package manager.
- Maven org.http4s:http4s-server_2.12: check the installed version with your package manager.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on org.http4s:http4s-server_2.11, org.http4s:http4s-client_2.12, org.http4s:http4s-server_2.13.0-M5, org.http4s:http4s-server_3, org.http4s:http4s-server_2.13, org.http4s:http4s-client_3, org.http4s:http4s-client_2.13, org.http4s:http4s-server_2.10, org.http4s:http4s-server_2.12.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-5vcm-3xc3-w7x3 (GitHub-reviewed advisory GHSA-5vcm-3xc3-w7x3 lists affected ranges: Maven org.http4s:http4s-client_2.12 >=0 =0.22.0 =0.23.0 =0 =0.22.0 =0.23.0 =0 =0.22.0 =0.23.0 =0 =0 =0 =0.22.0…)

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