How severe is CVE-2021-41084?

CVE-2021-41084 has a CVSS score of 4.7/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-41084?

CVE-2021-41084 affects the following packages: org.http4s:http4s-client_2.12 (Maven), org.http4s:http4s-client_2.12 (Maven), org.http4s:http4s-client_2.12 (Maven), org.http4s:http4s-client_2.13 (Maven), org.http4s:http4s-client_2.13 (Maven), org.http4s:http4s-client_2.13 (Maven), org.http4s:http4s-client_3 (Maven), org.http4s:http4s-client_3 (Maven), and 12 more. Ecosystems affected: Maven.

How do I fix CVE-2021-41084?

Update org.http4s:http4s-client_2.12 to 0.21.29 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-41084 is resolved across your whole dependency graph.

How do I detect CVE-2021-41084 in my Maven dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4s:http4s-client_2.12. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate CVE-2021-41084 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against CVE-2021-41084?

O3 pinpoints whether CVE-2021-41084 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2021-41084 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-41084, including 1 in-the-wild exploitation observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-41084?

CVE-2021-41084 has an EPSS (Exploit Prediction Scoring System) score of 1.2%, placing it in the 64th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

When was CVE-2021-41084 published, and has it been updated?

CVE-2021-41084 was published on September 21, 2021 and was last updated on April 10, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

☕ Maven

CVE-2021-41084

MEDIUM

Response Splitting from unsanitized headers

Also known asGHSA-5vcm-3xc3-w7x3

Published

Sep 21, 2021

Updated

Apr 10, 2026

Affected

20 pkgs

Patched

17 / 20

Exploits

1 known

EPSS Exploitation Probability

via FIRST.org ↗

1.2%probability of exploitation in next 30 days

Lower Risk64th percentile+0.74%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Blast Radius

20 pkgs affected

☕org.http4s:http4s-client_2.12☕org.http4s:http4s-client_2.12☕org.http4s:http4s-client_2.12☕org.http4s:http4s-client_2.13☕org.http4s:http4s-client_2.13☕org.http4s:http4s-client_2.13☕org.http4s:http4s-client_3☕org.http4s:http4s-client_3+12 more

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

Description

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.nameå), 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.

Affected Packages

20 total 17 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`org.http4s:http4s-client_2.12`	all versions	0.21.29
☕Maven	`org.http4s:http4s-client_2.12`	≥ 0.22.0&&< 0.22.5	0.22.5
☕Maven	`org.http4s:http4s-client_2.12`	≥ 0.23.0&&< 0.23.4	0.23.4
☕Maven	`org.http4s:http4s-client_2.13`	all versions	0.21.29
☕Maven	`org.http4s:http4s-client_2.13`	≥ 0.22.0&&< 0.22.5	0.22.5
☕Maven	`org.http4s:http4s-client_2.13`	≥ 0.23.0&&< 0.23.4	0.23.4

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

http4s is an open source scala interface for HTTP. In affected versions …

github.comNVDOct 2021

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4s:http4s-client_2.12. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update org.http4s:http4s-client_2.12 to 0.21.29 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-41084 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2021-41084 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2021-41084. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

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

O3 Security · Impact-Aware SCA

Is CVE-2021-41084 in your dependencies?

O3 detects CVE-2021-41084 across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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