CVE-2021-41084Network attack vector

HTTP message splitting via unsanitized fields

Published Sep 21, 2021 · Updated Aug 4, 2024

Improper input validation in http4s 0.21.28 and earlier allows remote attackers to alter HTTP messages via crafted header or URI fields. Header names, values, status reason phrases, URI paths, and registered names accept carriage-return, newline, or null characters that affected backends render as delimiters, enabling response or request splitting. Exploitation requires an application to place attacker-controlled data in those fields and use an affected backend; the impact is message integrity compromise rather than direct code execution.

CVSS severity4.7
Medium
EPSS probability1.23%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper input validation in http4s 0.21.28 and earlier allows remote attackers to alter HTTP messages via crafted header or URI fields. Header names, values, status reason phrases, URI paths, and registered names accept carriage-return, newline, or null characters that affected backends render as delimiters, enabling response or request splitting. Exploitation requires an application to place attacker-controlled data in those fields and use an affected backend; the impact is message integrity compromise rather than direct code execution.

The record

CVE
CVE-2021-41084
Published
Sep 21, 2021
Updated
Aug 4, 2024
Vendor
http4s
Product
http4s
Classifications
CWE-74, CWE-918, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 21, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=<= 0.21.28
  2. Affected versionversion=>= 0.22.0, < 0.22.5
  3. Affected versionversion=>= 0.23.0, < 0.23.4
  4. Affected versionversion=>= 1.0.0-M1, < 1.0.0-M27

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

http4s · http4sversion=<= 0.21.28; version=>= 0.22.0, < 0.22.5; version=>= 0.23.0, < 0.23.4; version=>= 1.0.0-M1, < 1.0.0-M27

Published CVSS scores

4.7NISTCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N
5.0NISTAV:N/AC:M/Au:N/C:N/I:P/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-74Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
CWE-918Server-Side Request Forgery (SSRF)
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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