CVE-2021-33621Network attack vector

Response splitting via unvalidated header newlines

Published Nov 18, 2022 · Updated Nov 4, 2025

HTTP response splitting in the Ruby cgi gem allows attackers to inject malicious HTTP response headers and body through untrusted input. Response-generation paths accept newline characters in untrusted header values, while CGI::Cookie#initialize fails to validate user-controlled contents before placing invalid attributes in Set-Cookie. The flaw is reachable only when an application passes untrusted input into HTTP response generation or CGI::Cookie construction, enabling deceptive responses or invalid cookie attributes.

CVSS severity8.8
High
EPSS probability2.35%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP response splitting in the Ruby cgi gem allows attackers to inject malicious HTTP response headers and body through untrusted input. Response-generation paths accept newline characters in untrusted header values, while CGI::Cookie#initialize fails to validate user-controlled contents before placing invalid attributes in Set-Cookie. The flaw is reachable only when an application passes untrusted input into HTTP response generation or CGI::Cookie construction, enabling deceptive responses or invalid cookie attributes.

The record

CVE
CVE-2021-33621
Published
Nov 18, 2022
Updated
Nov 4, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-74, CWE-436, T1190
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Nov 18, 2022CVE publishedPublication date reported by the CVE source.
  2. Nov 4, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

8.8GitHub Security AdvisoryCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
8.8Amazon LinuxCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
8.8Oracle LinuxCVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
8.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

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.

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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
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-74Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
CWE-436Interpretation Conflict
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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