What is CVE-2021-33621?

CVE-2021-33621 is a high-severity vulnerability in Ruby-Lang Cgi, classified under Improper Neutralization of Special Elements in Output Used by a Downstream Component (Injection). CVSS score: 8.8/10. Published 2022-11-18.

How severe is CVE-2021-33621?

High severity. CVSS v3 base score is 8.8 out of 10.

Is CVE-2021-33621 known to be exploited?

2 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Vulnerability in Ruby-Lang Cgi

CVE-2021-33621

The cgi gem before 0.1.0.2, 0.2.x before 0.2.2, and 0.3.x before 0.3.5 for Ruby allows HTTP response splitting. This is relevant to applications that use untrusted user input either to generate an HTTP response or to create a CGI::Cookie object.

Published 2022-11-18 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 8.8 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.024 (84.0th percentile), scored 2026-10-19.

Low probability of exploitation in the next 30 days, but still tracked. 84th percentile — 84.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.024 · newest: 0.024 · change: 0.000

Affected products

Ruby-Lang Cgi
Ruby-Lang Ruby
Fedoraproject Fedora — versions 35, 36, 37

Weakness classification (CWE)

CWE-74 — Improper Neutralization of Special Elements in Output Used by a Downstream Component (Injection)

Public proof-of-concept exploits

References

www.ruby-lang.org/en/news/2022/11/22/http-response-splitting-in-cgi-cve-2021-33… (Exploit, Third Party Advisory)
lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/… (vendor-advisory)
lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/… (vendor-advisory)
lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/… (vendor-advisory)
security.netapp.com/advisory/ntap-20221228-0004/ (Third Party Advisory)
lists.debian.org/debian-lts-announce/2023/06/msg00012.html (mailing-list)
security.gentoo.org/glsa/202401-27 (vendor-advisory)
af854a3a-2127-422b-91ae-364da2661108

Frequently asked questions

What is CVE-2021-33621?: CVE-2021-33621 is a high-severity vulnerability in Ruby-Lang Cgi, classified under Improper Neutralization of Special Elements in Output Used by a Downstream Component (Injection). CVSS score: 8.8/10. Published 2022-11-18.
How severe is CVE-2021-33621?: High severity. CVSS v3 base score is 8.8 out of 10.
Is CVE-2021-33621 known to be exploited?: 2 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.