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 month

HIGH 8.8UPGRADEMITIGATEUPGRADE

CVE-2021-33621 is a high-severity flaw (CVSS 8.8) affecting cgi, fedora, ruby. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

High severity
Public exploit reference

A fixed release or distro patch exists.

Risk facts

CVSS score	8.8 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	2.5% (top 16%)
CISA exploited list	No
Weakness	CWE-74
Published	2022-11-18
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: fedoraproject · ruby-lang

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2022-11-18 23:15 UTCPublished CVE record published
2026-06-17 03:54 UTCUpdated Record last modified

ruby-lang cgi UPGRADE

Affected: < 0.1.0.2 · >= 0.2.0 and < 0.2.2 · >= 0.3.0 and < 0.3.5

Fixed in: 0.1.0.2, 0.2.2, 0.3.5

Safe to downgrade to: a release before 0.2.0

1. Check installed version

apt-cache policy cgi || rpm -q cgi || cgi --version

2. UPGRADE to 0.1.0.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade cgi

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade cgi

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 0.2.0 (Debian / Ubuntu)

apt-cache madison cgi sudo apt install cgi=<VERSION_BEFORE_0.2.0> sudo apt-mark hold cgi

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list cgi sudo dnf downgrade cgi-<VERSION_BEFORE_0.2.0> sudo dnf versionlock add cgi

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

fedoraproject fedora MITIGATE

Affected: 35 · 36 · 37

Fixed in: no fixed release known

Safe to downgrade to: a release before 35

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

ruby-lang ruby UPGRADE

Affected: >= 2.7.0 and < 2.7.7 · >= 3.0.0 and < 3.0.5 · >= 3.1.0 and < 3.1.3

Fixed in: 2.7.7, 3.0.5, 3.1.3

Safe to downgrade to: a release before 2.7.0

1. Check installed version

apt-cache policy ruby || rpm -q ruby || ruby --version

2. UPGRADE to 2.7.7 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade ruby

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade ruby

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 2.7.0 (Debian / Ubuntu)

apt-cache madison ruby sudo apt install ruby=<VERSION_BEFORE_2.7.0> sudo apt-mark hold ruby

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list ruby sudo dnf downgrade ruby-<VERSION_BEFORE_2.7.0> sudo dnf versionlock add ruby

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-33621?

cgi: upgrade to 0.1.0.2, 0.2.2, 0.3.5; fedora: mitigate or apply the listed mitigation; ruby: upgrade to 2.7.7, 3.0.5, 3.1.3. Exact commands for each OS are on this page.

Is CVE-2021-33621 being exploited?

CVE-2021-33621 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-33621 affect?

CVE-2021-33621 affects cgi, fedora, ruby. Severity is high (CVSS 8.8). 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 ob

How urgent is CVE-2021-33621?

Priority 57/100. High severity; Public exploit reference

Software pages

Ruby vulnerabilities

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References

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