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CVE-2021-28834: Kramdown before 2.3.1 does not restrict Rouge formatters to the Rouge::Formatters namespace, and thus arbitrar

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CRITICAL 9.8REMOTEWATCHLISTUPGRADEMITIGATEMITIGATE

CVE-2021-28834 is a critical-severity flaw (CVSS 9.8) affecting kramdown, fedora, debian linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Kramdown before 2.3.1 does not restrict Rouge formatters to the Rouge::Formatters namespace, and thus arbitrary classes can be instantiated.

Why this priority

  • Critical severity
  • Public exploit reference
  • Remotely exploitable, no login, no user action
  • On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score9.8 (Critical)
Attack vectorNetwork (remote)
Attack complexityLow
Privileges neededNone
User interactionNone
EPSS exploit chance2.7% (top 14%)
CISA exploited listNo
Published2021-03-19
Last modified2026-06-17
Publisher (CNA): MITRE · Vendor: debian · fedoraproject · kramdown project

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

Am I affected?

Timeline

kramdown project kramdown UPGRADE
Affected: < 2.3.1
Fixed in: 2.3.1
Safe to downgrade to: no unaffected older release
1. Check installed version
apt-cache policy kramdown || rpm -q kramdown || kramdown --version
2. UPGRADE to 2.3.1 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade kramdown
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade kramdown
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
fedoraproject fedora MITIGATE
Affected: 32 · 33 · 34
Fixed in: no fixed release known
Safe to downgrade to: a release before 32
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.
debian debian linux MITIGATE
Affected: 10.0
Fixed in: no fixed release known
Safe to downgrade to: a release before 10.0
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.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.
RubyGems · kramdown · FIXED · 2.3.1 · vulnerable: >= 1.16.0, < 2.3.1 · source
bundle update kramdown bundle list | grep kramdown
Fedora ELN · python-discovery · FIXED · 1.9.0-1.eln159 · FEDORA-2026-7a881c0c99 · source
sudo dnf upgrade --advisory=FEDORA-2026-7a881c0c99
Fedora 46 · pipx · FIXED · 1.18.11-1.fc46 · FEDORA-2026-443a765287 · source
sudo dnf upgrade --advisory=FEDORA-2026-443a765287
Ubuntu 20.04 · ruby-kramdown · FIXED · 1.17.0-4ubuntu0.2
sudo apt update && sudo apt install --only-upgrade ruby-kramdown dpkg --compare-versions "$(dpkg-query -W -f='${Version}' ruby-kramdown)" ge "1.17.0-4ubuntu0.2" && echo PATCHED || echo STILL-VULNERABLE
Ubuntu 22.04 · ruby-kramdown · NOT AFFECTED
Ubuntu 22.04 · ruby-kramdown-rfc2629 · NOT AFFECTED
Ubuntu 20.04 · ruby-kramdown-rfc2629 · NOT AFFECTED
Debian 12 · ruby-kramdown · FIXED · 2.3.0-5
sudo apt update && sudo apt install --only-upgrade ruby-kramdown dpkg --compare-versions "$(dpkg-query -W -f='${Version}' ruby-kramdown)" ge "2.3.0-5" && echo PATCHED || echo STILL-VULNERABLE
Debian 13 · ruby-kramdown · FIXED · 2.3.0-5
sudo apt update && sudo apt install --only-upgrade ruby-kramdown dpkg --compare-versions "$(dpkg-query -W -f='${Version}' ruby-kramdown)" ge "2.3.0-5" && echo PATCHED || echo STILL-VULNERABLE
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-2021-28834?

kramdown: upgrade to 2.3.1; fedora: mitigate or apply the listed mitigation; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-28834 being exploited?

CVE-2021-28834 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-2021-28834 affect?

CVE-2021-28834 affects kramdown, fedora, debian linux. Severity is critical (CVSS 9.8). Kramdown before 2.3.1 does not restrict Rouge formatters to the Rouge::Formatters namespace, and thus arbitrary classes can be instantiated.

How urgent is CVE-2021-28834?

Priority 71/100. Critical severity; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities

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References

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