CVE-2021-28834 Remote code execution in Kramdown

What is CVE-2021-28834?

CVE-2021-28834 is a critical code injection vulnerability in Debian Debian Linux, published on 19 Mar 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

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

Which versions are affected by CVE-2021-28834?

2 open source packages ship an affected version, including kramdown. It affects 3 products.

Products include Debian Debian Linux, Fedoraproject Fedora, Kramdown Project Kramdown.

How do I fix CVE-2021-28834?

Upgrade kramdown to 2.3.1 or later.

Upgrade, and never evaluate templates, expressions or code built from user input. More about Code injection.

Is CVE-2021-28834 being exploited?

There is no known exploitation of CVE-2021-28834 in the wild and no public exploit code.

Other vulnerabilities in Debian Debian Linux

• CVE-2015-3306 ProFTPD Improper Access Control Exploited Critical 10.0
• CVE-2025-49113 RoundCube Webmail Deserialization of Untrusted Data Exploited Critical 9.9
• CVE-2026-24061 GNU InetUtils Argument Injection Exploited Critical 9.8
• CVE-2025-39682 Linux Kernel Improper Check for Unusual or Exceptional Conditions Exploited Critical 9.8
• CVE-2024-9680 Mozilla Firefox Use-After-Free Exploited Critical 9.8

All Debian Debian Linux vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as DEBIAN-CVE-2021-28834, DSA-4890-1, GHSA-52p9-v744-mwjj, OESA-2021-1159, UBUNTU-CVE-2021-28834, USN-6424-1.