CVE-2021-26813: markdown2 >=1.0.1.18, fixed in 2.4.0, is affected by a regular expression denial of service vulnerability

This week

HIGH 7.5REMOTEUPGRADEMITIGATE

CVE-2021-26813 is a high-severity flaw (CVSS 7.5) affecting markdown2, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

markdown2 >=1.0.1.18, fixed in 2.4.0, is affected by a regular expression denial of service vulnerability. If an attacker provides a malicious string, it can make markdown2 processing difficult or delayed for an extended period of time.

Why this priority

High severity
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	2.3% (top 17%)
CISA exploited list	No
Weakness	CWE-1333
Published	2021-03-03
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: fedoraproject · markdown2 project

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

Am I affected?

Timeline

2021-03-03 16:15 UTCPublished CVE record published
2026-06-17 03:43 UTCUpdated Record last modified

markdown2 project markdown2 UPGRADE

Affected: >= 1.0.1.18 and < 2.4.0

Fixed in: 2.4.0

Safe to downgrade to: a release before 1.0.1.18

1. Check installed version

apt-cache policy markdown2 || rpm -q markdown2 || markdown2 --version

2. UPGRADE to 2.4.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade markdown2

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade markdown2

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 1.0.1.18 (Debian / Ubuntu)

apt-cache madison markdown2 sudo apt install markdown2=<VERSION_BEFORE_1.0.1.18> sudo apt-mark hold markdown2

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

dnf --showduplicates list markdown2 sudo dnf downgrade markdown2-<VERSION_BEFORE_1.0.1.18> sudo dnf versionlock add markdown2

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: 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.

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-2021-26813?

markdown2: upgrade to 2.4.0; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-26813 being exploited?

CVE-2021-26813 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-2021-26813 affect?

CVE-2021-26813 affects markdown2, fedora. Severity is high (CVSS 7.5). markdown2 >=1.0.1.18, fixed in 2.4.0, is affected by a regular expression denial of service vulnerability. If an attacker provides a malicious string, it can make markdown2 processing difficult or delayed for an extended period of time.

How urgent is CVE-2021-26813?

Priority 57/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

References

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