CVE-2026-33347: league/commonmark is a PHP Markdown parser

Routine

MEDIUM 6.1UPGRADE

CVE-2026-33347 is a medium-severity flaw (CVSS 6.1) affecting commonmark. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

league/commonmark is a PHP Markdown parser. From version 2.3.0 to before version 2.8.2, the DomainFilteringAdapter in the Embed extension is vulnerable to an allowlist bypass due to a missing hostname boundary assertion in the domain-matching regex. An attacker-controlled domain like youtube.com.evil passes the allowlist check when youtube.com is an allowed domain. This issue has been patched in version 2.8.2.

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	6.1 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.2% (top 84%)
CISA exploited list	No
Weakness	CWE-79, CWE-185, CWE-918
Published	2026-03-24
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: thephpleague

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

Am I affected?

Timeline

2026-03-24 20:16 UTCPublished CVE record published
2026-06-17 10:37 UTCUpdated Record last modified

thephpleague commonmark UPGRADE

Affected: >= 2.3.0 and < 2.8.2

Fixed in: 2.8.2

Safe to downgrade to: a release before 2.3.0

1. Check installed version

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

2. UPGRADE to 2.8.2 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade commonmark

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 2.3.0 (Debian / Ubuntu)

apt-cache madison commonmark sudo apt install commonmark=<VERSION_BEFORE_2.3.0> sudo apt-mark hold commonmark

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

dnf --showduplicates list commonmark sudo dnf downgrade commonmark-<VERSION_BEFORE_2.3.0> sudo dnf versionlock add commonmark

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-2026-33347?

commonmark: upgrade to 2.8.2. Exact commands for each OS are on this page.

Is CVE-2026-33347 being exploited?

CVE-2026-33347 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-2026-33347 affect?

CVE-2026-33347 affects commonmark. Severity is medium (CVSS 6.1). league/commonmark is a PHP Markdown parser. From version 2.3.0 to before version 2.8.2, the DomainFilteringAdapter in the Embed extension is vulnerable to an allowlist bypass due to a missing hostname boundary assertion in the domain-matchi

How urgent is CVE-2026-33347?

Priority 25/100. No elevated risk signals yet.

Software pages

PHP vulnerabilities

Related CVEs

CVE-2026-30838 MEDIUM 6.1 UPGRADE league/commonmark is a PHP Markdown parser

References

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