Summaryinfo

A vulnerability identified as problematic has been detected in thephpleague commonmark up to 2.8.1. The impacted element is an unknown function of the component PHP Markdown Parser. The manipulation leads to cross site scripting. This vulnerability is referenced as CVE-2026-33347. Remote exploitation of the attack is possible. No exploit is available. You should upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in thephpleague commonmark up to 2.8.1. This issue affects some unknown functionality of the component PHP Markdown Parser. The manipulation with an unknown input leads to a cross site scripting vulnerability. Using CWE to declare the problem leads to CWE-79. The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Impacted is integrity. The summary by CVE is:

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.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-33347 since 03/18/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. It demands that the victim is doing some kind of user interaction. Neither technical details nor an exploit are publicly available. MITRE ATT&CK project uses the attack technique T1059.007 for this issue.

The vulnerability scanner Nessus provides a plugin with the ID 303380 (Linux Distros Unpatched Vulnerability : CVE-2026-33347), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 2.8.2 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at Tenable (303380). If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Productinfo

Vendor

• thephpleague

Name

• commonmark

Version

• 2.8.0
• 2.8.1

Website

• Product: https://github.com/thephpleague/commonmark/

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