medium CVSS 6.1

CVE-2026-33347·PHP vulnerability

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.

Severity
medium
Software
PHP
Fixed in
2.8.2
Published
2026-03-24

Affected versions

From: 2.3.0

Until: 2.8.2

Fixed in: 2.8.2

How to fix this CVE

Update the league/commonmark PHP library to version 2.8.2 or later to remediate the domain allowlist bypass vulnerability in the Embed extension. The vulnerability allows attackers to bypass domain filtering by crafting malicious domain names that pass incomplete regex validation. Ensure all PHP applications using this library are updated and tested in a staging environment before deploying to production.

sudo dnf update php php-common php-cli

Defensia detects this vulnerability

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement WAF rules to block or log HTTP requests containing suspicious domain patterns in parameters passed to endpoints using the Embed extension (e.g., pattern: /\b[a-z0-9.-]*\.(com|net|org|edu)\.(?!com|net|org|edu)[a-z0-9.-]+/i). Additionally, enforce strict Content Security Policy (CSP) headers to restrict iframe and embedded resource origins to explicitly whitelisted domains.

How to check if you are affected

  1. Check installed PHP version: php -v
  2. Verify league/commonmark library installation: composer show league/commonmark or check vendor/league/commonmark/composer.json for version
  3. Search application logs and web server logs for requests containing suspicious domain patterns (e.g., 'youtube.com.evil', 'example.com.attacker-domain') in Embed extension parameters
  4. Confirm remediation by running: php -r "echo (new ReflectionClass('League\\CommonMark\\Extension\\CommonMarkCoreExtension'))->getFileName();" and verify library version is 2.8.2 or higher

FAQ

What is CVE-2026-33347?

This vulnerability affects the Embed extension's DomainFilteringAdapter in league/commonmark, where a flawed regex pattern fails to properly validate domain boundaries. Attackers can register attacker-controlled domains like youtube.com.evil to bypass allowlist checks when youtube.com is whitelisted, potentially enabling malicious content embedding.

Is CVE-2026-33347 being actively exploited?

No, this vulnerability is not listed in CISA's Known Exploited Vulnerabilities catalog and no public exploits are currently available. However, the attack is straightforward and organizations should remediate promptly to prevent opportunistic exploitation.

What versions of PHP are affected by CVE-2026-33347?

The vulnerability affects league/commonmark versions 2.3.0 through 2.8.1. The PHP language itself is not affected; rather, any application using the vulnerable library versions is at risk.

How do I check if my server is vulnerable to CVE-2026-33347?

Run: composer show league/commonmark | grep Version. If the version is between 2.3.0 and 2.8.1, your installation is vulnerable. If the Embed extension is disabled or not in use, risk is reduced but the library should still be updated.

Does Defensia detect CVE-2026-33347?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If league/commonmark is detected on a monitored server, CVE-2026-33347 will appear in your dashboard with remediation steps.

Related PHP CVEs

CVE-2024-5932CVSS 10The GiveWP – Donation Plugin and Fundraising Platform plugin for WordPress is vulnerable to PHP Object Injection in all versions up to, and including, 3.14.1 via deserialization of untrusted input from the 'give_title' parameter. This makes it possible for unauthenticated attackers to inject a PHP Object. The additional presence of a POP chain allows attackers to execute code remotely, and to delete arbitrary files.
CVE-2025-46348CVSS 10YesWiki is a wiki system written in PHP. Prior to version 4.5.4, the request to commence a site backup can be performed and downloaded without authentication. The archives are created with a predictable filename, so a malicious user could create and download an archive without being authenticated. This could result in a malicious attacker making numerous requests to create archives and fill up the file system, or by downloading the archive which contains sensitive site information. This issue has been patched in version 4.5.4.
CVE-2025-62521CVSS 10ChurchCRM is an open-source church management system. Prior to version 5.21.0, a pre-authentication remote code execution vulnerability in ChurchCRM's setup wizard allows unauthenticated attackers to inject arbitrary PHP code during the initial installation process, leading to complete server compromise. The vulnerability exists in `setup/routes/setup.php` where user input from the setup form is directly concatenated into a PHP configuration template without any validation or sanitization. Any parameter in the setup form can be used to inject PHP code that gets written to `Include/Config.php`, which is then executed on every page load. This is more severe than typical authenticated RCE vulnerabilities because it requires no credentials and affects the installation process that administrators must complete. Version 5.21.0 patches the issue.
CVE-2025-47916CVSS 10Invision Community 5.0.0 before 5.0.7 allows remote code execution via crafted template strings to themeeditor.php. The issue lies within the themeeditor controller (file: /applications/core/modules/front/system/themeeditor.php), where a protected method named customCss can be invoked by unauthenticated users. This method passes the value of the content parameter to the Theme::makeProcessFunction() method; hence it is evaluated by the template engine. Accordingly, this can be exploited by unauthenticated attackers to inject and execute arbitrary PHP code by providing crafted template strings.
CVE-2026-28289CVSS 10FreeScout is a free help desk and shared inbox built with PHP's Laravel framework. A patch bypass vulnerability for CVE-2026-27636 in FreeScout 1.8.206 and earlier allows any authenticated user with file upload permissions to achieve Remote Code Execution (RCE) on the server by uploading a malicious .htaccess file using a zero-width space character prefix to bypass the security check. The vulnerability exists in the sanitizeUploadedFileName() function in app/Http/Helper.php. The function contains a Time-of-Check to Time-of-Use (TOCTOU) flaw where the dot-prefix check occurs before sanitization removes invisible characters. This vulnerability is fixed in 1.8.207.

References

Track CVEs across your fleet automatically

Defensia scans your Linux servers and tells you exactly which ones are running vulnerable versions — including CVE-2026-33347. Free for 1 server.

Get started free