CVE-2026-33347
Allowlist Bypass in league/commonmark Embed DomainFilteringAdapter
Publication date: 2026-03-24
Last updated on: 2026-04-08
Assigner: GitHub, Inc.
Description
Description
CVSS Scores
EPSS Scores
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Meta Information
Affected Vendors & Products
| Vendor | Product | Version / Range |
|---|---|---|
| thephpleague | commonmark | From 2.3.0 (inc) to 2.8.2 (exc) |
Helpful Resources
Exploitability
| CWE ID | Description |
|---|---|
| CWE-918 | The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination. |
| CWE-185 | The product specifies a regular expression in a way that causes data to be improperly matched or compared. |
| 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. |
Attack-Flow Graph
AI Powered Q&A
Can you explain this vulnerability to me?
The vulnerability exists in the league/commonmark PHP Markdown parser, specifically in the DomainFilteringAdapter of its Embed extension. Between versions 2.3.0 and before 2.8.2, the domain-matching regular expression used to enforce an allowlist of domains lacks a proper hostname boundary assertion. This flaw allows an attacker to bypass the allowlist by using crafted domains such as youtube.com.evil, which incorrectly passes the check when youtube.com is an allowed domain.
This issue was fixed in version 2.8.2 of league/commonmark.
How can this vulnerability impact me? :
This vulnerability can allow attackers to bypass domain allowlists and potentially embed or link to malicious or unauthorized content by using crafted domain names that appear to be allowed domains but are actually controlled by attackers.
As a result, users or applications relying on the domain allowlist for security may be exposed to phishing, malware distribution, or other malicious activities through seemingly trusted embedded content.
How does this vulnerability affect compliance with common standards and regulations (like GDPR, HIPAA)?:
I don't know
How can this vulnerability be detected on my network or system? Can you suggest some commands?
I don't know
What immediate steps should I take to mitigate this vulnerability?
To mitigate this vulnerability, you should upgrade the league/commonmark package to version 2.8.2 or later, where the issue has been patched.