CVE-2026-30838
MEDIUM
GHSA-4v6x-c7xx-hw9f
CVSS Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Lifecycle Timeline
3Tags
Description
league/commonmark is a PHP Markdown parser. Prior to version 2.8.1, the DisallowedRawHtml extension can be bypassed by inserting a newline, tab, or other ASCII whitespace character between a disallowed HTML tag name and the closing >. For example, <script\n> would pass through unfiltered and be rendered as a valid HTML tag by browsers. This is a cross-site scripting (XSS) vector for any application that relies on this extension to sanitize untrusted user input. All applications using the DisallowedRawHtml extension to process untrusted markdown are affected. Applications that use a dedicated HTML sanitizer (such as HTML Purifier) on the rendered output are not affected. This issue has been patched in version 2.8.1.
Analysis
The DisallowedRawHtml extension in PHP Commonmark (league/commonmark) versions prior to 2.8.1 can be bypassed by injecting whitespace characters between HTML tag names and closing brackets, allowing malicious scripts to pass sanitization filters and execute in user browsers. Applications relying solely on this extension to sanitize untrusted markdown input are vulnerable to cross-site scripting attacks, though those using additional HTML sanitizers are unaffected. …
Sign in for full analysis, threat intelligence, and remediation guidance.
Remediation
Within 30 days: Identify affected systems and apply vendor patches as part of regular patch cycle. Verify Content-Security-Policy and output encoding.
Sign in for detailed remediation steps.
Priority Score
Share
External POC / Exploit Code
Leaving vuln.today