Summary
Workarounds
- Set the
html_inputconfiguration option to'escape'or'strip'to disable all raw HTML, though this is a broader restriction than theDisallowedRawHtmlextension provides. - Pass the rendered HTML through a dedicated HTML sanitizer before serving it to users (always recommended)
Impact
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.
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Fixed in 2.8.1. The regex character class [ \/>] was changed to [\s\/>] to match all whitespace characters that browsers accept as valid tag name terminators.
Frequently Asked Questions
- What is CVE-2026-30838? CVE-2026-30838 is a medium-severity cross-site scripting (XSS) vulnerability in league/commonmark (composer), affecting versions >= 2.0.0, <= 2.8.0. It is fixed in 2.8.1. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- Which versions of league/commonmark are affected by CVE-2026-30838? league/commonmark (composer) versions >= 2.0.0, <= 2.8.0 is affected.
- Is there a fix for CVE-2026-30838? Yes. CVE-2026-30838 is fixed in 2.8.1. Upgrade to this version or later.
- Is CVE-2026-30838 exploitable, and should I be worried? Whether CVE-2026-30838 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-30838 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-30838? Upgrade
league/commonmarkto 2.8.1 or later.