CVE-2026-30838
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedleague/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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceThe DisallowedRawHtml extension in league/commonmark versions before 2.8.1 fails to block disallowed HTML tags when whitespace characters (newline, tab, or other ASCII whitespace) are inserted between the tag name and the closing >. For example, <script\n> passes through unfiltered and renders as valid HTML in browsers, creating an XSS vector.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 2.8.1CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- Required
- Scope
- Changed
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check the installed league/commonmark versionRun `composer show league/commonmark` or inspect the vendor/league/commonmark/composer.json file to find the version numberAffected if The version is lower than 2.8.1
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Verify the DisallowedRawHtml extension is in useCheck your Markdown parser configuration for calls to `->addExtension(new DisallowedRawHtmlExtension())` or similar extension registrationAffected if The DisallowedRawHtml extension is enabled and the commonmark version is below 2.8.1
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Test if whitespace-bypassed HTML tags are filteredParse a Markdown string containing `<script\n>` (script tag with newline before the closing bracket) using your configured parser and inspect the HTML outputAffected if The raw HTML tag `<script\n>` appears unescaped or unfiltered in the rendered output, indicating the bypass is active
You are affected if you use league/commonmark version below 2.8.1 with the DisallowedRawHtml extension enabled, and your parser allows the test input `<script\n>` to pass through unfiltered.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped2.8.1
Upgrade league/commonmark to version 2.8.1 or later to apply the patch. Alternatively, implement a dedicated HTML sanitizer such as HTML Purifier on the rendered output.
league/commonmark version 2.8.1
- Locate the composer.json file in the project that includes league/commonmark as a dependency
- Update the version constraint for league/commonmark to ^2.8.1 or 2.8.1
- Run 'composer update league/commonmark' to install the patched version
- Verify that DisallowedRawHtml extension now properly blocks tags like '<script\n>'
- Test the application to ensure no regressions were introduced
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation1.0 h
- Testing3.0 h
- Review / QA1.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-30838 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-30838 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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