CommonMark has DisallowedRawHtml extension bypass via whitespace in HTML tag names

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.1 · CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N · score source

EPSS

• CVE-2026-30838: 0.24% estimated exploitation activity in the next 30 days; percentile 15.88 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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. Patches 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. Workarounds - Set the htmlinput configuration option to 'escape' or 'strip' to disable all raw HTML, though this is a broader restriction than the DisallowedRawHtml extension provides. - Pass the rendered HTML through…

Published by the source

6 Mar 2026, 23:27 UTC

Last modified by the source

20 Mar 2026, 21:26 UTC

Source last checked

26 Oct 2026, 14:38 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-4v6x-c7xx-hw9f

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# Patch Next advisory brief: CVE-2026-30838

Advisory: CommonMark has DisallowedRawHtml extension bypass via whitespace in HTML tag names
IDs: CVE-2026-30838, GHSA-4v6x-c7xx-hw9f
Record: https://patchnext.com/advisories/cve-2026-30838 (revision 1, source checked 26 Oct 2026, 14:38 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: league/commonmark 2.8.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist league/commonmark: affected >= 2.0.0, <= 2.8.0; fixed in 2.8.1

## Suggested inspection (read-only)
- Packagist league/commonmark: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on league/commonmark.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-4v6x-c7xx-hw9f (GitHub-reviewed advisory GHSA-4v6x-c7xx-hw9f lists affected ranges: Packagist league/commonmark >=2.0.0 <=2.8.0)

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