CommonMark has DisallowedRawHtml extension bypass via whitespace in HTML tag names
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
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)
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Composer package | league/commonmark | >= 2.0.0, < 2.8.1 | 2.8.1 |
References
- CVSS vector
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- Weakness (CWE)
CWE-79- Platform
- Composer
- Package
league/commonmark- Disclosed
- 06.03.2026
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-4v6x-c7xx-hw9f - CC BY 4.0
Vulnerability history: league/commonmark
All published vulnerabilities of this package (Composer package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 18
- First → latest
- 09.2019 09.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 10
- Medium 8
Disclosures per year
- 2019: 1
- 2020: 0
- 2021: 0
- 2022: 1
- 2023: 0
- 2024: 1
- 2025: 1
- 2026: 14
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CommonMark has DisallowedRawHtml extension bypass via whitespace in HTML tag names this entry
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