CVE-2026-30838 — league/commonmark
CVE-2026-30838 is a Cross-site Scripting (XSS) vulnerability in league/commonmark. A fix is available for league/commonmark — see the affected versions and patch details below.
league/commonmark: DisallowedRawHtml extension bypass via whitespace in HTML tag names
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-30838.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
league/commonmarkReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
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 Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | league/commonmark | ≥ 2.0.0&&< 2.8.1 | 2.8.1composer require league/commonmark:^2.8.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for league/commonmark, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update league/commonmark to 2.8.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-30838 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-30838 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-30838. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Frequently Asked Questions
Is CVE-2026-30838 in your dependencies?
O3 Security finds CVE-2026-30838 across Packagist dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.