GHSA-3527-qv2q-pfvx
MEDIUMleague/commonmark contains a XSS vulnerability in Attributes extension
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.
Blast Radius
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
Summary
Cross-site scripting (XSS) vulnerability in the Attributes extension of the league/commonmark library (versions 1.5.0 through 2.6.x) allows remote attackers to insert malicious JavaScript calls into HTML.
Details
The league/commonmark library provides configuration options such as html_input: 'strip' and allow_unsafe_links: false to mitigate cross-site scripting (XSS) attacks by stripping raw HTML and disallowing unsafe links. However, when the Attributes Extension is enabled, it introduces a way for users to inject arbitrary HTML attributes into elements via Markdown syntax using curly braces.
As a result, even with the secure configuration shown above, an attacker can inject dangerous attributes into applications using this extension via a payload such as:
![](){onerror=alert(1)}
Which results in the following HTML:
<p><img onerror="alert(1)" src="" alt="" /></p>
Which causes the JS to execute immediately on page load.
Patches
Version 2.7.0 contains three changes to prevent this XSS attack vector:
- All attributes starting with
onare considered unsafe and blocked by default - Support for an explicit allowlist of allowed HTML attributes
- Manually-added
hrefandsrcattributes now respect the existingallow_unsafe_linksconfiguration option
Workarounds
If upgrading is not feasible, please consider:
- Disabling the
AttributesExtensionfor untrusted users - Filtering the rendered HTML through a library like HTMLPurifier
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | league/commonmark | ≥ 1.5.0&&< 2.7.0 | 2.7.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for league/commonmark. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update league/commonmark to 2.7.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-3527-qv2q-pfvx 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 pinpoints whether GHSA-3527-qv2q-pfvx is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-3527-qv2q-pfvx. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-3527-qv2q-pfvx in your dependencies?
O3 detects GHSA-3527-qv2q-pfvx across Packagist dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.