GHSA-3527-qv2q-pfvx is a medium-severity (CVSS 6.4) 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 contains a XSS vulnerability in Attributes extension
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 GHSA-3527-qv2q-pfvx.
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.
How urgent is this, really
GHSA-3527-qv2q-pfvx plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 376,715 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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
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 | 2.7.0composer require league/commonmark:^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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-3527-qv2q-pfvx can be triaged on real exposure rather than presence alone.
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.
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 GHSA-3527-qv2q-pfvx in your dependencies?
O3 Security finds GHSA-3527-qv2q-pfvx across Packagist dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.