Headline
GHSA-3527-qv2q-pfvx: league/commonmark contains a XSS vulnerability in Attributes extension
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
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:
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 on are considered unsafe and blocked by default
- Support for an explicit allowlist of allowed HTML attributes
- Manually-added href and src attributes now respect the existing allow_unsafe_links configuration option
Workarounds
If upgrading is not feasible, please consider:
- Disabling the AttributesExtension for untrusted users
- Filtering the rendered HTML through a library like HTMLPurifier
References
- GHSA-3527-qv2q-pfvx
- thephpleague/commonmark@f0d626c