CVE-2026-33889: ApostropheCMS: Stored XSS via CSS Custom Property Injection in `@apostrophecms/color-field` Escaping Style Tag Context

Vulnerability summary

CVE-2026-33889 is a medium (CVSS 5.4) Cross-site Scripting vulnerability in apostrophecms apostrophe. EPSS estimates a 0.26% chance of exploitation in the next 30 days, higher than 16.2% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 15-04-2026
   PublishedVulnerability first disclosed.
2. 16-04-2026
   Last modifiedVulnerability information updated.

Description

ApostropheCMS is an open-source Node.js content management system. Versions 4.28.0 and prior contain a stored cross-site scripting vulnerability in the @apostrophecms/color-field module, where color values prefixed with -- bypass TinyColor validation intended for CSS custom properties, and the launder.string() call performs only type coercion without stripping HTML metacharacters. These unsanitized values are then concatenated directly into <style> tags both in per-widget style elements rendered for all visitors and in the global stylesheet rendered for editors, with the output marked as safe HTML. An editor can inject a value which closes the style tag and executes arbitrary JavaScript in the browser of every visitor to any page containing the affected widget. This enables mass session hijacking, cookie theft, and privilege escalation to administrative control if an admin views draft content. This issue has been fixed in version 4.29.0.

• CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.