PatchSiren cyber security CVE debrief

CVE-2026-8981 Custom Block Builder CVE debrief

CVE-2026-8981 is a vulnerability in the Custom Block Builder WordPress plugin before version 4.3.0. The plugin does not consistently check the unfiltered_html capability across all paths that write to its block template code fields. This allows administrators on multisite installations (or single-site installs with DISALLOW_UNFILTERED_HTML defined) to inject arbitrary JavaScript that executes for any visitor of pages embedding the affected block. The vulnerability has a CVSS score of 3.5 and a severity of LOW.

Vendor: Custom Block Builder
Product: Custom Block Builder
CVSS: LOW 3.5
CISA KEV: Not listed in stored evidence
Original CVE published: 2026-06-09
Original CVE updated: 2026-06-09
Advisory published: 2026-06-09
Advisory updated: 2026-06-09

Who should care

Administrators of WordPress multisite installations or single-site installs with DISALLOW_UNFILTERED_HTML defined who use the Custom Block Builder plugin.

Technical summary

The Custom Block Builder WordPress plugin before 4.3.0 does not consistently check the unfiltered_html capability across all paths that write to its block template code fields, allowing administrators on multisite installations (or single-site installs with DISALLOW_UNFILTERED_HTML defined) to inject arbitrary JavaScript that executes for any visitor of pages embedding the affected block.

Defensive priority

LOW

Recommended defensive actions

Update the Custom Block Builder plugin to version 4.3.0 or later.
Review and restrict administrative access to the WordPress installation.

Evidence notes

The vulnerability was reported by WPScan.

Official resources

CVE-2026-8981 was published on [2026-06-09T06:16:54.067Z](https://www.cve.org/CVERecord?id=CVE-2026-8981) and modified on [2026-06-09T13:51:18.770Z](https://nvd.nist.gov/vuln/detail/CVE-2026-8981).