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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.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-8981 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-8981

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-8981 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-8981

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.