User Profile Builder WordPress Plugin Authentication Bypass (Type Confusion Admin Takeover)

Technical Analysis

CVE-2026-15826 stems from the plugin's wppb_log_in_user() function calling absint() on the return value of wp_insert_user() prior to performing an is_wp_error() check. When a registration request submits a username between 61-70 characters, WordPress core returns a WP_Error object instead of a user ID; absint() coerces this object into the integer 1, which corresponds to the default Administrator account on most WordPress installs. The plugin then generates a transient-backed autologin nonce tied to this coerced ID, allowing the attacker to authenticate as user ID 1 without credentials. This is a textbook type confusion / improper error-handling vulnerability requiring no authentication and no user interaction, rated CVSS 9.8. Organizations running AI agents or automation pipelines that manage WordPress sites (e.g., agents performing CMS content publishing, SEO management, or plugin administration via WP-CLI/REST API integrations) could have their credentials, API keys, or automation tokens exposed if an attacker pivots from admin takeover to install malicious plugins or exfiltrate stored secrets, making this agent-relevant in environments where WordPress is part of an agentic workflow or MCP-integrated toolchain.

Affected Systems

WordPress websites running the User Profile Builder plugin (including User Profile Builder Pro) versions up to and including 3.16.4; affects any WordPress core installation with default user registration enabled where the plugin is active.

Indicators of Compromise

• No known indicators of compromise published at this time; monitor for anomalous wp_insert_user() calls, unusual registration attempts with 61-70 character usernames, and unexpected admin logins via transient-based autologin nonces.

Remediation Steps

1. 1

   Update Plugin

   Immediately update User Profile Builder to a patched version above 3.16.4 once released by the vendor.

2. 2

   Disable User Registration Temporarily

   If a patch is unavailable, disable public user registration on affected WordPress sites to eliminate the attack vector.

3. 3

   Audit Admin Accounts

   Review user ID 1 and all administrator accounts for unauthorized changes, new admin users, or suspicious login activity.

4. 4

   Rotate Credentials and Keys

   Rotate WordPress admin passwords, API keys, and any automation/agent credentials that interact with the affected site.

5. 5

   Deploy WAF Rules

   Implement web application firewall rules to block registration requests with unusually long usernames (61-70 characters).

6. 6

   Monitor Logs

   Review server and plugin logs for evidence of exploitation attempts or successful unauthorized administrative access.