CVE-2026-16090 describes a stored cross-site scripting vulnerability in the GamiPress WordPress plugin (up to version 7.9.9.1). The vulnerability is due to improper neutralization of input during web page generation (CWE-79). Specifically, the 'heading_size' shortcode attribute in 'gamipress_achievement' does not properly sanitize or escape input, allowing authenticated users with contributor-level privileges or higher to inject arbitrary JavaScript code. The WordPress function wp_kses_post fails to mitigate this because the malicious input is stored within a shortcode attribute rather than as raw HTML, and it is rendered without escaping at page load time, leading to script execution in users' browsers.

An attacker with contributor-level or higher access can inject persistent malicious scripts into pages via the vulnerable shortcode attribute. These scripts execute in the context of any user viewing the affected page, potentially leading to session hijacking, privilege escalation, or other malicious actions limited to the scope of the injected script. The vulnerability has a CVSS 3.1 score of 6.4 (medium severity), indicating a moderate risk with network attack vector, low attack complexity, and no user interaction required.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, restrict contributor-level access to trusted users only and consider disabling or limiting use of the affected shortcode attribute. Monitor vendor channels for updates and apply patches promptly once released.