CVE-2026-121763 in Hummingbird Plugin
Summary
by MITRE • 11/09/2026
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') vulnerability in WPMU DEV Hummingbird hummingbird-performance allows Stored XSS.This issue affects Hummingbird: from n/a through 3.21.2.
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Analysis
by VulDB Data Team • 11/09/2026
The identified security flaw represents a classic instance of Improper Neutralization of Input During Web Page Generation, commonly known as Cross-Site Scripting or XSS. Specifically, this vulnerability manifests as a Stored XSS within the WPMU DEV Hummingbird performance optimization plugin for WordPress. Unlike reflected XSS attacks where malicious scripts are executed via manipulated URLs, stored vulnerabilities persist on the target server because user-supplied input is saved in a database or other persistent storage mechanism without adequate sanitization. When the compromised data is subsequently retrieved and rendered to users by the web application, the embedded script executes within the context of the victim's browser session. This specific vulnerability affects versions of Hummingbird from its initial release through version 3.21.2, indicating a long-standing exposure window for sites utilizing these older iterations of the plugin.
From a technical perspective, the root cause lies in the failure to properly sanitize or encode user-controlled input before it is included in dynamically generated web pages. In the context of WordPress plugins like Hummingbird, this often occurs when settings, configuration options, or cached data entered by administrators are stored directly into the database and later echoed back onto admin dashboard pages without applying appropriate output encoding functions such as esc_html or wp_kses_post. Because the payload is stored server-side, it does not require complex social engineering to trigger for every visitor; rather, any user with access to the affected page will have their session compromised upon loading the malicious content. The attacker typically needs a low-privilege account capable of saving settings within the plugin interface to inject the persistent script, which then becomes available to all subsequent viewers or administrators accessing that specific configuration screen.
The operational impact of this vulnerability is severe due to its persistence and potential for widespread exploitation. Successful exploitation allows an authenticated attacker with minimal privileges to execute arbitrary JavaScript code in the context of other users' browsers. This can lead to session hijacking, where the attacker steals administrative cookies to gain full control over the WordPress installation. It also enables credential harvesting by injecting fake login forms or keyloggers that capture usernames and passwords as they are typed. Furthermore, attackers may use this access to deface websites, redirect traffic to malicious domains for phishing campaigns, or install additional malware such as web shells. The persistence of the attack vector means that remediation is not immediate; even after a user clears their browser cache, the vulnerability remains active on the server until the plugin is updated and existing stored payloads are purged from the database.
This flaw aligns with CWE-79, which defines Improper Neutralization of Input During Web Page Generation: Cross-site Scripting (XSS), and maps to MITRE ATT&CK techniques such as T1059 Command and Scripting Interpreter for executing arbitrary code within the browser environment, and potentially T1534 Internal Spearphishing if used to target other users on the same network. To mitigate this risk, site administrators must immediately update the Hummingbird plugin to version 3.21.3 or later, where input validation and output encoding have been corrected by the developers. Additionally, implementing a Web Application Firewall can provide an additional layer of defense by filtering out malicious script injections before they reach the application logic. Regular security audits and adherence to secure coding standards that mandate strict type checking and context-aware escaping are essential preventive measures against this class of vulnerability in future development cycles.