CVE-2021-33851: 
WordPress vulnerability analysis and mitigation

A Stored Cross-Site Scripting (XSS) vulnerability was identified in WordPress Customize Login Image plugin version 3.4, tracked as CVE-2021-33851. The vulnerability was discovered on November 30, 2021, and was officially reported on December 2, 2021. This security flaw affects the WordPress plugin 'Customize Login Image' and allows attackers to inject malicious JavaScript code that executes when users access the WordPress login page (CSW ZeroDay).

The vulnerability exists in the 'Custom Logo Link' field (clilogourl parameter) of the plugin's settings. When an attacker injects malicious JavaScript code into this field, the payload gets stored and executes whenever a user opens the WordPress login page at /wp-login.php. The vulnerability was confirmed in version 3.4 of the Customize Login Image plugin (CSW ZeroDay).

The vulnerability allows attackers to inject malicious code into the vulnerable variable and exploit the application through Cross-Site Scripting. Attackers can modify the code to obtain session information of other users and potentially compromise user machines. The XSS payload executes whenever users access the login page of the WordPress application (CSW ZeroDay).

The vulnerability was fixed by the WordPress team on December 7, 2021, and the plugin was reopened for download on the same day. Recommended security measures include performing context-sensitive encoding of untrusted input, implementing input validation for special characters, explicitly setting character set encoding for each page, and encoding dynamic output elements. Additionally, the default Cross-Site Scripting mitigation settings in wp.config file can help prevent XSS attacks (CSW ZeroDay).