CVE-2021-34640 is a Reflected Cross-Site Scripting (XSS) vulnerability identified in the Securimage-WP-Fixed WordPress plugin, specifically in versions up to and including 3.5.4. The vulnerability arises from the insecure use of the PHP global variable $_SERVER['PHP_SELF'] within the ~/securimage-wp.php file. This variable contains the filename of the currently executing script and can be manipulated by an attacker to inject arbitrary JavaScript code into the web page output. Because the plugin fails to properly sanitize this input before reflecting it back to the user, an attacker can craft a malicious URL that, when visited by a victim, executes injected scripts in the victim's browser context. This can lead to theft of cookies, session tokens, or other sensitive information, as well as unauthorized actions performed on behalf of the user. The vulnerability requires no authentication but does require user interaction, as the victim must visit the maliciously crafted URL. The CVSS v3.1 base score is 6.1 (medium severity), reflecting the network attack vector, low attack complexity, no privileges required, but user interaction is necessary. The impact affects confidentiality and integrity but not availability. No known exploits in the wild have been reported, and no official patches are linked in the provided information, though updating or patching the plugin is recommended. This vulnerability is classified under CWE-79, a common and well-understood XSS category.

For European organizations using WordPress websites with the Securimage-WP-Fixed plugin version 3.5.4 or earlier, this vulnerability poses a moderate risk. Exploitation could allow attackers to hijack user sessions, steal sensitive data, or perform actions on behalf of authenticated users, potentially leading to data breaches or unauthorized access to internal systems. This is particularly concerning for organizations handling personal data under GDPR, as exploitation could result in non-compliance and regulatory penalties. Additionally, compromised websites can damage organizational reputation and trust. Since the vulnerability requires user interaction via a crafted URL, phishing campaigns could be used to target employees or customers, increasing the risk of successful exploitation. The scope is limited to websites using this specific plugin version, but given WordPress's popularity in Europe, the impact could be significant in sectors relying on this plugin for CAPTCHA or security features.

European organizations should immediately audit their WordPress installations to identify the presence of the Securimage-WP-Fixed plugin and verify the version in use. If version 3.5.4 or earlier is detected, the plugin should be updated to a fixed version if available, or replaced with a more secure alternative CAPTCHA plugin that properly sanitizes user input. In the absence of an official patch, organizations can implement web application firewall (WAF) rules to detect and block suspicious requests containing malicious payloads targeting the PHP_SELF parameter. Additionally, applying Content Security Policy (CSP) headers can help mitigate the impact of XSS by restricting the execution of unauthorized scripts. Regular security training to raise awareness about phishing and suspicious links can reduce the risk of user interaction with malicious URLs. Finally, monitoring web server logs for unusual requests and implementing strict input validation and output encoding practices in custom code can further reduce exposure.