The vulnerability identified as CVE-2025-66507 affects 1Panel, an open-source web-based control panel used for Linux server management. Versions 2.0.13 and below improperly enforce CAPTCHA verification on the client side by trusting a parameter controlled by the client without adequate server-side validation. This design flaw corresponds to CWE-602 (Client-Side Enforcement of Server-Side Security), CWE-807 (Reliance on Untrusted Inputs in a Security Decision), and CWE-290 (Authentication Bypass by Spoofing). An unauthenticated attacker can exploit this by manipulating the parameter to disable CAPTCHA, allowing automated login attempts to proceed unchecked. Since CAPTCHA is intended to prevent automated brute-force attacks, its bypass significantly increases the risk of account takeover (ATO) without requiring any user interaction or prior authentication. The vulnerability has a CVSS 3.1 base score of 7.5, reflecting its high severity due to network attack vector, low attack complexity, no privileges required, and no user interaction needed. Although no known exploits are currently reported in the wild, the vulnerability's nature makes it a prime target for automated attacks. The issue is resolved in 1Panel version 2.0.14, which implements proper server-side validation of CAPTCHA enforcement parameters.

For European organizations, this vulnerability poses a significant risk to the confidentiality and integrity of server management accounts. Successful exploitation can lead to unauthorized access to Linux servers managed via 1Panel, potentially allowing attackers to manipulate server configurations, deploy malware, or exfiltrate sensitive data. The bypass of CAPTCHA protections facilitates automated brute-force attacks, increasing the likelihood of account takeover, especially if weak or reused credentials are in use. This can disrupt business operations, compromise customer data, and lead to regulatory non-compliance under GDPR due to unauthorized access incidents. Organizations relying on 1Panel for critical infrastructure management are particularly vulnerable, as attackers gaining control over server management interfaces can escalate attacks within internal networks. The lack of required authentication or user interaction lowers the barrier for exploitation, increasing the threat landscape.

European organizations using 1Panel should immediately upgrade to version 2.0.14 or later to ensure the vulnerability is patched. Until patching is possible, organizations should implement compensating controls such as restricting access to the 1Panel interface via IP whitelisting or VPNs to limit exposure. Enforce strong, unique passwords and consider multi-factor authentication (MFA) for all accounts to reduce the risk of account takeover. Monitor authentication logs for unusual login attempts or patterns indicative of automated attacks. Additionally, review and harden server-side validation logic to ensure no client-controlled parameters can bypass security controls. Conduct regular security assessments and penetration tests focusing on web management interfaces to detect similar flaws. Finally, educate administrators about the risks of client-side enforcement of security controls and the importance of server-side validation.