CVE-2026-39772 in Captcha Plugin
Summary
by MITRE • 10/06/2026
Unauthenticated Bypass Vulnerability in Captcha by BestWebSoft <= 5.2.8 versions.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 10/06/2026
The vulnerability identified as an unauthenticated bypass of the CAPTCHA mechanism within BestWebSoft plugins, specifically affecting versions up to and including 5.2.8, represents a critical failure in access control logic that undermines one of the primary defenses against automated abuse on web applications. This flaw is rooted in how the application validates user input during form submissions intended for actions such as login, registration, or password recovery. Instead of strictly enforcing the presence and correctness of the CAPTCHA token before processing sensitive operations, the underlying code allows requests to proceed without valid verification under certain conditions. This architectural weakness effectively neutralizes the anti-bot protection designed to prevent automated scripts from performing repetitive tasks, thereby exposing the application to a range of malicious activities that rely on high-volume, low-cost automation.
From a technical perspective, this issue aligns with CWE-287, which describes Improper Authentication, and more specifically relates to weaknesses in verifying credentials or security tokens during authentication processes. The root cause typically involves insufficient validation checks where the server-side logic fails to cross-reference the submitted CAPTCHA solution against the session data or expected values before granting access or executing state-changing operations. In many WordPress-based environments utilizing BestWebSoft plugins, this bypass allows attackers to submit forms without ever solving a challenge-response test. This omission in input validation creates an open channel for automated tools to interact with protected endpoints as if they were human users, effectively rendering the CAPTCHA implementation decorative rather than functional.
The operational impact of this vulnerability is significant due to its direct facilitation of brute-force attacks and credential stuffing campaigns. Attackers can leverage automated scripts to attempt thousands or millions of password combinations against user accounts without being throttled or blocked by rate-limiting mechanisms that are often tied to CAPTCHA triggers. This capability drastically reduces the time and resources required for an attacker to compromise legitimate user credentials, leading to potential unauthorized access, data exfiltration, and account takeover. Furthermore, if the affected forms handle password resets or email verifications, the vulnerability can be exploited to trigger mass notification emails, resulting in denial of service through resource exhaustion or facilitating phishing campaigns by generating a high volume of suspicious-looking authentication links sent to targeted individuals.
This attack vector is well-documented within the MITRE ATT&CK framework under techniques such as T1110, which covers Brute Force attacks, and potentially T1596, Search Open Technical Databases for vulnerabilities that attackers use to identify targets. The ability to bypass CAPTCHA also supports reconnaissance activities by allowing automated enumeration of valid usernames through error message analysis or response time differences during login attempts. Consequently, the security posture of any website relying on these vulnerable plugin versions is severely compromised, as the fundamental assumption that a human user has interacted with the form cannot be trusted.
To mitigate this risk, immediate action must be taken to update all affected BestWebSoft plugins to version 5.2.9 or later, where the validation logic for CAPTCHA tokens has been corrected to ensure strict enforcement before processing requests. Administrators should verify that no custom code overrides have disabled these security checks and review server logs for signs of automated traffic patterns indicative of brute-force attempts prior to patching. Additionally, implementing network-level rate limiting and IP-based blocking can provide a secondary layer of defense against residual threats while the update is deployed. It is also advisable to enforce strong password policies and multi-factor authentication where possible to reduce the impact if credentials are compromised through other means. Regular security audits and vulnerability scanning should be integrated into the maintenance cycle to detect similar misconfigurations in third-party components before they can be exploited by malicious actors seeking to automate interactions with web applications.