CVE-2015-6250 in simple-php-captcha
Summary
by MITRE
simple-php-captcha before commit 9d65a945029c7be7bb6bc893759e74c5636be694 allows remote attackers to automatically generate the captcha response by running the same code on the client-side.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 12/27/2022
The vulnerability identified as CVE-2015-6250 affects the simple-php-captcha library, which is commonly used for generating CAPTCHA images to prevent automated bot access on web applications. This weakness stems from a fundamental flaw in the cryptographic implementation where the CAPTCHA generation algorithm can be reverse-engineered and replicated on the client-side. The vulnerability exists because the library uses predictable and deterministic methods for generating CAPTCHA values, making it susceptible to automated attacks that can bypass the security mechanism designed to distinguish human users from bots.
The technical flaw manifests in the way the CAPTCHA system handles its internal state and randomization processes. When the simple-php-captcha library generates a CAPTCHA image, it employs algorithms that, while appearing random, actually follow predictable patterns that can be reconstructed through reverse engineering. Attackers can analyze the CAPTCHA generation process and replicate the same algorithm on their client systems to automatically determine the correct CAPTCHA response without human intervention. This represents a critical failure in the security model where the system's ability to authenticate human users becomes compromised due to insufficient entropy and predictable generation mechanisms.
The operational impact of this vulnerability is significant as it completely undermines the security purpose of CAPTCHA systems in web applications. Remote attackers can now automate the process of bypassing CAPTCHA protections across multiple websites that utilize the simple-php-captcha library, leading to potential abuse of services such as comment spam, account creation, form flooding, and other automated attacks. The vulnerability affects the core security controls designed to prevent automated access, making it easier for malicious actors to conduct large-scale attacks without manual intervention, which can result in service degradation, data integrity issues, and increased operational costs for defenders.
This vulnerability aligns with CWE-330, which addresses the use of insufficiently random values, and represents a failure in proper random number generation within security-critical components. From an attack perspective, it maps to techniques described in the ATT&CK framework under credential access and privilege escalation tactics where attackers can bypass authentication mechanisms through automated means. The vulnerability also relates to CWE-287, which deals with improper handling of authentication tokens, as the CAPTCHA serves as an authentication token that can be automatically generated and validated. Organizations using this library should immediately update to versions that implement proper cryptographic randomization, ensure the use of secure random number generators, and consider implementing additional layers of authentication verification such as rate limiting, IP reputation checks, or more sophisticated CAPTCHA systems that cannot be easily reverse-engineered.
The remediation approach requires updating to the fixed version of simple-php-captcha after commit 9d65a945029c7be7bb6bc893759e74c5636be694, which addresses the predictable generation algorithm by implementing proper cryptographic randomization. Security teams should also conduct comprehensive audits of all applications using this library to identify and remediate any other potential security flaws in authentication and validation mechanisms. Additionally, implementing multi-factor authentication and additional verification layers can help mitigate the impact of such vulnerabilities while ensuring that security controls remain robust against automated attack vectors.