CVE-2018-20745 in Yii
Summary
by MITRE
Yii 2.x through 2.0.15.1 actively converts a wildcard CORS policy into reflecting an arbitrary Origin header value, which is incompatible with the CORS security design, and could lead to CORS misconfiguration security problems.
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Analysis
by VulDB Data Team • 07/03/2023
The vulnerability identified as CVE-2018-20745 affects the Yii 2 web application framework version 2.x through 2.0.15.1 and represents a critical security flaw in the Cross-Origin Resource Sharing implementation. This issue stems from how the framework handles wildcard CORS policies, specifically when converting these policies into reflecting arbitrary Origin header values. The flaw fundamentally undermines the security model that CORS is designed to enforce, creating potential attack vectors that could compromise web application security.
The technical root cause of this vulnerability lies in the improper handling of CORS policy configuration within the Yii 2 framework. When developers configure a wildcard CORS policy, the system should enforce strict security boundaries by either rejecting requests from arbitrary origins or implementing proper origin validation mechanisms. However, the vulnerable implementation actively converts the wildcard policy into a mechanism that reflects back the Origin header value from incoming requests, effectively allowing any origin to access the application resources. This behavior directly contradicts the fundamental CORS security principle that access control should be explicitly defined and controlled rather than automatically reflecting client-provided values.
From an operational perspective, this vulnerability creates significant security risks for applications using the affected Yii 2 versions. Attackers could exploit this misconfiguration to bypass CORS protections that are typically in place to prevent cross-site request forgery attacks and unauthorized data access. The impact extends beyond simple data exposure, as it could enable more sophisticated attacks such as credential theft, data manipulation, or privilege escalation within the application context. The vulnerability is particularly dangerous because it operates at the framework level, meaning that all applications built on affected Yii 2 versions could be compromised without requiring specific application-level exploitation.
Security practitioners should understand that this vulnerability aligns with CWE-346, which addresses "Origin Validation Error" in web applications, and represents a direct violation of the CORS security model. The issue also maps to ATT&CK technique T1566, specifically the "Phishing" sub-technique, as attackers could leverage the compromised CORS configuration to craft more effective phishing attacks against authenticated users. Organizations should immediately update to Yii 2.0.15.2 or later versions where this vulnerability has been patched, as the fix properly implements CORS security controls that prevent automatic reflection of Origin headers. Additional mitigations include implementing explicit CORS policy configurations, avoiding wildcard origins in production environments, and conducting thorough security testing of CORS implementations to ensure proper origin validation and access control enforcement.
The broader implications of this vulnerability highlight the critical importance of proper security implementation in web frameworks, as flaws in core security mechanisms can have widespread consequences across all applications built on affected platforms. This issue demonstrates how seemingly minor implementation details in security controls can lead to significant vulnerabilities that undermine the entire security posture of web applications relying on the framework.