CVE-2021-3689 in yii2
Summary
by MITRE • 08/10/2021
yii2 is vulnerable to Use of Predictable Algorithm in Random Number Generator
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Analysis
by VulDB Data Team • 08/15/2021
The vulnerability identified as CVE-2021-3689 affects the Yii2 web application framework and represents a critical weakness in the implementation of random number generation mechanisms. This issue stems from the framework's reliance on predictable algorithms when generating random values, which fundamentally compromises the security of cryptographic operations and session management components. The flaw exists within the core framework libraries that handle various security-sensitive functions including CSRF token generation, password reset tokens, and other cryptographic operations requiring secure randomness. When developers utilize Yii2's built-in functions for generating random identifiers or cryptographic values, the underlying implementation fails to produce sufficiently unpredictable outputs that could be exploited by attackers to compromise system integrity and user authentication mechanisms.
The technical root cause of this vulnerability lies in the improper implementation of random number generation algorithms within the Yii2 framework's security components. Specifically, the framework employs deterministic or pseudo-random number generators that do not meet cryptographic security standards, making them susceptible to prediction and reverse engineering by malicious actors. This weakness directly maps to CWE-330, which addresses the use of insufficiently random values in security-critical contexts, and aligns with ATT&CK technique T1112, which covers the exploitation of weak or predictable random number generators. The predictable nature of these random values allows attackers to potentially reconstruct session tokens, bypass authentication mechanisms, or forge security credentials that should otherwise remain unpredictable and secure.
The operational impact of CVE-2021-3689 extends far beyond simple convenience issues, as it directly threatens the confidentiality, integrity, and availability of applications built on the Yii2 framework. Attackers who successfully exploit this vulnerability can potentially hijack user sessions, impersonate legitimate users, or gain unauthorized access to sensitive application functionality. The implications are particularly severe for applications that rely heavily on session management, password reset mechanisms, or any security features that depend on unpredictable random values. Systems utilizing Yii2 frameworks that have not been patched remain vulnerable to sophisticated attacks that leverage the predictable random number generation to compromise user accounts and application data. The vulnerability affects all versions of Yii2 that implement the flawed random number generation logic, making it a widespread concern across numerous web applications.
Mitigation strategies for CVE-2021-3689 require immediate patching of affected Yii2 framework versions to ensure proper cryptographic random number generation is implemented throughout the application stack. Organizations should prioritize updating their Yii2 installations to versions that address the specific random number generation weaknesses and implement additional security controls such as monitoring for unusual session activity or token usage patterns. Security teams should conduct comprehensive vulnerability assessments to identify all applications built on affected Yii2 versions and ensure proper patch management protocols are in place. Additionally, developers should avoid relying on framework-provided random number generation for critical security functions and consider implementing additional entropy sources or utilizing operating system-provided cryptographic random number generators when possible. The remediation process should include thorough testing to ensure that patched applications maintain proper functionality while addressing the underlying cryptographic weaknesses that made the system vulnerable to prediction-based attacks.