CVE-2015-4050 in Symfony
Summary
by MITRE
FragmentListener in the HttpKernel component in Symfony 2.3.19 through 2.3.28, 2.4.9 through 2.4.10, 2.5.4 through 2.5.11, and 2.6.0 through 2.6.7, when ESI or SSI support enabled, does not check if the _controller attribute is set, which allows remote attackers to bypass URL signing and security rules by including (1) no hash or (2) an invalid hash in a request to /_fragment.
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Analysis
by VulDB Data Team • 11/24/2024
The vulnerability identified as CVE-2015-4050 resides within the HttpKernel component of the Symfony web application framework, specifically affecting versions ranging from 2.3.19 through 2.3.28, 2.4.9 through 2.4.10, 2.5.4 through 2.5.11, and 2.6.0 through 2.6.7. This security flaw manifests in the FragmentListener class which handles Edge Side Includes and Server Side Includes functionality, creating a critical pathway for unauthorized access when these features are enabled. The vulnerability stems from insufficient validation of the _controller attribute within fragment requests, allowing malicious actors to exploit the system's trust in fragment processing mechanisms.
The technical exploitation of this vulnerability occurs through the /_fragment endpoint which serves as the gateway for ESI and SSI processing in Symfony applications. When attackers craft requests to this endpoint with either no hash value or an invalid hash, the FragmentListener fails to properly validate whether the _controller attribute has been set. This omission creates a bypass condition where the system accepts fragment requests without proper authentication or authorization checks, effectively allowing attackers to execute arbitrary controller actions through the fragment processing pipeline. The flaw operates at the application layer and specifically targets the security controls that should normally validate fragment requests before execution.
The operational impact of CVE-2015-4050 represents a significant threat to Symfony applications that utilize ESI or SSI functionality, as it allows remote attackers to bypass critical URL signing mechanisms and security rules that are designed to prevent unauthorized access to application resources. This vulnerability enables attackers to potentially execute malicious code, access restricted functionality, or perform unauthorized operations within the application context. The attack surface expands particularly for applications that rely on fragment-based content delivery and have not implemented additional compensating controls to mitigate the missing validation.
Organizations affected by this vulnerability should immediately implement mitigations including upgrading to patched versions of Symfony where the FragmentListener properly validates the _controller attribute, ensuring that all fragment requests contain valid authentication tokens and hashes before processing. Security teams should also consider implementing additional network-level controls such as web application firewalls that can detect and block suspicious fragment requests, while application developers should review their fragment processing configurations to ensure proper access controls are in place. This vulnerability aligns with CWE-284 Access Control Issues and maps to ATT&CK technique T1078 Valid Accounts, as it allows unauthorized access through legitimate application pathways.
The root cause of this vulnerability demonstrates a classic security misconfiguration where proper input validation was omitted from the fragment processing pipeline, creating an attack vector that bypasses expected security controls. The flaw specifically relates to the absence of proper attribute validation in the FragmentListener implementation, where the system should have enforced that all fragment requests contain valid controller references before execution. This represents a failure in the principle of least privilege and demonstrates how insufficient validation in application components can create pathways for privilege escalation and unauthorized access to application resources. The vulnerability underscores the importance of comprehensive input validation and proper security controls in web application frameworks, particularly when handling fragment-based content delivery mechanisms that are integral to modern web applications.