CVE-2015-4520 in Firefox
Summary
by MITRE
Mozilla Firefox before 41.0 and Firefox ESR 38.x before 38.3 allow remote attackers to bypass CORS preflight protection mechanisms by leveraging (1) duplicate cache-key generation or (2) retrieval of a value from an incorrect HTTP Access-Control-* response header.
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Analysis
by VulDB Data Team • 10/22/2024
The vulnerability described in CVE-2015-4520 represents a critical security flaw in Mozilla Firefox browsers that undermines the Cross-Origin Resource Sharing (CORS) preflight protection mechanisms. This weakness affects versions prior to Firefox 41.0 and Firefox ESR 38.x before 38.3, creating a significant risk for web applications that rely on CORS for security boundary enforcement. The vulnerability stems from improper handling of HTTP headers during CORS preflight requests, specifically through two distinct attack vectors that bypass the intended security controls.
The technical implementation of this vulnerability involves two primary exploitation methods that leverage cache management flaws within Firefox's HTTP handling. The first method exploits duplicate cache-key generation, where the browser incorrectly processes multiple requests with identical cache keys, leading to improper header validation. This allows attackers to manipulate the caching mechanism to retrieve cached responses that should have been filtered out by CORS preflight checks. The second method targets the retrieval of values from incorrect HTTP Access-Control-* response headers, where Firefox incorrectly processes or retrieves header information from cached responses that do not match the current request context. Both approaches fundamentally undermine the CORS protocol's integrity by allowing unauthorized cross-origin resource access.
The operational impact of this vulnerability extends beyond simple data leakage, as it enables sophisticated attacks that can bypass security controls designed to prevent malicious cross-origin requests. Attackers can exploit this flaw to perform unauthorized access to resources that should be restricted by CORS policies, potentially leading to data exfiltration, privilege escalation, or further exploitation of web applications. The vulnerability particularly affects web applications that rely heavily on CORS for security enforcement, as it allows attackers to circumvent the preflight mechanism that should validate cross-origin requests before they are executed. This creates a dangerous situation where legitimate security controls are rendered ineffective, enabling attackers to access protected resources without proper authorization.
This vulnerability maps directly to CWE-284, which addresses improper access control in software systems, and relates to ATT&CK technique T1071.004 for application layer protocol manipulation. The flaw demonstrates how cache-related security issues can be exploited to bypass network security controls, highlighting the importance of proper header validation and cache management in web browsers. Organizations should implement immediate mitigations including updating to patched versions of Firefox, implementing additional application-level CORS validation, and monitoring for suspicious cross-origin requests. Network administrators should also consider implementing web application firewalls that can detect and block anomalous CORS header patterns. The vulnerability underscores the critical need for comprehensive security testing of browser components and cache mechanisms, as these systems form fundamental parts of web security infrastructure that, when compromised, can undermine entire application security models.