CVE-2017-11173 in rack-cors
Summary
by MITRE
Missing anchor in generated regex for rack-cors before 0.4.1 allows a malicious third-party site to perform CORS requests. If the configuration were intended to allow only the trusted example.com domain name and not the malicious example.net domain name, then example.com.example.net (as well as example.com-example.net) would be inadvertently allowed.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 12/12/2022
The vulnerability identified as CVE-2017-11173 resides within the rack-cors gem version 0.4.0 and earlier, representing a critical flaw in Cross-Origin Resource Sharing implementation that enables malicious third-party sites to bypass intended access controls. This issue stems from improper handling of domain name matching within regular expressions used by the gem's CORS configuration mechanism, creating a security gap that allows unauthorized cross-origin requests to succeed when they should be blocked. The flaw manifests specifically in how the gem constructs regular expressions for domain validation, failing to properly anchor the patterns to prevent partial matches that could be exploited by attackers.
The technical root cause of this vulnerability lies in the missing anchors within the generated regular expressions, which allows for substring matching rather than exact domain matching. When administrators configure CORS policies to permit requests only from trusted domains such as example.com, the flawed implementation fails to ensure that only exact domain matches are accepted. Instead, malicious actors can craft domain names that contain the legitimate domain as a substring, such as example.com.example.net or example.com-example.net, which would incorrectly match the regex pattern and grant unauthorized access. This represents a classic example of improper input validation and regex construction that violates fundamental security principles for web application development.
The operational impact of this vulnerability extends beyond simple access control bypass, creating potential security risks that could lead to data exposure, privilege escalation, or further exploitation within affected applications. Attackers leveraging this flaw can perform unauthorized CORS requests against applications that rely on rack-cors for security, potentially accessing sensitive resources or performing actions that should be restricted to legitimate domains. The vulnerability affects any application using rack-cors version 0.4.0 or earlier that has configured CORS policies, making it particularly dangerous in environments where multiple applications share common infrastructure or where CORS policies are not rigorously tested for edge cases. This issue directly relates to CWE-20, which describes improper input validation, and CWE-1107, which addresses regular expression vulnerabilities.
Mitigation strategies for CVE-2017-11173 require immediate action to upgrade the rack-cors gem to version 0.4.1 or later, where the vulnerability has been patched by implementing proper regex anchoring. Organizations should conduct comprehensive security reviews of their CORS configurations to identify any potential misconfigurations that could be exploited, particularly focusing on domain matching patterns and ensuring that all domain restrictions are properly enforced. Additionally, implementing comprehensive testing procedures for CORS policies, including edge case testing with subdomain variations and partial matches, helps prevent similar vulnerabilities from being introduced in future configurations. The fix addresses this vulnerability by ensuring that regular expressions used for domain validation are properly anchored, preventing the substring matching behavior that enabled the exploitation. This remediation aligns with ATT&CK technique T1071.004, which covers application layer protocol manipulation, and demonstrates the importance of proper input validation and pattern matching in security implementations. Organizations should also consider implementing additional security controls such as Content Security Policy headers and regular security scanning to detect similar vulnerabilities in other components of their web application infrastructure.