CVE-2017-20146 in handlers
Summary
by MITRE • 12/28/2022
Usage of the CORS handler may apply improper CORS headers, allowing the requester to explicitly control the value of the Access-Control-Allow-Origin header, which bypasses the expected behavior of the Same Origin Policy.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 04/12/2025
The vulnerability identified as CVE-2017-20146 represents a critical flaw in Cross-Origin Resource Sharing (CORS) implementation that directly undermines the fundamental security principle of the Same Origin Policy. This weakness allows malicious actors to manipulate CORS headers in ways that bypass intended security controls, creating potential attack vectors for cross-site request forgery and data exfiltration. The issue manifests when applications use CORS handlers that permit external parties to dictate the values of Access-Control-Allow-Origin headers, effectively removing the browser-based security restrictions that protect against unauthorized cross-origin requests.
The technical flaw underlying CVE-2017-20146 stems from improper input validation and header manipulation within CORS processing mechanisms. When a web application accepts user-supplied data to construct CORS headers, particularly the Access-Control-Allow-Origin value, it creates an opportunity for attackers to inject malicious origins that can bypass origin validation checks. This vulnerability typically occurs in applications that dynamically generate CORS headers based on request parameters or user input without proper sanitization or validation of the origin values. The flaw is categorized under CWE-346 known as "Improper Verification of Source of a Communication Channel" which directly relates to the failure to properly validate the legitimacy of origin sources in web applications.
The operational impact of this vulnerability extends beyond simple security bypasses to encompass potential data breaches and unauthorized access to sensitive resources. Attackers can exploit this weakness to perform cross-site scripting attacks, steal session cookies, access protected APIs, and exfiltrate sensitive information from authenticated users. The vulnerability is particularly dangerous in environments where applications handle sensitive data or provide privileged access to resources, as it can enable attackers to circumvent security controls that should prevent unauthorized cross-origin access. This flaw can be leveraged in conjunction with other attack vectors to amplify the impact of security breaches, making it a significant concern for organizations relying on CORS-based security mechanisms.
Mitigation strategies for CVE-2017-20146 require implementing strict validation and sanitization of CORS header values, eliminating user-controlled input from CORS configuration, and enforcing a whitelist approach for allowed origins. Organizations should avoid dynamically constructing Access-Control-Allow-Origin headers from external input and instead use static values or predefined lists of trusted origins. Security measures should include implementing proper CORS policies that only allow specific, known origins rather than permitting arbitrary values. The solution aligns with ATT&CK technique T1566 which covers social engineering tactics, as attackers often exploit such vulnerabilities through manipulation of web application behavior to gain unauthorized access. Additionally, implementing Content Security Policy headers and regular security testing can help detect and prevent exploitation of this vulnerability, while following the principle of least privilege in CORS implementation reduces potential attack surface and maintains robust security posture against cross-origin threats.