Original Post: Part 1- Everything You Need to Know About Browser Security Policies — SOP, CORS. | by vikram naidu | Jun, 2024
CORS, or Cross-Origin Resource Sharing, is a mechanism that permits resource sharing between different origins, bypassing the Same-Origin Policy (SOP). This is crucial for modern web applications that need to communicate with APIs and services located on different domains.
How CORS Works
CORS is managed through HTTP headers. Key headers include:
- Origin Header: Sent by browsers in cross-origin requests to indicate the origin of the request.
- Access-Control-Allow-Origin: Sent by the server in the response to specify which origins can access the resource.
- Access-Control-Allow-Credentials: Indicates whether the request can include credentials like cookies or HTTP authentication.
Misconfigurations can lead to security vulnerabilities, for example, allowing any origin (*) to access resources along with credentials.
Key Points
- CORS itself is not a vulnerability but improper configurations can be.
- Using wildcard (
*) in Access-Control-Allow-Origin can expose sensitive data to any domain. - Setting
Access-Control-Allow-Credentialsto true with*can allow any origin to access user credentials.
Examples and Considerations
A properly configured server will allow trusted domains only. For instance, a server at https://api.example.com should permit access only from https://client.example.com. An improper configuration, like allowing any origin with credentials, can expose sensitive data such as user details and authentication tokens.
Mitigations
- Restrict allowed origins to trusted domains.
- Control credential inclusion.
- Limit allowed HTTP methods and headers.
- Use specific, secure configurations for various headers like
Access-Control-Max-AgeandAccess-Control-Expose-Headers. - Validate and sanitize the
Originheader.
Proper configuration and cautious handling of CORS headers are essential to secure web applications from unauthorized access and data leaks.
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