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CORS (Cross-Origin Resource Sharing) is a browser mechanism that allows restricted resources on a web page to be requested from another domain outside the domain from which the resource originated. However, misconfigurations in CORS can lead to severe security vulnerabilities, allowing attackers to steal sensitive data such as tokens, cookies, or personal information.
How Attackers Exploit CORS Misconfiguration
- Allowing Any Site to Access Data (Access-Control-Allow-Origin: *)
Some servers are configured to allow any external site to request data using:Access-Control-Allow-Origin: *
This means any site, even one controlled by an attacker, can send requests and access protected data.
Example Attack Code:
fetch("https://victim/api/info", {
credentials: "include"
})
.then(response => response.json())
.then(data => console.log(data)); // Attacker views the data!
2. Allowing Specific Origin Without Validation (Access-Control-Allow-Origin: null)
Some servers accept any origin written in the request without validation, like:
Access-Control-Allow-Origin: null
This can allow attackers to use iframes or other techniques to trick users into visiting their site and then execute requests on the target server.
3. Allowing All Origins with Credentials (Access-Control-Allow-Credentials: true)
If the server has:
Access-Control-Allow-Origin: * Access-Control-Allow-Credentials: true
Any external site can send requests and use the user’s cookies containing session tokens, allowing easy theft of protected data.
How to Protect Your Site from CORS Misconfiguration
- Specify Allowed Origins Precisely: Instead of using
*, specify the domains that are allowed:Access-Control-Allow-Origin: https://trusted-site.com
-
Avoid Using `Access-Control-Allow-Credentials: true` with
*: If you must use cookies, specify the origin clearly. -
Validate User-Requested Values: Ensure that no one can modify the origin in the request and exploit it.
-
Regularly Test CORS Settings: Use tools like Burp Suite or CORS Scanner to ensure everything is secure.
You Should Know:
Practical Commands and Steps:
1. Testing CORS Misconfiguration with cURL:
curl -H "Origin: https://attacker.com" -I https://victim.com/api/info
Check the response headers for `Access-Control-Allow-Origin`.
2. Using Burp Suite to Test CORS:
- Intercept the request with Burp Suite.
- Modify the `Origin` header to a different domain.
- Observe if the server responds with `Access-Control-Allow-Origin: *` or the modified domain.
3. Automating CORS Testing with Python:
import requests
url = "https://victim.com/api/info"
headers = {"Origin": "https://attacker.com"}
response = requests.get(url, headers=headers)
print(response.headers.get("Access-Control-Allow-Origin"))
4. Linux Command to Monitor CORS Headers:
tcpdump -i eth0 -s 0 -A 'tcp port 80 and host victim.com' | grep -i "Access-Control-Allow-Origin"
5. Windows Command to Check CORS Headers:
Invoke-WebRequest -Uri "https://victim.com/api/info" -Headers @{"Origin"="https://attacker.com"} | Select-Object -ExpandProperty Headers
What Undercode Say:
CORS misconfigurations can lead to severe data breaches if not properly managed. By understanding how attackers exploit these vulnerabilities and implementing precise CORS policies, you can significantly reduce the risk. Regularly testing your CORS settings and using tools like Burp Suite or custom scripts can help ensure your server remains secure. Always validate user inputs and avoid using wildcard origins with credentials to protect sensitive data effectively.
Further Reading:
References:
Reported By: Amir Abdelnaby – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
