Cis Azure Compute 2.3.17

Ensure cross-origin resource sharing does not allow all origins

Description

Cross-origin resource sharing (CORS) is a security feature that controls how applications interact with resources hosted on different domains.

Rationale

Restrict CORS to only trusted origins to help enforce proper access control and reduce exposure to malicious cross-origin requests.

Impact

Configuration is required to ensure that the appropriate origins have access.

Setting up a proper CORS policy can be fairly complex and an incorrect setting could permit Cross-Site Request Forgery (CSRF). The "caveat" is that if the app being deployed is a PUBLIC API, a wildcard "*" CORS policy is absolutely necessary.

Audit Procedure

Using Azure Portal

1. Go to App Service or Function App.
2. Click the name of a function app.
3. Under API, click CORS.
4. Ensure Allowed Origins does not include *.
5. Repeat steps 1-4 for each function app.

Using Azure CLI

Run the following command to list function apps:

az functionapp list

For each function app, run the following command to get the CORS allowed origins setting:

az functionapp show --resource-group <resource-group-name> --name <function-app-name> --query siteConfig.cors.allowedOrigins

Ensure that the response does not include *.

Expected Result

The CORS allowed origins should not include a wildcard * entry.

Remediation

Using Azure Portal

1. Go to App Service or Function App.
2. Click the name of a function app.
3. Under API, click CORS.
4. Under Allowed Origins, delete the entry that equals *.
5. Specify the origins that should be allowed to make cross-origin calls.
6. Click Save.
7. Repeat steps 1-6 for each function app requiring remediation.

Using Azure CLI

For each function app requiring remediation, run the following command to remove the wildcard origin:

az functionapp cors remove --resource-group <resource-group-name> --name <function-app-name> --allowed-origins "*"

Use the following command to specify the origins that should be allowed:

az functionapp cors add --resource-group <resource-group-name> --name <function-app-name> --allowed-origins <allowed-origins>

Default Value

By default, Allowed Origins is set to https://portal.azure.com.

References

1. https://learn.microsoft.com/en-gb/azure/app-service/app-service-web-tutorial-rest-api
2. https://learn.microsoft.com/en-us/cli/azure/functionapp/cors
3. https://cheatsheetseries.owasp.org/cheatsheets/HTTP_Headers_Cheat_Sheet.html
4. https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/CORS
5. https://cheatsheetseries.owasp.org/cheatsheets/Cross-Site_Request_Forgery_Prevention_Cheat_Sheet.html

Profile

Level 2 | Automated