Cis Azure Compute 2.2.17

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.
2. Click the name of an app.
3. Under Deployment, click Deployment slots.
4. Click the name of a deployment slot.
5. Under API, click CORS.
6. Ensure Allowed Origins does not include *.
7. Repeat steps 1-6 for each app and deployment slot.

Using Azure CLI

Run the following command to list apps:

az webapp list

For each app, run the following command to list deployment slots:

az webapp deployment slot list --resource-group <resource-group-name> --name <app-name>

For each deployment slot, run the following command to get the CORS allowed origins setting:

az resource show --name web --resource-group <resource-group-name> --namespace Microsoft.Web --resource-type config --parent sites/<app-name>/slots/<deployment-slot-name> --query properties.cors.allowedOrigins

Ensure that the response does not include *.

Expected Result

The cors.allowedOrigins property should not contain the wildcard *. It should either be empty/not configured or contain only specific trusted origin URLs.

Remediation

Using Azure Portal

1. Go to App Service.
2. Click the name of an app.
3. Under Deployment, click Deployment slots.
4. Click the name of a deployment slot.
5. Under API, click CORS.
6. Under Allowed Origins, delete the entry that equals *.
7. Specify the origins that should be allowed to make cross-origin calls.
8. Click Save.
9. Repeat steps 1-8 for each app and deployment slot requiring remediation.

Using Azure CLI

For each deployment slot requiring remediation, run the following command to update the allowed origins array to contain only the origins that should be allowed:

az resource update --name web --resource-group <resource-group-name> --namespace Microsoft.Web --resource-type config --parent sites/<app-name>/slots/<deployment-slot-name> --set properties.cors.allowedOrigins="['<allowed-origin>']"

Default Value

By default, CORS is not configured.

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/webapp
3. https://learn.microsoft.com/en-us/cli/azure/resource
4. https://cheatsheetseries.owasp.org/cheatsheets/HTTP_Headers_Cheat_Sheet.html
5. https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/CORS
6. https://cheatsheetseries.owasp.org/cheatsheets/Cross-Site_Request_Forgery_Prevention_Cheat_Sheet.html

Profile

Level 2 | Automated