docs: add warning and best practices for url_for(..., _external=True)… #5722
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Thanks for working on this! I really appreciate how clearly the risks and recommendations were described. This patch aligns well with the concerns I originally raised in issue #5718, glad to see it resolved! |
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| When generating external URLs using :func:`url_for` with the ``_external=True`` argument, | ||
| Flask constructs the URL using the requeust's ``Host`` header by default. If your application |
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It always uses the Host header, not only by default. That's why the config needs to be set.
| **Best Practices:** | ||
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| - Always set :data:`SERVER_NAME` in your configuration for production deployments. | ||
| - Consider using :data:`trusted_hosts` to restrict which hosts are accepted. |
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The Request.trusted_hosts attribute on the individual request is not the correct place to start. Instead, use the global TRUSTED_HOSTS config. The attribute is used to modify that per-request.
| **Best Practices:** | ||
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| - Always set :data:`SERVER_NAME` in your configuration for production deployments. | ||
| - Consider using :data:`trusted_hosts` to restrict which hosts are accepted. |
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Don't "consider" using it, do use it, it's the only thing that actually protects from the issue.
| critical when generating links for password resets or other sensitive actions that may be | ||
| sent to users. | ||
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| .. warning:: |
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This is the same statement as the previous paragraph, just restated in a warning block and bold now.
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| **Best Practices:** | ||
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| - Always set :data:`SERVER_NAME` in your configuration for production deployments. |
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SERVER_NAME is only used outside of requests, where the injection isn't relevant because there's not request or Host header anyway.
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| When generating external URLs using :func:`url_for` with the ``_external=True`` argument, | ||
| Flask constructs the URL using the requeust's ``Host`` header by default. If your application | ||
| does not explicitly set the :data:`SERVER_NAME` configuration or use :data:`trusted_hosts`, |
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Both of these are wrong, see further on.
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Heads up, all this information is wrong in various ways. Do not use AI, especially in regards to information about security. |
Description of the Change
This pull request adds a warning and best practices to the Web Security documentation regarding the use of url_for(..., _external=True) without setting SERVER_NAME or trusted_hosts. The new section explains the risk of host header injection and provides recommendations for safer configuration. This aims to improve developer awareness and help prevent potential security vulnerabilities, as discussed in #5718.
How it Addresses the Issue
Relevant Issue
fixes #5718