security: Stop automatically adding URLs from server-side load fetch calls to dependencies

[tcc-sejohnson]

Closes #9803. This is a breaking change for security purposes.

This PR prevents server load function from implicitly depending on URLs passed to fetch. This is because dependencies from server load functions have to be sent to the client in order to be invalidated, which can leak secrets (if they're in the search params, for example).

If you understand the risk and want to keep the old behavior, you can set config.kit.dangerZone.trackServerFetchesPotentiallyExposingSecrets to true.

TODO:

• Tests
• Docs

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

[changeset-bot]

🦋 Changeset detected

Latest commit: 487aa22

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Minor

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[Rich-Harris]

I think we can trim the option name down to trackServerFetches — we can explain the details in the inline documentation, and the dangerZone makes it clear that this is 'know what you're doing' territory

Suggested change

	/**
	* Automatically add server-side `fetch`ed URLs to the `dependencies` map of `load` functions. This will expose secrets
	* to the client if your URL contains them.
	*/
	trackServerFetchesPotentiallyExposingSecrets?: boolean;
	/**
	* Mark server `load` functions that call `fetch(url)` as depending on `url`, allowing you to
	* call `invalidate(url)` from the client. The URL, including e.g. sensitive query parameters,
	* will be visible to the client. This option will be removed in version 2; use `depends` instead
	*/
	trackServerFetches?: boolean;

[tcc-sejohnson]

I dunno, I like explicitly requiring users to acknowledge what they're doing -- there's no guarantee they understand why this is dangerous. Happy to make the change, but I personally think the annoyance of typing a gigantic and explicit config value is worth it, especially since it's not going to be around forever.