Linked to issue #32209.
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [x] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation
There are three Trusted Types violations that are fixed in this PR:
### 1. ban-element-innerhtml-assignments: maintain--tab-focus.ts
The innerHTML assignment here is unsafe as a string is being used that could contain an XSS attack. The solution chosen was to replace the string containing HTML with programmatically-created DOM elements. This removes the Trusted Types violation as there is no longer a string passed in that can contain an XSS attack.
Notes on solution:
- The `<svg>` tag is omitted completely since the original snippet returns fragment.firstChild.firstChild. The first firstChild omits the `<div>`, and the second firstChild omits the `<svg>`, so to remove unnecessary code the created elements start at the foreignObject level.
- The reason createElementNS is used instead of createElement is because the ‘foreignObject’ element is a separate namespace from the default HTML elements. The documentation for this command is found [here](https://developer.mozilla.org/en-US/docs/Web/API/Document/createElementNS).
The code was tested to be equivalent by rendering both the original code and the re-written code in a browser to see if they evaluate to the same thing in the DOM. The DOM elements styles were then compared to ensure that they were identical.
### 2. ban-window-stringfunctiondef: packages/next/lib/recursive-delete.ts
The setTimeout function caused a Trusted Types violation because if a string is passed in as the callback, XSS can occur. The solution to this problem is to ensure that only function callbacks can be passed to setTimeout. There is only one call to the sleep function and it does not involve a string callback, so this can be enforced without breaking the application logic. In the process of doing this, promisify has been removed and the promise has been created explicitly.
The code was tested in a sample application to ensure it behaved as expected.
### 3. ban-window-stringfunctiondef: packages/next/client/dev/fouc.ts
This file also uses setTimeout, so the call was wrapped in a `safeSetTimeout` call that specifies that the callback argument is not a string.
Linked to issue #32209.
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [x] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation
There is one tsec violation that is fixed in this PR:
### 1. ban-script-src-assignment: route-loader.ts
XSS can occur with the line script.src = src in appendScript(src, script) if src can be controlled by a malicious user. From tracing through the code, it was determined that src comes from the function `getFilesForRoute(route)`. The behaviour of this function differs depending on the environment (development vs. production), but in both cases the function will construct strings that lead to valid file paths. These strings depend on two variables: `assetPrefix` and `route`, but due to the nature of the constructed strings it was determined that the scripts here are safe to use. Thus, the solution was to promote these strings to `TrustedScriptURL`s. This is the Trusted Types way of declaring that the script URL passed to the DOM sink is safe from DOM XSS attacks.
To create a `TrustedScriptURL`, a policy needs to be created. This policy was put in its own file: `client/trusted-types.ts`. This policy has the name `nextjs`. If this name should be changed to something else, feel free to change it now. However, once it is released to the public and application developers begin using it, it may be harder to change the value since any application developers with a custom policy name allowlist would now need to update their `next.config.js` headers to allow this new name.
The code was tested in a sample application to ensure it behaved as expected.
