Migration Assessment Scan: Browser File Handling

Learn how to mitigate issues with Browser File Handling during migration.

Overview

The Browser File Handling settings on the Web Applications in SharePoint impact how you can browse certain file types. The source environment allowed you to change this setting from Strict to Permissive. The Permissive setting enables you to open all file types within the browser. However, in the target environment, the Strict setting is enforced and cannot be modified. As a result, you may find some file types will not open in the browser post migration. For example, *.htm and *.html files in document libraries will no longer open in the browser. Users are prompted to download the files.

The main reason for the change is that the Strict setting is more secure. There's a potential elevation of privilege scenario where a malicious user with contributor access to a site could create an HTML file that contains JavaScript that runs against a different site collection that they don't have permissions to. They then have a user that does have permissions to browse the page, which results in the elevated user executing the JavaScript and accessing the data that the malicious user was after.

Data Migration

Data is migrated, but the behavior with the HTM and HTML files change from opening within the browser to prompting the user to download. If you have an HTM or HTML page as the target of a Page View web part, when the page renders, you get a prompt to download the HTM or HTML file. The Page Viewer web part renders as an iframe, so there's a background request for the HTM or HTML page, which results in the download prompt. You're unable to rename the file extension in the browser. However, if you rename the files using SharePoint Designer, you'll get a prompt to fix the URL reference.

Preparing for Migration

The provided report contains a list of all the HTM and HTML files in your environment. Contact the site owners to ensure they're aware of the issue. If the files are required to open in the browser, rename them to *.aspx. Uploading an ASPX file requires Designer access to a site collection, which reduces the footprint of the risk to people that have more permissions than Contribute. A contributor is able to create wiki pages on some document libraries, which are technically ASPX pages, however, the contributor permissions restrict the user's ability to add or configure web parts that would expose a cross site scripting attack on these pages. For example, the following will occur for a contributor attempting to add web parts to an ASPX page:

• Content Editor web part isn't available as an option.

• Script Editor web part is available, but won't allow a user to submit anything with <script> tags.

• Page Viewer web part doesn't allow a contributor to modify the URL setting. Blocking them from pointing the user to a malicious page.

Options for renaming the file extension of a file:

• Open the site in SharePoint Designer and rename the file.

• Programmatically rename files using SPFile.MoveTo() via CSOM.

Post Migration

Validate that your pages render as expected.

Scan Result Reports

The following table describes the columns in the BrowserFileHandling-detail.csv report.

This scan report contains all the *.htm and *.html files that will be impacted by the Browser File Handling change from Permissive to Strict.