Microsoft CVE-2026-48796

Summary

FolderSchemeHandlerFactory was intended to restrict served files to a configured rootFolder, but its path validation used a raw string prefix check. A request could escape to a sibling directory whose full path starts with the root folder path, allowing files outside the configured root to be served.

Details

In affected versions, FolderSchemeHandlerFactory canonicalized rootFolder, decoded the request path, combined it with the root, and then allowed the file when:

filePath.StartsWith(rootFolder, StringComparison.OrdinalIgnoreCase)

This does not enforce a directory boundary. For example, /tmp/app/www2/secret.txt starts with /tmp/app/www, but www2 is a sibling of www, not a child. The same issue applies on Windows, for example C:\app\www2\secret.txt starts with C:\app\www.

The affected code was reviewed at commit b5fef3bb4bc58798c95170078c41de92cfe9066e, assembly version 147.0.100.

PoC

Set rootFolder to a directory named www and create a sibling directory named www2:

<temp>/www/index.html
<temp>/www2/secret.txt

Register FolderSchemeHandlerFactory for <temp>/www, then request:

https://folderschemehandlerfactory.test/..%2fwww2/secret.txt

The request path is URL-decoded to ../www2/secret.txt, combined with <temp>/www, and canonicalized to:

<temp>/www2/secret.txt

Because <temp>/www2/secret.txt starts with <temp>/www as a string prefix, the affected check passes and secret.txt is served from outside rootFolder.

Expected vulnerable result: HTTP 200 with the contents of <temp>/www2/secret.txt.

Expected fixed result: 404 or equivalent not-found response because the resolved file is outside rootFolder.

Impact

Applications using FolderSchemeHandlerFactory for a custom scheme or registered HTTP/HTTPS scheme may expose local files outside the intended served directory. This is most relevant when sensitive sibling directories share the root path prefix, such as www/www2, public/public_backup, or static/static-secrets.

An attacker must be able to cause the embedded browser to request URLs handled by the affected scheme registration.