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EUVD-2026-356484

fkie_cve-2026-48796

CefSharp provides .NET bindings for the Chromium Embedded Framework for Windows Forms and Windows Presentation Foundation applications. Prior to version 148.0.90, CefSharp/SchemeHandler/FolderSchemeHandlerFactory.cs used filePath.StartsWith(rootFolder, StringComparison.OrdinalIgnoreCase) to decide whether a decoded and canonicalized request path remained inside rootFolder. That raw prefix test did not enforce a directory boundary, so a request such as ..%2fwww2/secret.txt could escape a configured www directory into a sibling www2 directory whose path shared the same string prefix. Applications that register FolderSchemeHandlerFactory for a custom scheme or an HTTP or HTTPS scheme can therefore serve local files outside the intended root when an attacker can cause the embedded browser to request the crafted URL. The issue affects both Unix-style paths such as /tmp/app/www2 and Windows paths such as C:\app\www2, and the fix appends a directory separator to the normalized root before comparison while rejecting null bytes and alternate data stream syntax. This issue is fixed in version 148.0.90.

GHSA-85jm-cwp2-mvpv — CefSharp.Common: `FolderSchemeHandlerFactory` path boundary check can expose files outside the configured root folder

Affected: NuGet: CefSharp.Common

### 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:

```csharp 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`:

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

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

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

```text <temp>/www2/secret.txt ```

Because `<temp>/www2/secret.txt` starts with `<temp>/www` as a string prefix, the affected check p…