CVE-2026-48796: 
C# vulnerability analysis and mitigation

CVE-2026-48796 is a path traversal vulnerability in the FolderSchemeHandlerFactory component of CefSharp.Common (NuGet) that allows files outside the configured rootFolder to be served to an embedded browser. The flaw stems from a raw string prefix check that fails to enforce directory boundaries, enabling requests to escape to sibling directories whose full path happens to start with the root folder string. It affects CefSharp.Common versions prior to 148.0.90 (assembly version < 147.0.100). The vulnerability was first published on May 29, 2026, and added to the GitHub Advisory Database on June 30, 2026. It carries a CVSS v3.1 base score of 5.3 (Moderate) (GitHub Advisory, CefSharp Advisory).

The root cause is CWE-22 (Improper Limitation of a Pathname to a Restricted Directory — Path Traversal). In affected versions, FolderSchemeHandlerFactory canonicalizes rootFolder, URL-decodes the request path, combines it with the root, and then validates the result using filePath.StartsWith(rootFolder, StringComparison.OrdinalIgnoreCase). This string prefix check does not enforce a directory separator boundary: a path like /tmp/app/www2/secret.txt satisfies the prefix check against /tmp/app/www even though www2 is a sibling directory, not a child. The same logic flaw applies on Windows (e.g., C:\app\www2\secret.txt starts with C:\app\www). Exploitation requires the attacker to cause the embedded browser to issue a crafted URL with a URL-encoded path traversal sequence (e.g., ..%2f) targeting a sibling directory that shares the root path prefix (GitHub Advisory, CefSharp Advisory).

Successful exploitation allows an attacker to read arbitrary local files located in sibling directories that share the configured root folder's path prefix (e.g., www/www2, public/public_backup, static/static-secrets). The impact is limited to confidentiality — there is no integrity or availability impact. This is most dangerous in applications where sensitive configuration files, credentials, or private data reside in directories adjacent to the web root served by the embedded CefSharp browser (GitHub Advisory, CefSharp Advisory).

A public proof-of-concept is included in the official advisory, demonstrating exploitation by requesting https://folderschemehandlerfactory.test/..%2fwww2/secret.txt against a scheme registered for a www root folder. Exploitation requires high attack complexity (the attacker must be able to influence URLs processed by the embedded browser) and user interaction, limiting opportunistic exploitation. There is no evidence of in-the-wild exploitation, no known threat actor attribution, and the CVE status remains Reserved. No EPSS score or CISA KEV listing has been reported (GitHub Advisory, CefSharp Advisory).

1. Identify target application: Locate a desktop application embedding CefSharp (versions < 148.0.90) that uses FolderSchemeHandlerFactory to serve local files via a custom or HTTP/HTTPS scheme registered to a directory (e.g., /tmp/app/www).
2. Enumerate sibling directories: Determine whether sibling directories exist whose names share the root folder's string prefix (e.g., /tmp/app/www2, /tmp/app/www-backup) and contain sensitive files.
3. Craft malicious URL: Construct a URL with a URL-encoded path traversal sequence targeting the sibling directory, e.g., https://folderschemehandlerfactory.test/..%2fwww2/secret.txt.
4. Cause the browser to request the URL: Induce the embedded CefSharp browser to navigate to or load the crafted URL — for example, via a malicious web page, a crafted link, or by influencing application logic that controls navigation.
5. Receive file contents: The FolderSchemeHandlerFactory URL-decodes the path to ../www2/secret.txt, resolves it to /tmp/app/www2/secret.txt, passes the flawed StartsWith check, and returns the file contents with HTTP 200, exposing the sensitive file (GitHub Advisory, CefSharp Advisory).

• Network: HTTP requests to a CefSharp-registered scheme containing URL-encoded path traversal sequences (e.g., ..%2f, ..%252f) in the URL path, particularly targeting paths outside the registered root directory.
• Logs: Application or web server logs showing requests to scheme-handled URLs with ../ or encoded equivalents resolving to sibling directories; HTTP 200 responses for resources not under the configured rootFolder.
• File System: Unexpected access timestamps on files in sibling directories adjacent to the configured web root (e.g., www2/, public_backup/, static-secrets/) that should not be served by the application.

Upgrade CefSharp.Common to version 148.0.90 or later, which fixes the path boundary check by ensuring the resolved file path is a proper child of rootFolder (not merely a string prefix match). No configuration-based workaround is documented; the fix requires updating the NuGet package. As an interim measure, ensure that no sensitive sibling directories share a string prefix with the configured rootFolder, and restrict the application's file system permissions to limit exposure (GitHub Advisory, CefSharp Advisory).

The vulnerability was reported by security researcher sondt99 and published by CefSharp maintainer amaitland. No significant broader media coverage or notable public social media discussion has been identified beyond the official advisory (CefSharp Advisory).