Snyk agent scan: Starlette: SSRF and NTLM credential theft via UNC paths in StaticFiles on Windows (CVE-2026-48818)

The vulnerability in Starlette's StaticFiles on Windows arises from the use of os.path.realpath to resolve requested paths. When a UNC path (e.g., \\attacker.com\share) is provided, os.path.realpath triggers an SMB connection to the remote host, causing the server to send NTLMv2 authentication credentials. This happens before the path containment check rejects the request, resulting in credential leakage despite a 404 HTTP response. The issue affects the default follow_symlink=False configuration, while follow_symlink=True uses os.path.abspath, which does not perform I/O and is not vulnerable. This SSRF allows any client to cause the server to authenticate to an attacker-controlled SMB server, potentially enabling offline cracking or relay attacks. The vulnerability impacts applications running on Windows that serve static files with StaticFiles, including those using frameworks like FastAPI.

The vulnerability allows unauthenticated remote attackers to cause the server to initiate SMB connections to attacker-controlled hosts, leaking the service account's NTLMv2 credentials. This credential disclosure can lead to offline cracking or relay attacks, compromising internal network security. Additionally, attackers can infer internal SMB hosts by timing responses. The HTTP response to the triggering request is a benign 404, so exploitation may go unnoticed. The impact is limited to Windows environments running affected versions of StaticFiles in the default configuration.

A patch is available for affected versions. Applications not running on Windows are not affected. On Windows, it is recommended to serve static files through dedicated web servers such as nginx or IIS instead of using StaticFiles. Alternatively, blocking outbound SMB traffic (port 445) from the application host prevents credential leakage even if UNC paths are resolved. Using the follow_symlink=True configuration branch also avoids the vulnerability as it uses os.path.abspath, which does not trigger SMB connections.