CVE-2026-48818: Starlette: SSRF and NTLM credential theft via UNC paths in StaticFiles on Windows

Synopsis

This is a Server-Side Request Forgery (SSRF) vulnerability in the Starlette ASGI framework's StaticFiles component on Windows. An unauthenticated remote attacker can send a crafted HTTP request containing a UNC path (for example, \\attacker.com\share) that causes the Windows host to initiate an outbound SMB connection to an attacker-controlled server before Starlette returns a 404. The outbound SMB handshake exposes the service account's NTLMv2 credential hash, which the attacker can crack offline or relay to authenticate elsewhere on the network. No fix version has been published yet; HarborGuard tracks this advisory and will make a patched-image rebuild available as soon as an upstream fix is released.

HarborGuard Coverage

Exploit Conditions

• Network reachabilityRequired

  The attacker must be able to send HTTP requests to the Starlette StaticFiles endpoint over the network; no local access is needed.

• AuthenticationNot required

  No account or credential is needed; the malicious UNC path request can be sent by any unauthenticated HTTP client.

• Victim interactionNot required

  No user action is required; the vulnerable path resolution happens automatically when Starlette processes the attacker's request.

• Attack complexityDetail

  Exploitation is reliable and condition-free on any Windows host running an affected Starlette version with the default follow_symlink=False configuration; no race condition or special memory layout is required.

Blast Radius

• The service account's NTLMv2 credential hash is captured by the attacker's SMB listener and can be cracked offline to recover the plaintext password.
• The captured hash can be relayed immediately in an NTLM relay attack to authenticate to other Windows services on the internal network without cracking.
• Any resource accessible to the service account, such as file shares, internal APIs, or domain-joined systems, becomes reachable to the attacker after a successful relay or crack.
• Confidentiality of the host environment is compromised; integrity and availability are not directly affected by this vulnerability.