attacksurface.md

Attack Surface

This is an evolving document, describing currently known attack surface, a few mitigations, and several open questions. This is a work in progress. We document our current understanding with the intent of improving both our understanding and our security posture over time.

WireGuard for Windows consists of four components: a kernel driver, and three separate interacting userspace parts.

WireGuardNT

WireGuardNT is a kernel driver. It exposes:

• A miniport driver to the ndis stack, meaning any process on the system that can access the network stack in a reasonable way can send and receive packets, hitting those related ndis handlers.
• A UDP port parsing WireGuard packets.
• There are also various ndis OID calls, accessible to certain users, which hit further code.
• A PNP and Close notifier added to the NDIS device file.
• IOCTLs are added to the NDIS device file, and those IOCTLs are restricted to O:SYD:P(A;;FA;;;SY)(A;;FA;;;BA)S:(ML;;NWNRNX;;;HI). The IOCTL allows userspace to get and set configuration, adapter state, and read log messages from a ring buffer.

Tunnel Service

The tunnel service is a userspace service running as Local System, responsible for creating WireGuardNT adapters and configuring them. It exposes:

• A global mutex is used for WireGuardNT interface creation, with the same DACL as the pipe, but first CreatePrivateNamespace is called with a "Local System" SID.
• After some initial setup, it uses AdjustTokenPrivileges to remove all privileges, except for SeLoadDriverPrivilege, so that it can remove the interface when shutting down. This latter point is rather unfortunate, as SeLoadDriverPrivilege can be used for all sorts of interesting escalation. Future work includes forking an additional process or the like so that we can drop this from the main tunnel process.

Manager Service

The manager service is a userspace service running as Local System, responsible for starting and stopping tunnel services, and ensuring a UI program with certain handles is available to Administrators. It exposes:

• Extensive IPC using unnamed pipes, inherited by the UI process.
• A readable CreateFileMapping handle to a binary ringlog shared by all services, inherited by the UI process.
• It listens for service changes in tunnel services according to the string prefix "WireGuardTunnel$".
• It manages DPAPI-encrypted configuration files in C:\Program Files\WireGuard\Data, which is created with O:SYG:SYD:PAI(A;OICI;FA;;;SY)(A;OICI;FA;;;BA), and makes some effort to enforce good configuration filenames.
• The actual DPAPI-encrypted configuration files are created with O:SYG:SYD:PAI(A;;FA;;;SY)(A;;SD;;;BA).
• It uses WTSEnumerateSessions and WTSSESSION_NOTIFICATION to walk through each available session. It then uses WTSQueryUserToken to get the token belonging to each session and then determines whether or not it is an administrator token. To determine that, it calls CheckTokenMembership(CreateWellKnownSid(WinBuiltinAdministratorsSid)) on a duplicated impersonation token, as well as and calling GetTokenInformation(TokenElevation) on it. If either of these are false, then it fetched the linked token using GetTokenInformation(TokenLinkedToken) and queries the same. Only then does it spawn the UI process as that the elevated user token, passing it three unnamed pipe handles for IPC and the log mapping handle, as described above.
• In the event that the administrator has set HKLM\Software\WireGuard\LimitedOperatorUI to 1, sessions are started for users that are a member of group S-1-5-32-556 (determined sing CheckTokenMembership(CreateWellKnownSid(WinBuiltinNetworkConfigurationOperatorsSid)) on it and its linked token), with a more limited IPC interface, in which these non-admin users are denied private keys and tunnel editing rights. (This means users can potentially DoS the IPC server by draining notifications too slowly, or exhausting memory of the manager by spawning too many watcher go routines, or by sending garbage data that Go's gob decoder isn't expecting.)

UI

The UI is a process running for each user who is in the Administrators group (per the above), running with the elevated high integrity linked token. It exposes:

• Since the UI process is executed with an elevated token, it runs at high integrity and should be immune to various shatter attacks, modulo the great variety of clever bypasses in the latest Windows release.
• It uses AdjustTokenPrivileges to remove all privileges.
• It renders highlighted config files to a msftedit.dll control, which typically is capable of all sorts of OLE and RTF nastiness that we make some attempt to avoid.

Updates

A server hosts the result of b2sum -l 256 *.msi > list && signify -S -e -s release.sec -m list && upload ./list.sec, with the private key stored on an HSM. The MSIs in that list are only the latest ones available, and filenames fit the form wireguard-${arch}-${version}.msi. The updater, running as part of the manager service, downloads this list over TLS and verifies the signify Ed25519 signature of it. If it validates, then it finds the first MSI in it for its architecture that has a greater version. It then downloads this MSI from a predefined URL to a randomly generated (256-bits) file name inside C:\Windows\Temp with permissions of O:SYD:PAI(A;;FA;;;SY)(A;;FR;;;BA), scheduled to be cleaned up at next boot via MoveFileEx(MOVEFILE_DELAY_UNTIL_REBOOT), and verifies the BLAKE2b-256 signature. If it validates, then it calls WinTrustVerify(WINTRUST_ACTION_GENERIC_VERIFY_V2, WTD_REVOKE_WHOLECHAIN) on the MSI. If it validates, then it executes the installer with msiexec.exe /qb!- /i, using the elevated token linked to the IPC UI session that requested the update. Because msiexec requires exclusive access to the file, the file handle is closed in between the completion of downloading and the commencement of msiexec. Hopefully the permissions of C:\Windows\Temp are good enough that an attacker can't replace the MSI from beneath us.