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### WireGuard for Windows 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.

#### Wintun

Wintun 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.
  - There are also various ndis OID calls, accessible to certain users, which hit further code.
  - IOCTLs are added to the NDIS device file, and those IOCTLs are restricted to `SDDL_DEVOBJ_SYS_ALL`. The IOCTL allows userspace to register a pair of rings and event objects, which Wintun then locks the pages of with a double mapping and takes a reference to the event object. It parses the contents of the ring to send and receive layer 3 packets, each of which it minimally parses to determine IP family.

### Tunnel Service

The tunnel service is a userspace service running as Local System, responsible for creating UDP sockets, creating Wintun adapters, and speaking the WireGuard protocol between the two. It exposes:

  - A listening pipe in `\\.\pipe\ProtectedPrefix\Administrators\WireGuard\%s`, where `%s` is some basename of an already valid filename. Its DACL is set to `O:SYD:(A;;GA;;;SY)`. If the config file used by the tunnel service is not DPAPI-encrypted and it is owned by a SID other than "Local System" then an additional ACE is added giving that file owner SID access to the named pipe. This pipe gives access to private keys and allows for reconfiguration of the interface, as well as rebinding to different ports (below 1024, even). Clients who connect to the pipe run `GetSecurityInfo` to verify that it is owned by "Local System".
  - A global mutex is used for Wintun interface creation, with the same DACL as the pipe, but first CreatePrivateNamespace is called with a "Local System" SID.
  - It handles data from its two UDP sockets, accessible to the public Internet.
  - It handles data from Wintun, accessible to all users who can do anything with the network stack.
  - 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 Local System's local appdata directory, and makes some effort to enforce good configuration filenames.
  - It uses `WTSEnumerateSessions` and `WTSSESSION_NOTIFICATION` to walk through each available session. It then uses `WTSQueryUserToken`, and then calls `GetTokenInformation(TokenGroups)` on it. If one of the returned group's SIDs matches `IsWellKnownSid(WinBuiltinAdministratorsSid)`, and has attributes of either `SE_GROUP_ENABLED` or `SE_GROUP_USE_FOR_DENY_ONLY` and calling `GetTokenInformation(TokenElevation)` on it or its `TokenLinkedToken` indicates that either is elevated, then it spawns 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.

### 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.