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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.
  • A virtual file in \\Device\WINTUN%d, whose permissions are set to SDDL_DEVOBJ_SYS_ALL. Presumably this means only the "Local System" user can open the file and do things, but it might be worth double checking that. It sends and receives layer 3 packets, and does minimal parsing of the IP header in order to determine packet family. It also does more complex struct alignment pointer arithmetic, as it can send and receive several packets at a time in a single bundle.

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\WireGuard\%s, where %s is some basename of an already valid filename. Its permissions are set to O:SYD:(A;;GA;;;SY), which presumably means only the "Local System" user can access it and do things, but it might be worth double checking that. This pipe gives access to private keys and allows for reconfiguration of the interface, as well as rebinding to different ports (below 1024, even).
  • 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.