Wednesday, August 10, 2022

Micropatches For "KrbRelay" Local Privilege Escalation Vulnerability (Wontfix/0day)


by Mitja Kolsek, the 0patch Team

Update 10/21/2022: Microsoft silently fixed this issue with October 2022 Updates. No CVE ID was assigned.

"KrbRelay" is a tool for forced authentication issue in Windows that can be used by a low-privileged domain user to take over a Windows computer, potentially becoming a local or domain admin within minutes. The tool, based on James Forshaw's research, was developed by security researcher cube0x0, and was later wrapped by Mor Davidovich into another tool called "KrbRelayUp" that further automated attack steps for escalating privileges.

KrbRelay provides various options to launch different versions of attack; some of these options were already known under the name RemotePotato0, for which we already had patches before. What was new for us with KrbRelay was its capability to launch a local service (running in session 0) via RPC and exploit it for leaking Local System credentials through forced authentication. In order to be exploitable, a service must allow authentication over the network, and just two such services were identified on affected Windows versions:

  1. ActiveX Installer Service, identified by CLSID 90f18417-f0f1-484e-9d3c-59dceee5dbd8; and
  2. RemoteAppLifetimeManager.exe, identified by CLSID 0bae55fc-479f-45c2-972e-e951be72c0c1.

Microsoft does not fix forced authentication issues unless an attack can be mounted anonymously. Our customers unfortunately can't all disable relevant services or implement mitigations without breaking production, so it is on us to provide them with such patches.

For the purpose of identifying vulnerabilities we decided to name the vulnerability exposing the above services "KrbRelay", as other attack vectors provided by the tool were already blocked by our existing patches for RemotePotato0. We decided to inject our patch logic at the point where a local unprivileged attacker launches the exploitable service, because such patch would be fairly simple - and we like it simple: it's harder to make mistakes.

Our patch, source code shown below, resides in rpcss.dll and checks whether someone is trying to launch one of the above services via RPC; in such case, if the requestor's token is elevated, we allow it, otherwise not. This is the same approach as we used with patching RemotePotato0.

MODULE_PATH "..\Affected_Modules\rpcss.dll_10.0.17763.3113_Srv2019_64-bit_u202207\rpcss.dll"
VULN_ID 7416

    PIT Advapi32.dll!GetTokenInformation,ntdll!_strnicmp,rpcss.dll!0x68ccd
    ; memory representation:    17 84 f1 90 f1 f0 4e 48 9d 3c 59 dc ee e5 db d8
    ; clsid:                    90f18417-f0f1-484e-9d3c-59dceee5dbd8

        call VAR                       
        dd 0x90f18417                 ; CIeAxiInstallerService Class
        dw 0xf0f1, 0x484e
        db 0x9d, 0x3c, 0x59, 0xdc, 0xee, 0xe5, 0xdb, 0xd8
        pop rcx                       ; rcx => clsid in memory respresentation
        mov rdx, [rbx]                ; ClientToken hadle
        mov r8, 16                    ; length to compare
        call PIT__strnicmp            ; Compares the specified number of characters
                                      ; of two strings without regard to case
        cmp rax, 0                    ; rax == 0 string are equal
        jne CONTINUE                  ; if rax != 0 continue normal code flow

        mov rdx, [rbx+8]
        mov rdx, [rdx]
        mov rcx, [rdx+40h]            ; current session token, TokenHandle
        mov rdx, 14h                  ; TokenInformationClass, TokenElevation
        sub rsp, 30h                  ; home space + vars
        lea r8, [rsp+30h]             ; TokenInformation
        mov qword[rsp+30h], 0         ; memset
        mov r9, 4                     ; TokenInformationLength
        lea rax, [rsp+28h]            ; ReturnLength address
        mov [rsp+20h], rax            ; pointer to address
        call PIT_GetTokenInformation  ; The GetTokenInformation function retrieves a
                                      ; specified type of information about an access token
        add rsp, 30h                  ; restore stack pointer
        cmp byte[rsp], 0              ; token elevated?
        je PIT_0x68ccd                ; if elevated(1) continue normal code flow



Micropatch Availability

While this vulnerability has no official vendor patch and could be considered a "0day", Microsoft seems determined not to fix relaying issues such as this one; therefore, this micropatch is not provided in the FREE plan but requires a PRO or Enterprise license.

The micropatch was written for the following Versions of Windows with all available Windows Updates installed: 

  1. Windows 10 v21H2
  2. Windows 10 v21H1
  3. Windows 10 v20H2
  4. Windows 10 v2004
  5. Windows 10 v1909
  6. Windows 10 v1903
  7. Windows 10 v1809
  8. Windows 10 v1803
  9. Windows 7 (no ESU, ESU year 1, ESU year 2)
  10. Windows Server 2008 R2 (no ESU, ESU year 1, ESU year 2)
  11. Windows Server 2012
  12. Windows Server 2012 R2
  13. Windows Server 2016
  14. Windows Server 2019 
  15. Windows Server 2022 
This micropatch has already been distributed to, and applied on, all online 0patch Agents in PRO or Enterprise accounts (unless Enterprise group settings prevent that). 

If you're new to 0patch, create a free account in 0patch Central, then install and register 0patch Agent from, and email for a trial. Everything else will happen automatically. No computer reboot will be needed.

To learn more about 0patch, please visit our Help Center

We'd like to thank James Forshaw and cube0x0 for sharing details about this vulnerability and sharing a tool, which allowed us to create a micropatch and protect our users. We also encourage security researchers to privately share their analyses with us for micropatching.

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