Lab - Extract An Executable From A Pcap

Extracting an Executable from a PCAP File: A Deep Dive for Malware Analysts

Analyzing network traffic is crucial in cybersecurity investigations, particularly when dealing with malware. Packet capture (PCAP) files offer a detailed record of network communication, often containing malicious executables hidden within the data stream. This comprehensive guide delves into the process of extracting executables from PCAP files, outlining the tools, techniques, and challenges involved. We’ll cover various scenarios, from straightforward HTTP downloads to more complex, obfuscated transfer methods.

Understanding PCAP Files and Network Protocols

Before diving into extraction, it's vital to understand the structure of PCAP files and the relevant network protocols. PCAP (Packet Capture) files store network packets captured by tools like Wireshark, tcpdump, or others. These packets contain various fields, including source and destination IP addresses, ports, protocols, and the actual payload data. The payload is where the executable usually resides.

Key Protocols Involved:

• HTTP: The most common method for transferring executables. The executable is typically embedded within the body of an HTTP GET or POST request. Identifying these requests within the PCAP is straightforward using filters.

• HTTPS: Similar to HTTP, but encrypted. Decrypting HTTPS traffic requires access to the SSL/TLS keys, which are often not readily available without a man-in-the-middle attack or other specialized techniques.

• FTP: File Transfer Protocol, another common method for transferring files, including executables. Identifying FTP commands and data transfer in a PCAP is relatively easy.

• SMB/CIFS: Used for file sharing within Windows networks. Executables might be transferred using SMB/CIFS protocols, requiring careful analysis of the protocol data units.

• DNS: While not directly transferring executables, DNS queries can sometimes reveal the location of malicious files on a remote server.

• Custom Protocols: Malware authors often use custom protocols to obfuscate their communication and evade detection. These require detailed reverse engineering of the network communication patterns.

Tools for PCAP Analysis and Executable Extraction

Several tools can aid in analyzing PCAP files and extracting embedded executables. Choosing the right tool depends on the complexity of the PCAP file and the sophistication of the transfer method.

1. Wireshark: The Foundation

Wireshark is a powerful and versatile network protocol analyzer. Its ability to dissect network packets and display detailed information makes it invaluable for PCAP analysis. Wireshark can filter traffic based on various criteria (protocol, IP addresses, ports, etc.), which significantly reduces the amount of data that needs to be manually examined.

Key Wireshark Features for Executable Extraction:

• Filtering: Essential for isolating relevant traffic containing the executable.
• Packet Details: Examining packet details reveals the protocol used, payload size, and other crucial information.
• Follow TCP Stream: Reconstructs the entire TCP stream, allowing you to view the complete executable data.
• Exporting Data: Enables saving the extracted data into various file formats.

2. tcpdump: Command-Line Power

Tcpdump is a command-line network packet analyzer. While less visually intuitive than Wireshark, it provides powerful filtering capabilities and is often integrated into automated analysis scripts.

3. Specialized Tools:

Various other specialized tools offer specific capabilities, such as:

• Foremost: Recovers files from various data sources, including PCAP files. It can be particularly effective when the executable is fragmented or embedded within other data.

• Binwalk: A firmware analysis tool, but also useful for detecting and extracting embedded files within PCAP payloads, especially useful when dealing with embedded executables disguised within larger files.

Step-by-Step Guide to Extracting Executables

The process of extracting an executable varies depending on the network protocol and the way the executable is transferred. Let's break down common scenarios:

Scenario 1: HTTP Download

This is the simplest scenario. The executable is downloaded via HTTP, often via a GET request.

1. Open the PCAP in Wireshark: Load the PCAP file into Wireshark.
2. Filter HTTP traffic: Use the filter http to display only HTTP traffic. You might need more specific filters depending on the context. For example, http.request.method == "GET" will isolate GET requests.
3. Locate the download: Examine the HTTP responses. Look for responses with large payload sizes, especially those containing file extensions like .exe, .dll, .scr, etc.
4. Follow the TCP stream: Right-click on the relevant HTTP response and select "Follow TCP Stream." This will display the complete content of the response in a text editor.
5. Save the executable: Copy the raw data to a new file and save it with the appropriate extension (e.g., .exe). If the content isn't readily identifiable, further analysis might be required to determine the correct extension based on file signatures.

Scenario 2: HTTPS Download (Encrypted)

Extracting from HTTPS requires more advanced techniques because the data is encrypted.

1. Obtain the SSL/TLS Keys: Access to the private key is needed for decryption. This is often challenging and may require specific tools and expertise.
2. Use Specialized Tools: Specialized tools like Wireshark with plugins or dedicated decryption tools can help decrypt the HTTPS traffic. This process may also involve using SSL certificates and other relevant information.
3. Proceed as in Scenario 1: Once decrypted, follow the steps for HTTP download.

Scenario 3: FTP Transfer

Similar to HTTP, but with different commands.

1. Filter FTP traffic: Use the filter ftp to isolate FTP traffic.
2. Identify the data transfer: Locate the FTP DATA transfer commands.
3. Follow the data stream: Examine the data being transferred to identify the executable.
4. Save the executable: Similar to Scenario 1, save the extracted data as a file.

Scenario 4: Obfuscation and Custom Protocols

This is the most complex scenario.

1. Deep Packet Inspection: Carefully analyze the packets, paying close attention to unusual patterns or data structures.
2. Protocol Reverse Engineering: Reverse engineering might be necessary to understand the custom protocol used by the malware.
3. Scripting and Automation: Python, along with libraries like Scapy, can be useful for automating the analysis and extraction of obfuscated data.

Challenges and Considerations

Extracting executables from PCAP files can present several challenges:

• Encryption: HTTPS traffic requires decryption, which is not always feasible.
• Fragmentation: Executables might be fragmented across multiple packets.
• Obfuscation: Malware authors often use obfuscation techniques to hide the executable.
• Compression: Executables are sometimes compressed before transmission. Decompression may be needed.
• Data Encoding: The data might be encoded or encrypted in various ways.

Advanced Techniques and Best Practices

• Use of Scripting Languages: Python, with libraries like scapy or dpkt, allows automation and advanced analysis. These scripts can filter and process PCAP data much faster and more efficiently than manual analysis.
• Hashing and Signature Analysis: After extracting the executable, hash it to compare it against known malware databases. This can identify the malware family and reveal its malicious capabilities.
• Sandbox Analysis: Run the extracted executable in a virtual environment to analyze its behaviour and determine the extent of its malicious actions without risking your actual system.
• Dynamic Analysis: Use dynamic analysis tools to study the malware's actions as it interacts with a system.
• Static Analysis: Use tools like IDA Pro to analyze the executable's code without running it. This can reveal the malware's inner workings and identify its malicious intent.

Conclusion

Extracting executables from PCAP files is a critical step in malware analysis. The process can be straightforward in simple scenarios, but more complex when dealing with encrypted, obfuscated, or custom protocols. Mastering the techniques and tools described in this guide, combined with good analytical skills and an understanding of network protocols, is essential for cybersecurity professionals. Remember to always conduct your analysis in a safe and controlled environment.