The Android Malware Handbook by Qian Han & Salvador Mandujano & Sebastian Porst & V.S. Subrahmanian & Sai Deep Tetali & Yanhai Xiong

Author:Qian Han & Salvador Mandujano & Sebastian Porst & V.S. Subrahmanian & Sai Deep Tetali & Yanhai Xiong
Language: eng
Format: epub, pdf
Publisher: No Starch Press
Published: 2024-11-15T00:00:00+00:00

Decrypting the Command-and-Control Communications

As in the previous chapter, one of the most important properties of the malware we’ll want to reverse engineer is its encrypted communication with the command-and-control server. Breaking the encryption will allow us to better understand the commands it supports. Rather than using static analysis and code reading to find out how this communication works, we’ll use dynamic analysis. Between tcpdump, logcat, and Frida, we have all the tools we need.

After our earlier analysis using tcpdump and Wireshark, we know that the first connection the malware makes is an HTTP POST request to http://simpleyo5.tk/ping. In this request, the malware sends a JSON object with four entries, as shown in Listing 4-6.

{ "hash": "c9KjsZ9C7He6VRmwPMY9YpRrW8H9UFIITKB7umfOUyo=", "id": "9hbTqZU/XYXD8Z1hftmY0N63NltNY2+ihQOnUHrg9T1B/C...", "iv": "M0KcSRwOMvKnbNd4TE719Q==", "type": "request_verify" }

Listing 4-6: The JSON payload posted to the /ping URL

It’s still unclear what these values are. The one plaintext entry, "type": "request_verify", suggests that the purpose of this connection is to request that the malware client be verified. We’ve shortened the value of id here; originally, it had more than 7,000 bytes. Its length indicates that it might be the message’s main payload. The value of hash is unknown at this point, as is the value of iv, which has a noteworthy name. Could this be the initialization vector used to encrypt the payload? Later, we’ll confirm this to be the case.

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