Becoming the Hacker by Adrian Pruteanu

Author:Adrian Pruteanu
Language: eng
Format: epub
Publisher: Packt Publishing

Since this is a custom TCP protocol, we have to intercept the traffic using Wireshark or tcpdump, as opposed to Burp or ZAP. With Wireshark running, we can capture and inspect the TCP stream of data of our interaction with the DeserLab server, as the following figure shows:

Figure 8.7: TCP stream of data

We can see the entire conversation in a hex dump format by analyzing the packet capture (pcap) generated by our packet sniffer. In the preceding figure, the data sent is the stream printed in light gray, while the darker parts represents the server response.

While the data may be a bit hard to read, each byte has a purpose. We can see the familiar ac ed header and the various inputs the client has sent, such as name and string. You'll also notice that the string value is a serialized HashRequest object. This is a Java class implemented by both the server and the client. Serialization is used to instantiate an object that will calculate the hash of a given input and store it in one of its properties. The packets we've just captured are the serialized representation of this object being transmitted from the client to the server and vice versa. The server-serialized object also contains an extra property: the generated hash.

When the server receives the client-generated serialized object, containing the inputted string to be hashed, it will deserialize the bytes coming in over the wire and attempt to cast them to the HashRequest class.

Since DeserLab is open-source, we can inspect the deserialization process on the server component by looking at its source code hosted on GitHub:

[...] oos = new ObjectOutputStream(clientSock.getOutputStream()); //Read a HashRequest object request = (HashRequest)ois.readObject(); //Generate a hash request.setHash(generateHash(request.getData())); oos.writeObject(request); [...]

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