Question

Programmer RJ tries to design a network protocol to enable a client to check whether a server is still active. He creates a protocol in this way. The client generates a random string and sends the string together with the string length to the server. If the server is still active, the server will echo the string back to the client so that the client knows that the connection is not lost. The server uses the string process function below to process the string. The function first creates a buffer of the size equal to strlen, which is the string length sent by the client, and then it copies the input string (the one sent by the client) of size strlen to the buffer. The echo_string function sends the content in the buffer to the client. This code has a vulnerability that can be exploited by an adversary to cause severe consequences. Please identify the vulnerability, discuss a solution, and explain your reason.

void string_process(unsigned char *string, unsigned int strlen) {
unsigned char *buffer;
buffer = (unsigned char *)malloc(strlen);
memcpy(buffer, string, strlen);
echo_string(buffer, strlen);
return;
}
```"

Answer 1

The function string_process is vulnerable to a buffer overflow attack because it does not verify the length of the string against the provided length.

Step-by-step explanation:

The vulnerability in the function string_process is a potential buffer overflow. This can occur because the function allocates a buffer based on the strlen provided by the client and blindly copies the string into it without verifying that the actual string size matches the provided length.

If an adversary sends a string length smaller than the actual size of the string, the memcpy can write past the end of the allocated buffer, potentially overwriting important data or control information, such as return addresses or pointers. This breach can lead to arbitrary code execution or crashes, leading to a Denial of Service (DoS) attack.

A solution to this vulnerability is to ensure that the length of the input string matches the length specified by the client before copying it into the buffer. One can also use safer functions like strncpy, which includes a length parameter and ensures that the destination buffer is not overflowed.

Alternatively, dynamically determining the length of the input string using functions like strlen can prevent this issue, as the buffer will only be as large as is necessary for the actual string size.

The solution is to validate the string length or use safer functions like strncpy to prevent overflow.