Buffer overflow to RCE

Aditya Chauhan
3 min readFeb 20, 2023

Buffer overflow to achieve remote code execution (RCE) is a serious security vulnerability in web applications. Attackers can exploit this vulnerability to execute their code on the targeted system and gain complete control over it. In this blog, we will explain how buffer overflow can be exploited to achieve RCE with a practical example.

Example Scenario

Let's consider an example scenario where a web application has a search bar that takes user input and searches a database for matching records. The search query is passed to the server via an HTTP GET request. The server then returns the matching records to the user.

The server-side code for the search functionality is written in C and uses a buffer to store the search query. The buffer has a size of 128 bytes. The search function is called with the user input as a parameter. However, the server does not perform any input validation or bounds checking, making it vulnerable to buffer overflow attacks.

Exploiting Buffer Overflow to Achieve RCE

An attacker can exploit this vulnerability by sending a search query that is larger than the buffer size. The excess data overflows the buffer and overwrites adjacent memory locations, including the function pointer that controls the program's execution.

The following steps outline how an attacker can exploit this vulnerability to achieve RCE:

Step 1: Identify the vulnerable input: The first step is to identify the input that is vulnerable to buffer overflow. In our example scenario, the search query is the vulnerable input.

Step 2: Craft the payload: The attacker needs to craft a payload that will trigger the buffer overflow. The payload should be designed to overwrite the function pointer with the address of the attacker's code. In our example, the attacker can craft a payload that overflows the buffer and overwrites the return address of the function with the address of the attacker's code.

Step 3: Send the payload: The payload is then sent to the vulnerable input, which triggers the buffer overflow. If the payload is successful, the program will jump to the attacker's code.

Example Exploit Code

The following is an example exploit code that overflows the buffer and overwrites the return address with the address of the attacker's code:

#include <stdio.h>
#include <string.h>

void search(char* query) {
char buffer[128];
strcpy(buffer, query);
printf("Search query: %s\n", buffer);
}

void exploit() {
// Attacker's code goes here
}

int main(int argc, char** argv) {
char* query = "A"; // Start with a small query
while (strlen(query) < 128) {
search(query);
query = strcat(query, "A"); // Add one "A" to the query each iteration
}
exploit();
return 0;
}

In this example, the search function takes a query as a parameter, copies it to a buffer, and prints the search query. The exploit function contains the attacker's code.

The main function starts with a small query and repeatedly calls the search function with the query string concatenated with one "A" character. This will cause the buffer to overflow and overwrite the return address with the address of the exploit function.

Conclusion

Buffer overflow to achieve RCE is a serious security vulnerability in web applications. In this blog, we explained how an attacker can exploit this vulnerability with a practical example. It is essential to prevent buffer overflow vulnerabilities by implementing secure coding practices, proper input validation, and exploit mitigation techniques. Web application developers must understand the risks associated with buffer overflow and take appropriate measures to prevent it.

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Aditya Chauhan

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