🚨🎯 TryHackMe Flag Vault 2 | Format String Vulnerability Exploitation | Hackfinity CTF 🚨🎯

Summary

This video demonstrates how to exploit a format string vulnerability in a C program to access a hidden flag stored in memory. The presenter walks through the use of various format specifiers like '%s', '%p', and '%x' to read values from the stack and locate the flag. By manipulating the format string, the attacker digs deeper into the memory and eventually uncovers the flag, which is stored in a 200-byte buffer. The process highlights the importance of proper input sanitization and how attackers can abuse such vulnerabilities to gain unauthorized access to sensitive data.

Takeaways

• 😀 The script discusses a format string injection vulnerability in a C-based binary program.
• 😀 The vulnerability allows an attacker to manipulate user input, potentially accessing sensitive information from memory (like the flag).
• 😀 The program takes user input as a username, and if it contains format specifiers (e.g., %s, %p), they can be exploited to read from the stack.
• 😀 The attack method relies on the improper use of the 'printf' function, which treats user input as a format string, enabling attackers to print memory contents.
• 😀 Format specifiers such as %s (string) and %p (pointer) are used to access stack data in the exploitation process.
• 😀 The attacker navigates through the stack by experimenting with different combinations of format specifiers to locate the flag in memory.
• 😀 In hexadecimal form, the flag appears reversed due to little-endian format, which must be taken into account when interpreting the flag's value.
• 😀 The attacker tries various techniques like using %x and %s, printing values and navigating deeper into the stack to find the flag's location.
• 😀 The final successful extraction of the flag occurs when the attacker locates it at position 5 in the stack, printing its contents with a specific format specifier.
• 😀 The process of exploitation highlights the importance of secure handling of user input, particularly when used in functions like 'printf' that interpret format specifiers.
• 😀 The video demonstrates how multiple approaches (like using the 'p', 's', 'x', and 'lx' format specifiers) can be leveraged to exploit a format string vulnerability and retrieve sensitive data.

Q & A

What is the main vulnerability being discussed in the script?

The main vulnerability discussed is a format string injection, where user input is passed to a `printf()` function as a format string, allowing an attacker to read or manipulate memory from the stack.

How does the format string injection work in this context?

In this context, the user provides input (such as a username), which is directly passed into the `printf()` function. If the input includes format specifiers like `%p`, `%s`, or `%x`, they allow the program to read values from the stack, potentially exposing sensitive data, such as a flag.

What is the role of the `printf()` function in this vulnerability?

The `printf()` function is vulnerable because it uses the user-provided input directly as a format string. This enables an attacker to control the output and potentially read from the stack by using format specifiers.

Why does the attacker use the `%p` format specifier?

The `%p` format specifier is used to print memory addresses (pointers) stored on the stack. By injecting this specifier, the attacker can gain insight into the stack's contents, potentially locating the flag stored there.

What happens when the attacker uses the `%s` format specifier?

The `%s` format specifier is used to print a string from the stack. By adding multiple `%s` specifiers, the attacker can navigate through the stack and locate the flag if it is stored there as a string.

How does the attacker determine the position of the flag in memory?

The attacker experiments with different numbers of format specifiers (`%s`) to traverse the stack. By doing this, they identify the specific position where the flag is stored. In this case, the flag is found at the 5th position using `%s`.

What is the significance of hexadecimal (`%x`) and pointer (`%p`) format specifiers in this exploit?

The `%x` format specifier prints hexadecimal values from the stack, while the `%p` specifier prints pointers. Both are useful for exploring memory content in different formats, helping the attacker locate the flag or other sensitive data.

What is little-endian format, and why is it relevant in this context?

Little-endian format refers to the way data is stored in memory, where the least significant byte is stored first. It is relevant here because the flag is stored in little-endian format, and the attacker must account for this when reading the memory values to correctly interpret the flag.

Why does the attacker repeatedly try different combinations of format specifiers like `%p`, `%s`, and `%x`?

The attacker tries different combinations of format specifiers to explore the stack in various ways. Each specifier reveals different types of data (e.g., memory addresses, strings, hexadecimal values), helping the attacker locate the flag by reading the correct stack locations.

How does the attacker ultimately find the flag?

The attacker eventually locates the flag by using a combination of `%s` format specifiers, navigating through the stack. They identify the flag when the correct memory location is reached, and recognize the reversed string `issue try hack me` stored in the memory.