How do Attackers Exploit Buffer Overflows?

Attackers exploit buffer overflow vulnerabilities by writing more data into a buffer (a temporary storage area) than it can hold. This excess data spills into adjacent memory, corrupting it. 

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If done correctly, a buffer overflow attack can let an attacker execute malicious code, crash a system, or escalate privileges. 

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Buffer Overflow Vulnerability

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A buffer is a temporary storage space in memory, typically used to hold data before processing. Buffers exist in:

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• User inputs (e.g., forms, command-line arguments)
• Network packets
• Files loaded into memory
• Function arguments

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A buffer overflow vulnerability arises when a program doesn’t properly validate input sizes before writing data into a buffer. If an attacker can write beyond the intended space, they can overwrite:

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1. Adjacent data (corrupting information or variables)
2. Control structures (altering program flow)
3. Executable code (injecting their own malicious instructions)

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This is where things get dangerous. If an attacker controls what gets written past the buffer’s limit, they can manipulate the program into doing whatever they want.

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Types of Buffer Overflow Attacks

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These are the ways through which an attacker can manipulate your memory buffers.

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1. Stack Buffer Overflow (Most Common)

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This is the classic buffer overflow attack. It happens when a function writes beyond a local buffer stored on the stack.

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How it Works:

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• Local variables in a function are stored in a stack frame.
• The return address (which tells the program where to go next) is also stored in the stack.
• If an attacker overwrites the return address, they can redirect execution to malicious code they’ve injected.

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Example: A Simple C Program with a Buffer Overflow

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

void vulnerable_function(char *user_input) {
	char buffer[20];  // Small fixed-size buffer
	strcpy(buffer, user_input);  // No bounds checking!
	printf("You entered: %s\n", buffer);
}

int main(int argc, char *argv[]) {
	if (argc > 1) {
    	vulnerable_function(argv[1]);
	}
	return 0;
}

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What’s wrong?

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• strcpy(buffer, user_input); blindly copies user input into buffer without checking length.
• If the user enters more than 20 characters, it overflows into adjacent memory.
• If crafted properly, an attacker can overwrite the return address and hijack execution.

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Exploitation (Return Address Hijacking)

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• The attacker fills the buffer with junk data.
• They overwrite the return address to point to their own malicious code.
• When the function returns, execution jumps to the attacker’s payload.

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This is how many classic exploits (e.g., old-school Windows/Linux exploits) worked before modern protections. 

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2. Heap-Based Buffer Overflow

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Not all buffers are on the stack. Some are dynamically allocated on the heap using malloc() or new.

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How It Works:

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• Heap-based overflows corrupt adjacent heap metadata or function pointers.
• Attackers overwrite these pointers to redirect execution to malicious code.

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Why It’s Dangerous:

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• More subtle than stack overflows (stack protection mechanisms are more common).
• Can be used to corrupt memory management and bypass security checks.

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3. Format String Vulnerabilities (Related Exploit)

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While not strictly a buffer overflow, format string attacks exploit functions like printf() when used incorrectly.

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Example:

printf(user_input);  // Dangerous! No format specifier

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If the user enters %x %x %x %x, they can leak memory contents (including stack values like return addresses). This can be a stepping stone to a buffer overflow attack.

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How Attackers Exploit Buffer Overflows (Step-by-Step Attack Chain)

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Attackers don’t just stumble upon a vulnerability; they analyze, craft, and execute a payload to exploit the weakness effectively.

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1. Find a Vulnerable Program

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Before launching a buffer overflow attack, an attacker must identify a target that is susceptible to buffer overflows.

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What They Look For:

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• Old Software: Legacy applications (especially those written in C or C++) that were developed before security practices like bounds checking became common.
• Poorly Written Code: Programs that use dangerous functions (e.g., strcpy, gets, sprintf) without validating input lengths.
• IoT Devices & Embedded Systems: Many IoT devices still use lightweight, insecure C-based applications with little to no exploit mitigations.
• Network Services: Any application that processes user input over the network (e.g., web servers, mail servers, VPNs) is a prime target.
• Custom Business Applications: Many in-house enterprise applications don’t go through rigorous security testing, making them vulnerable.

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How They Find Vulnerabilities:

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• Reverse Engineering: Using tools like IDA Pro, Ghidra, or Radare2, attackers analyze binary code for weaknesses.
• Fuzzing: Automated testing tools (e.g., AFL++, libFuzzer) feed large amounts of malformed input to see if they cause crashes, hinting at buffer overflow vulnerabilities.
• Public Databases: Attackers often scan the Common Vulnerabilities and Exposures (CVE) database to find programs known to have buffer overflow issues.

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Once they find a weak target, the next step is figuring out how to exploit it.

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2. Determine Memory Layout

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Once a vulnerable program is found, an attacker must analyze how its memory is structured. This is crucial because the goal is to overwrite specific parts of memory to alter program execution.

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How Attackers Map Memory:

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• Disassembling the Program: Using a debugger (e.g., GDB, WinDbg), they look at how the program handles memory, especially local buffers.
• Finding the Buffer’s Location: They determine the exact memory addresses where the buffer, return address, and function pointers are stored.
• Examining the Stack & Heap: If the overflow is stack-based, they identify where the return address is. If it’s heap-based, they look for function pointers or heap metadata.
• Analyzing Compiler Protections: Some programs use stack canaries, ASLR, and DEP—attackers must figure out ways to bypass them.

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Tools Used for Memory Analysis:

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• GDB (GNU Debugger) – Debugs Linux binaries.
• WinDbg – Debugs Windows programs.
• Radare2 / IDA Pro – Disassemblers that help analyze how a program uses memory.

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Once they understand where to write malicious data, they move on to crafting the exploit.

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3. Craft a Malicious Payload

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Now comes the fun part (for the attacker): constructing the buffer overflow exploit. The goal is to overwrite key parts of memory and inject malicious shellcode (a small program that executes arbitrary commands).

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Steps in Crafting a Buffer Overflow Payload:

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1. Fill the Buffer: Attackers send a string that is exactly large enough to overflow the buffer.
2. Overwrite the Return Address: By precisely calculating how many bytes are needed, they replace the function’s return address with an address that points to their shellcode.
3. Inject Shellcode: The attacker embeds assembly instructions (e.g., a small script to open a shell) into the buffer.
4. Align Memory (NOP Sleds): Since memory addresses may shift slightly, attackers insert NOP (No-Operation) instructions to increase reliability.

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Example Payload (Injecting a Reverse Shell in C):

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char exploit[] =
  "\x90\x90\x90\x90" // NOP sled
  "\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\x99\xb0\x0b\xcd\x80";
// Shellcode to execute /bin/sh

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This shellcode spawns a shell, allowing the attacker to control the system.

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4. Execute the Payload

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Once the payload is ready, the attacker triggers the exploit by sending their crafted input to the program.

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What Happens Next?

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• The program writes too much data into the buffer.
• The return address is overwritten, redirecting execution to the attacker's shellcode.
• The shellcode runs, opening a backdoor or giving admin access.

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This gives the attacker full control over the system, allowing them to steal data, install malware, or escalate privileges.

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Buffer Overflow Prevention; What Deters the Attacker?

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To stop attackers, modern security techniques limit buffer overflows and mitigate their effects. Here’s how:

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1. Bounds Checking & Secure Coding

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• Always validate input lengths before writing to buffers.
• Use safe functions like strncpy() instead of strcpy().
• Use memory-safe languages like Python, Rust, or Java, which don’t allow direct memory access.

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Example of Secure Code:

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void safe_function(char *input) {
	char buffer[20];
	strncpy(buffer, input, sizeof(buffer) - 1);
	buffer[sizeof(buffer) - 1] = '\0';  // Null-terminate to avoid overflows
}

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This ensures no more than 19 characters are copied, preventing buffer overflows.

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2. Stack Canaries (Buffer Overflow Detection)

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• A random “canary” value is placed before return addresses.
• If an overflow occurs, the canary value gets overwritten, and the program detects the attack and crashes.

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How It Works:

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3. ASLR (Address Space Layout Randomization)

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• Randomizes memory locations, making it harder for attackers to predict where buffers and return addresses are.
• Breaks classic stack overflow exploits that rely on fixed memory addresses.

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4. DEP (Data Execution Prevention)

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• Prevents execution of shellcode in memory areas marked as non-executable.
• Attackers must find more complex return-oriented programming (ROP) methods.

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5. Compiler-Based Protections

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• GCC and Clang have buffer overflow protection (-fstack-protector).
• Microsoft’s /GS flag helps prevent stack-based attacks.

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6. Runtime Exploit Mitigation (Firewalls, Sandboxing, IDS/IPS)

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• Intrusion Detection Systems (IDS) can recognize and block buffer overflow attack patterns.
• Sandboxing limits what a vulnerable program can access, reducing damage.

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