I. Common Buffer Overflows:
1. Stack-Based:
Scenario: Consider a web application that processes user login credentials. If the username field does not have sufficient input validation, an attacker may send a forged username that exceeds the stack’s assigned buffer size. This can overwrite important data such as the return address, allowing the attacker to divert program execution and perhaps obtain unauthorized access.
Code Example:
void login(char *username, char *password) {
char buffer[20]; // Vulnerable buffer for username
strcpy(buffer, username); // No bounds checking
// ... (authenticate user based on buffer contents)
if (authenticated) {
// Grant access
} else {
// Login failed
}
}
Scenario: Consider a file compression program that dynamically allocates heap memory to hold compressed data. If the utility fails to validate the size of compressed input, an attacker can provide a maliciously designed file that overflows the allocated buffer. This can overwrite adjacent memory areas, potentially holding more program data or code, resulting in unexpected behavior or code execution vulnerabilities.
Code Example:
void compress_file(char *filename) {
FILE *fp = fopen(filename, "rb");
char *buffer = malloc(1024); // Fixed buffer size
// No input validation
fread(buffer, 1, 1024, fp); // Can overflow if file is larger
// ... (compress data in buffer)
free(buffer);
}
3. Integer Overflow:
Scenario: A banking application computes account balances and interest rates. If the calculations use unsigned integers with no overflow protection, an attacker can modify input numbers (for example, deposit amounts) to produce an overflow, resulting in inaccurate calculations and substantial financial losses.
Code Example:
unsigned int calculate_interest(unsigned int balance, unsigned int rate) {
unsigned int interest = balance * rate;
// No overflow check
return interest;
}
4. Format String:
Scenario: A system logging utility records user operations using formatted messages. If the format string is not properly sanitized, an attacker can use malicious format specifiers such as %p to leak sensitive information such as memory addresses or system pointers, thereby aiding in future attacks.
Code Example:
void log_action(char *username, int action_id) { printf("User %s performed action %d\n", username, action_id); }
Less Common Buffer Overflows:
1. Off-by-One Buffer Overflow:
Scenario: Consider a network packet processing library iterating over packet data. A apparently tiny error of one byte might result in overwriting nearby memory, possibly disclosing sensitive information or disturbing critical system activities. This can occur as a result of indexing problems or loop conditions that are off by one.
Code Example:
void process_packet(char *data, int size) { for (int i = 0; i <= size; i++) { // Off-by-one loop // Process each byte of data } }
2. Buffer Underflow:
Scenario: Consider a security patch that tries to overwrite harmful code inserted into a buffer. If the patch estimates the initial byte erroneously, it may underflow the buffer, changing valid code and resulting in unforeseen consequences such as system crashes or unusual behavior.
Code Example:
void patch_buffer(char *buffer, int offset, int value) { buffer[offset - 1] = value; // Underflows if offset is 0 }
3. Double Free:
Scenario: A memory management library mishandles pointer operations, resulting in the releasing of the same memory block twice. This can corrupt the memory heap, resulting in software crashes, data loss, or potential attacker exploitation via dangling pointers or corrupt memory regions.
Code Example:
void allocate_and_free() {
char *ptr = malloc(10);
free(ptr);
free(ptr); // Double free
}
4. Dangling Pointer:
Scenario: A program dynamically allocates memory but does not maintain track of the allocated pointers. If memory is deallocated but the pointer is still in use, it becomes a dangling pointer, and accessing it results in unpredictable behavior, crashes, or potential security issues.
Code Example:
void use_pointer() {
char *ptr = malloc(10);
free(ptr);
ptr[0] = 'a'; // Accessing freed memory
}
Advanced Buffer Overflow Techniques:
1. Return-Oriented Programming (ROP):
Scenario: Consider a function that accepts user input but lacks sufficient boundaries checking. An attacker might use a long string to overrun the buffer and overwrite the return address on the stack. Instead of injecting their own complete instructions, they use existing, harmless code fragments (gadgets) located in memory (such as instructions to transfer data or add values). By properly chaining these devices together, they can achieve their goal (for example, executing illegal code) without explicitly introducing malicious instructions.
Code Example:
void vulnerable_function(char *input) {
char buffer[10]; // Stack buffer with limited space
strcpy(buffer, input); // No bounds checking!
}
// Attacker injects malicious input:
char payload[] = "...; mov eax, 0xdeadbeef; ret; ..."; // Gadget chain
vulnerable_function(payload); // Stack overflow, gadgets executed
2. Buffer Overflow Gadgets:
Scenario:
The attacker creates a buffer overflow payload with strategically positioned code snippets. These snippets, known as gadgets, are harmless chunks of code that already exist in memory. When an overflow occurs, these gadgets are performed sequentially, allowing the attacker to carry out their nefarious goal (for example, circumventing security checks or executing arbitrary code).
Code Example:
// Harmless gadgets in memory (example):
void gadget1() { // Moves a value to a register }
void gadget2() { // Calls a specific function }
// Attacker's payload:
char payload[] = "...; gadget1; gadget2; ..."; // Chain to achieve goal
// Stack overflow:
// gadget1() -> gadget2() -> ... (attacker's control)
Buffer Overflow Protection Techniques:
1. Input Validation:
Thoroughly check user input before using it to ensure it adheres to expected size and format constraints. This prevents buffer overflows before they can occur.
int safe_function(char *input) {
if (strlen(input) >= sizeof(buffer)) {
return -1; // Input too long, reject
}
strcpy(buffer, input); // Safe copy
return 0;
}
2. Secure Coding Practices:
Utilize libraries and techniques that make exploitation more difficult, such as:
-
- Address Space Layout Randomization (ASLR): Randomizes the memory layout of key components (code, stack, heap) at load time, making it harder for attackers to predict memory addresses for gadgets.
-
- Data Execution Prevention (DEP): Marks data regions as non-executable, preventing even injected code from running.
3. Memory Protection Mechanisms:
- Use hardware-based capabilities such as Memory Protection Units (MPUs) to prevent unauthorized memory access and execution.
- Example (ARM architecture): Memory regions and access rights can be defined using MPUs. Unauthorized attempts result in exceptions.
For a foundational understanding of buffer overflow vulnerabilities, visit hackedyou.org/buffer-overflows-a-beginners-guide-for-ethical-hackers-and-pentesters and dive into the comprehensive insights provided.
Disclaimer: These examples are for educational purposes only, and actual exploitation attempts are unethical and illegal.
