CWE-121: Stack-based Buffer Overflow

What is Stack-based Buffer Overflow?

• Overview: A stack-based buffer overflow occurs when data written to a buffer on the stack exceeds the buffer's allocated size, potentially overwriting adjacent memory locations, including return addresses or other control data.

• Exploitation Methods:

• Attackers exploit this by inputting more data than the buffer is designed to hold, causing it to overwrite critical stack data.
• Common techniques include injecting malicious code into the stack or altering control flow to execute attacker-controlled instructions.

• Security Impact:

• Direct consequences include unexpected behavior, crashes, or arbitrary code execution.
• Potential cascading effects involve taking control of the system, data corruption, and unauthorized access to sensitive information.
• Business impact can result in service downtime, data breaches, loss of customer trust, and legal liabilities.

• Prevention Guidelines:

• Implement bounds checking to ensure that data written to a buffer does not exceed its size.
• Adopt security best practices such as using functions that limit input size (e.g., strncpy instead of strcpy).
• Utilize recommended tools and frameworks like StackGuard, AddressSanitizer, and modern compilers with built-in protections to detect and mitigate buffer overflows.

Corgea can automatically detect and fix Stack-based Buffer Overflow in your codebase. [Try Corgea free today](https://corgea.app).

Technical Details

Likelihood of Exploit: High

Affected Languages: C, C++

Affected Technologies: Not specified

There are generally several security-critical data on an execution stack that can lead to arbitrary code execution. The most prominent is the stored return address, the memory address at which execution should continue once the current function is finished executing. The attacker can overwrite this value with some memory address to which the attacker also has write access, into which they place arbitrary code to be run with the full privileges of the vulnerable program. Alternately, the attacker can supply the address of an important call, for instance the POSIX system() call, leaving arguments to the call on the stack. This is often called a return into libc exploit, since the attacker generally forces the program to jump at return time into an interesting routine in the C standard library (libc). Other important data commonly on the stack include the stack pointer and frame pointer, two values that indicate offsets for computing memory addresses. Modifying those values can often be leveraged into a "write-what-where" condition.

Vulnerable Code Example

Sure, let's address the issues and improve the code examples for CWE-121 (Stack-based Buffer Overflow).

C Example

// vulnerable_program.c {7-9}
#include <stdio.h>
#include <string.h>

void vulnerableFunction(const char *input) {
    char buffer[10]; // Fixed-size buffer allocated on the stack
    // Potential stack-based buffer overflow if input exceeds buffer size
    strcpy(buffer, input); // Unsafe copy without bounds checking
    printf("Buffer content: %s\n", buffer);
}

int main() {
    // Example input that causes overflow
    char largeInput[] = "This input is too long!";
    vulnerableFunction(largeInput);
    return 0;
}

Explanation

In this vulnerable code example, the function vulnerableFunction uses strcpy to copy the input to a fixed-size buffer without checking the length of the input. If the input exceeds the buffer's capacity, it will result in a stack-based buffer overflow, which can lead to undefined behavior, including potential security vulnerabilities such as overwriting the return address.

How to fix Stack-based Buffer Overflow?

To fix a stack-based buffer overflow, the goal is to prevent writing more data to a buffer than it can hold. This can be achieved by:

1. Using safer alternatives: Replace strcpy() with strncpy() or snprintf(), which allow specifying the maximum number of bytes to copy.
2. Performing bounds checking: Explicitly check the length of the input before copying it to the buffer.
3. Using dynamic memory allocation: Allocate memory on the heap instead of the stack, as it is more flexible and reduces the risk of overwriting stack memory.

Implementing these practices will help prevent buffer overflow vulnerabilities in C.

Fixed Code Example

// fixed_program.c {9-11, 17-18}
#include <stdio.h>
#include <string.h>

void safeFunction(const char *input) {
    char buffer[10]; // Fixed-size buffer allocated on the stack
    // Safe copy with bounds checking using strncpy
    strncpy(buffer, input, sizeof(buffer) - 1); // Copy with limit one less than buffer size
    buffer[sizeof(buffer) - 1] = '\0'; // Ensure null-termination
    printf("Buffer content: %s\n", buffer);
}

int main() {
    // Example input handled safely
    char largeInput[] = "This input is too long!";
    safeFunction(largeInput); // Call to safe function
    return 0;
}

Explanation

In the fixed code example, we use strncpy() to safely copy the input to the buffer, specifying a maximum number of bytes to copy that is one less than the buffer size. This ensures that the buffer will not overflow. Additionally, we manually set the last character of the buffer to \0 to ensure null-termination, which is crucial for string operations in C. This approach effectively prevents stack-based buffer overflows.