CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer

What is Improper Restriction of Operations within the Bounds of a Memory Buffer?

• Overview: Improper Restriction of Operations within the Bounds of a Memory Buffer occurs when a program performs actions on a memory buffer, such as reading or writing, outside of its allocated boundaries. This can lead to accessing unintended memory locations, potentially affecting other variables or program data.

• Exploitation Methods:

• Attackers can exploit this vulnerability to gain unauthorized access to sensitive information or to execute arbitrary code.
• Common attack patterns include buffer overflow exploits, where attackers supply input that exceeds buffer limits to overwrite critical memory areas.

• Security Impact:

• Direct consequences of successful exploitation include crashing the application, corrupting data, or executing malicious code.
• Potential cascading effects involve compromising system integrity, leading to unauthorized data access or privilege escalation.
• Business impact can range from data breaches and loss of customer trust to financial losses and regulatory penalties.

• Prevention Guidelines:

• Specific code-level fixes include using secure functions that check buffer boundaries, such as strncpy() instead of strcpy() in C/C++.
• Security best practices involve regular code reviews, static analysis, and adopting a defensive programming mindset.
• Recommended tools and frameworks include using memory-safe languages, employing compiler flags like Stack Smashing Protector (SSP), and utilizing address space layout randomization (ASLR).

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Technical Details

Likelihood of Exploit: High

Affected Languages: C, C++, Assembly

Affected Technologies: Not specified

Certain languages allow direct addressing of memory locations and do not automatically ensure that these locations are valid for the memory buffer that is being referenced.

Vulnerable Code Example

// Vulnerable code demonstrating CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer

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

void copy_input(char *input) {
    char buffer[10]; // Fixed size buffer

    // Vulnerable: No bounds checking on input size
    strcpy(buffer, input); // Can overflow if input is larger than buffer
    printf("Input copied to buffer: %s\n", buffer);
}

int main() {
    char user_input[50];
    printf("Enter some text: ");
    fgets(user_input, sizeof(user_input), stdin); // Reads up to 49 characters plus null terminator
    user_input[strcspn(user_input, "\n")] = '\0'; // Remove newline character

    copy_input(user_input);

    return 0;
}

How to fix Improper Restriction of Operations within the Bounds of a Memory Buffer?

To fix this vulnerability, it is crucial to ensure that any data copied into a buffer respects the buffer's size limit. This can be achieved by using safer string manipulation functions that include bounds checking, such as strncpy instead of strcpy. Additionally, always ensure that the buffer is large enough to hold incoming data, including the null terminator. Moreover, when reading user input, truncate the input to prevent buffer overflow.

Fixed Code Example

// Fixed code addressing CWE-119 by implementing bounds checking

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

void copy_input(char *input) {
    char buffer[10]; // Fixed size buffer

    // Fixed: Use strncpy with explicit bounds checking
    strncpy(buffer, input, sizeof(buffer) - 1); // Copy up to 9 characters
    buffer[sizeof(buffer) - 1] = '\0';  // Ensure null termination
    printf("Input copied to buffer: %s\n", buffer);
}

int main() {
    char user_input[50];
    printf("Enter some text: ");
    fgets(user_input, sizeof(user_input), stdin); // Reads up to 49 characters plus null terminator
    user_input[strcspn(user_input, "\n")] = '\0'; // Remove newline character

    copy_input(user_input);

    return 0;
}

Explanation:

• strncpy Usage: The vulnerable use of strcpy has been replaced with strncpy, which takes the size of the destination buffer minus one as an argument. This ensures that no more than the specified number of characters are copied, thus preventing buffer overflow.
• Null Termination: After using strncpy, explicitly setting the last character to '\0' ensures the buffer is null-terminated, which is crucial for safety and to avoid undefined behavior.
• fgets: Properly using fgets to limit the input size ensures that the input cannot overflow the receiving buffer, and the newline character is stripped to ensure consistent buffer handling.

By adhering to these practices, the program becomes resilient to buffer overflow attacks, thereby maintaining memory safety.