CWE-806: Buffer Access Using Size of Source Buffer

What is Buffer Access Using Size of Source Buffer?

• Overview: This vulnerability occurs when software incorrectly uses the size of the source buffer to read from or write to a destination buffer, potentially allowing memory access outside the intended bounds, leading to buffer overflows.

• Exploitation Methods:

• Attackers can exploit this vulnerability by providing input larger than what the destination buffer can handle, causing it to overflow.
• Common attack patterns include sending specially crafted data that exceeds buffer limits to execute arbitrary code or crash the application.

• Security Impact:

• Direct consequences of successful exploitation include application crashes, data corruption, or unauthorized code execution.
• Potential cascading effects might involve further system compromise or unauthorized access to sensitive data.
• Business impact could range from service downtime and data breaches to loss of customer trust and financial penalties.

• Prevention Guidelines:

• Specific code-level fixes include ensuring destination buffers are large enough to hold the source data and using functions that limit operation to the buffer's size, such as strncpy() instead of strcpy() in C.
• Security best practices involve consistently validating input sizes and employing safe buffer handling techniques.
• Recommended tools and frameworks include static code analysis tools to detect buffer overflows and adopting memory-safe languages or libraries when possible.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: C, C++

Affected Technologies: Not specified

Vulnerable Code Example

C Example: CWE-806

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

void copyString(char *dest, const char *src) {
    // Vulnerable: Using the size of the source buffer can lead to buffer overflow
    // if the source string is larger than the destination buffer.
    size_t srcSize = sizeof(src); // Incorrectly uses sizeof on pointer
    strncpy(dest, src, srcSize); // Potentially writes beyond the bounds of `dest`
}

int main() {
    char largeBuffer[100] = "This is a very long string that could potentially cause issues if not handled properly.";
    char smallBuffer[20];

    // Call the vulnerable function
    copyString(smallBuffer, largeBuffer); // Undefined behavior: buffer overflow
    printf("Copied string: %s\n", smallBuffer);

    return 0;
}

How to fix Buffer Access Using Size of Source Buffer?

Fixed Code Example

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

void copyString(char *dest, size_t destSize, const char *src) {
    // Properly use strlen to find the length of the source string
    size_t srcLength = strlen(src);
    // Ensure we do not exceed the destination size, leaving space for null terminator
    size_t numToCopy = (srcLength < destSize - 1) ? srcLength : destSize - 1;
    strncpy(dest, src, numToCopy);
    dest[numToCopy] = '\0'; // Explicitly null-terminate the destination buffer
}

int main() {
    char largeBuffer[100] = "This is a very long string that could potentially cause issues if not handled properly.";
    char smallBuffer[20];

    // Call the fixed function
    copyString(smallBuffer, sizeof(smallBuffer), largeBuffer); // Safe: buffer overflow mitigated
    printf("Copied string: %s\n", smallBuffer);

    return 0;
}

Key Changes:

• Line 9: Updated function signature to include destSize for safe copying.
• Line 12: Replaced sizeof(src) with strlen(src) to correctly determine the length of the source string.
• Line 14: Calculated numToCopy to ensure it does not exceed destSize - 1.
• Line 16: Added an explicit null terminator to dest to maintain string integrity.

This approach ensures that the destination buffer is not overrun, preventing buffer overflow vulnerabilities.