CWE-680: Integer Overflow to Buffer Overflow

What is Integer Overflow to Buffer Overflow?

• Overview: Integer Overflow to Buffer Overflow (CWE-680) occurs when an application calculates the necessary memory to allocate using integer arithmetic, but an overflow causes less memory to be allocated than intended, leading to buffer overflow vulnerabilities.

• Exploitation Methods:

• Attackers can exploit this by inputting values that cause the integer calculation to overflow, resulting in insufficient memory allocation.
• Common attack patterns include providing large input sizes or values that exceed expected ranges to trigger the overflow during the allocation process.

• Security Impact:

• Direct consequences include the possibility of writing data beyond the allocated memory buffer, leading to memory corruption.
• Potential cascading effects involve system crashes, arbitrary code execution, or unauthorized data access.
• Business impact can include data breaches, service outages, and damage to brand reputation.

• Prevention Guidelines:

• Specific code-level fixes involve validating input sizes and ensuring calculations do not overflow; use safe integer libraries or data types with well-defined overflow behavior.
• Security best practices include implementing rigorous input validation, using memory-safe programming languages when possible, and performing regular security audits.
• Recommended tools and frameworks include static analysis tools to detect potential overflows and using memory-safe libraries that handle allocations securely.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

Python Example

def allocate_buffer(size):
    # Calculate the total size needed for the buffer
    total_size = size * 4  # Each int is assumed to be 4 bytes
    # Potential Integer Overflow: If size is too large, total_size might wrap around
    buffer = [0] * total_size  # Allocate buffer
    return buffer

# Example usage
large_size = 2**30  # Large size that could cause overflow
buffer = allocate_buffer(large_size)

Explanation

In the above code, the allocate_buffer function calculates the total_size by multiplying size by 4. If size is a sufficiently large integer, this multiplication can cause an integer overflow, leading to an incorrect total_size that is smaller than expected. This can result in insufficient memory allocation and potentially a buffer overflow when the buffer is used. The vulnerability arises because Python's list allocation does not inherently handle integer overflow, even though Python integers themselves are of arbitrary precision.

How to fix Integer Overflow to Buffer Overflow?

To fix this issue, it's important to:

1. Check for Integer Overflow: Before performing operations that could lead to overflow, validate the input size to ensure that the result of the calculation will not exceed the maximum allowable integer for memory allocation.

2. Use Safe Data Types or Libraries: In languages where integer overflow is a concern, use data types or libraries that handle overflow checks.

3. Limit Input Size: Impose limits on the input size to avoid calculations that could lead to overflow.

4. Error Handling: Implement proper error handling to deal with invalid input sizes gracefully.

Fixed Code Example

def allocate_buffer(size):
    # Check for potential overflow
    if size > (2**31 - 1) // 4:
        raise ValueError("Size is too large, potential integer overflow detected.")
    
    # Safe calculation after check
    total_size = size * 4
    
    # Allocate buffer safely
    buffer = [0] * total_size
    return buffer

# Example usage with safe size
try:
    large_size = 2**30
    buffer = allocate_buffer(large_size)
except ValueError as e:
    print(f"Error: {e}")

Explanation

In the fixed code:

• Overflow Check: We introduced a check to ensure that size is not large enough to cause an overflow when multiplied by 4. The condition (2**31 - 1) // 4 represents the maximum safe value for size to prevent overflow when multiplied by 4. If size exceeds this threshold, a ValueError is raised.

• Error Handling: The code now includes exception handling to gracefully handle cases where the input size is invalid. This prevents the application from crashing and allows for a controlled response to such errors, maintaining the application's stability and security.