C Code Security Review Guide

TL;DR

This guide provides a practical checklist for reviewing C code security. It covers common vulnerabilities and how to identify them, focusing on buffer overflows, format string bugs, integer issues, memory leaks, and input validation.

1. Setting Up Your Review Environment

1. Static Analysis Tools: Use tools like clang-tidy, cppcheck, or Coverity Scan to automatically detect potential vulnerabilities before runtime.
2. Dynamic Analysis Tools: Employ tools such as Valgrind (specifically Memcheck) and AddressSanitizer (ASan) during testing to find memory errors at runtime.
3. Code Editor/IDE: A good code editor with syntax highlighting and linting features can help spot issues visually.

2. Buffer Overflows

Buffer overflows occur when a program writes data beyond the allocated boundaries of a buffer.

1. Identify Potential Buffers: Look for character arrays (char[]) and other fixed-size buffers used to store input data.
2. Check Input Lengths: Ensure that all input operations are bounded by the size of the destination buffer. Use functions like strncpy, snprintf, or fgets instead of unsafe alternatives like strcpy and gets.
3. Example (Unsafe):

   char buffer[10];
   strcpy(buffer, userInput); // Vulnerable to overflow if userInput is longer than 9 characters.
   

4. Example (Safe):

   char buffer[10];
   sncpy(buffer, userInput, sizeof(buffer) - 1);
   buffer[sizeof(buffer) - 1] = ''; // Ensure null termination.
   

3. Format String Bugs

Format string bugs happen when user-controlled data is used directly as the format string in functions like printf, fprintf, and sprintf.

1. Avoid User Input as Format Strings: Never use user input directly as the format string.
2. Use Fixed Format Strings: Always provide a fixed format string.

   printf("%s", userInput); // Safe
   

3. Example (Unsafe):

   char userInput[256];
   printf(userInput); // Vulnerable if userInput contains format specifiers like %x or %n.
   

4. Integer Issues

Integer issues can lead to overflows, underflows, and other unexpected behavior.

1. Check for Overflow/Underflow: When performing arithmetic operations, especially with user input, verify that the result will not exceed the maximum or fall below the minimum value of the data type.
2. Use Safe Integer Functions: Consider using libraries or functions designed to handle integer overflows safely (e.g., compiler-specific built-ins).
3. Example (Potential Overflow):

   int a = INT_MAX; // Maximum value for int.
   a++; // May cause overflow, leading to unexpected results.
   

5. Memory Leaks

Memory leaks occur when dynamically allocated memory is not properly freed.

1. Track Allocations and Deallocations: For every call to malloc, calloc, or realloc, ensure there’s a corresponding call to free.
2. Use Tools like Valgrind: Run your code with Valgrind (Memcheck) to detect memory leaks and other memory errors.

   valgrind --leak-check=full ./your_program
   

6. Input Validation

Input validation is crucial for preventing many security vulnerabilities.

1. Validate All Inputs: Check all user inputs, including command-line arguments, environment variables, and data from files or networks.
2. Whitelisting vs. Blacklisting: Prefer whitelisting (allowing only known good input) over blacklisting (blocking known bad input).
3. Sanitize Inputs: Remove or escape potentially harmful characters before using the input.

   // Example of basic sanitization (replace dangerous chars):
   char *sanitize_input(const char *input) {
     // ... implementation to remove/escape special characters...
   }
   

7. General Best Practices

• Principle of Least Privilege: Run your program with the minimum necessary privileges.
• Regular Updates: Keep your compiler, libraries, and operating system up to date to benefit from security patches.
• Code Reviews: Conduct regular code reviews by multiple developers.