Fixing Buffer Overflows (Big Endian)

TL;DR

Buffer overflows happen when a program writes data beyond the allocated space for a buffer, potentially overwriting important memory. On Big Endian systems, this means you need to be especially careful about how you construct your overflow payload – most significant byte first! This guide shows you how to identify and fix these vulnerabilities.

Understanding Buffer Overflows on Big Endian

Big Endian architecture stores the most significant byte of a multi-byte value at the lowest memory address. This is different from Little Endian, where the least significant byte comes first. When exploiting or mitigating buffer overflows, this impacts how you build your payload to correctly overwrite target values (like return addresses).

Identifying Buffer Overflows

1. Fuzzing: Use fuzzers like AFL (American Fuzzy Lop) to send a large amount of random data to the program. Monitor for crashes or unexpected behavior.
2. Static Analysis: Tools like Coverity, SonarQube, or even manual code review can identify potentially vulnerable functions (e.g., strcpy, gets, sprintf) that don’t perform bounds checking.
3. Dynamic Analysis: Use debuggers (GDB, LLDB) to step through the program and observe memory writes. Look for situations where data is written beyond buffer boundaries.

Fixing Buffer Overflows

1. Use Safe Functions: Replace unsafe functions with their safer counterparts:
   • Instead of strcpy(dest, src), use strncpy(dest, src, sizeof(dest) - 1); dest[sizeof(dest)-1] = '';
   • Instead of gets(buffer), use fgets(buffer, sizeof(buffer), stdin).
   • Instead of sprintf(buffer, format_string, ...), use snprintf(buffer, sizeof(buffer), format_string, ...).
2. Bounds Checking: Always check the length of input data before copying it into a buffer.

   if (strlen(src) >= sizeof(dest)) {
     // Handle error - input too long!
   } else {
     strcpy(dest, src);
   }

3. Stack Canaries: Enable stack canaries during compilation. These are random values placed on the stack before return addresses. If a buffer overflow overwrites the canary, the program detects it and terminates.

   gcc -fstack-protector-all your_program.c -o your_program

4. Address Space Layout Randomization (ASLR): ASLR randomizes the base addresses of key memory regions (e.g., stack, heap, libraries). This makes it harder for attackers to predict where to overwrite return addresses.

   gcc -fPIE -pie your_program.c -o your_program

5. Data Execution Prevention (DEP) / NX Bit: DEP prevents code execution from data segments, making it harder for attackers to inject and execute malicious code.

Payload Construction on Big Endian Systems

When crafting an exploit payload for a buffer overflow on a Big Endian system, remember the byte order. You need to arrange the bytes of your target address (e.g., return address) in reverse order compared to Little Endian.

1. Determine Target Address: Find the memory address you want to overwrite (often a return address).
2. Convert to Bytes: Convert the target address into its byte representation.
3. Reverse Byte Order: Reverse the order of the bytes. For example, if your target address is 0x12345678:
   • Little Endian: 78 56 34 12
   • Big Endian: 12 34 56 78
4. Construct Payload: Build your payload, including the reversed bytes of the target address. You’ll also need padding to reach the correct offset in memory.

Example (Conceptual)

Let’s say you want to overwrite a return address at offset 100 with the address 0x41424344 on a Big Endian system.

payload = b"A" * 100 + b"x44x43x42x41"  # Reversed bytes of 0x41424344

Send this payload to the vulnerable program.

Testing

1. Debugging: Use a debugger (GDB, LLDB) to verify that your payload correctly overwrites the target memory location.
2. Reproducibility: Ensure that the exploit is reproducible and reliable.