Buffer Overflow

Buffer Overflow

What Is Buffer Overflow?

Buffer overflow is a software coding error or vulnerability that can be exploited by hackers to gain unauthorized access to corporate systems. It is one of the best-known software security vulnerabilities yet remains fairly common. This is partly because buffer overflows can occur in various ways, and the techniques used to prevent them are often error-prone.

Buffers are sequential sections of computing memory that hold data temporarily as it is transferred between locations. Also known as a buffer overrun, buffer overflow occurs when the amount of data in the buffer exceeds its storage capacity. The extra data overflows into adjacent memory locations, corrupting or overwriting the data in those locations.

What Is a Buffer Overflow Attack?

A buffer overflow attack occurs when an attacker exploits the coding error to carry out malicious actions and compromise the affected system. The attacker manipulates the application’s execution path and overwrites elements of its memory, altering the program’s execution path to damage files or expose data.

Characteristics of Buffer Overflow Attacks:

• Violates programming language constraints and overwrites buffer boundaries.
• Results from memory manipulation and mistaken assumptions about data size or composition.

Vulnerability Occurs When Code:

• Relies on external data to control behavior.
• Depends on data properties enforced beyond its immediate scope.
• Is so complex that programmers cannot accurately predict its behavior.

Buffer Overflow Exploits

The techniques a hacker uses depend on the target’s architecture and operating system. Attackers issue extra data containing malicious code, enabling them to:

• Trigger additional actions.
• Send new instructions to the application.

Example:

If an attacker knows a program’s memory layout, they may input data that cannot be stored by the buffer, overwriting memory locations with malicious code to take control of the program.

Buffer overflow exploits can corrupt a web application’s execution stack, execute arbitrary code, and take over a machine. Vulnerabilities can exist in:

• Application servers
• Web servers
• Custom web application code

Buffer Overflow Consequences

Common Consequences:

1. System Crashes: Leads to crashes, lack of availability, or infinite loops.
2. Access Control Loss: Arbitrary code may bypass security policies.
3. Further Security Issues: Exploitation of other vulnerabilities and subversion of security services.

Types of Buffer Overflow Attacks

1. Stack-Based Buffer Overflows:

The most common form. Data containing malicious code overwrites the stack buffer, including its return pointer, handing control to the attacker.

2. Heap-Based Buffer Overflows:

More difficult to execute. Involves flooding a program’s memory space beyond the allocated runtime memory.

3. Format String Attack:

Occurs when applications process input data as a command or fail to validate input effectively. Allows attackers to execute code, read stack data, or cause segmentation faults.

Vulnerable Programming Languages

Applications written in interpreted languages like Java and Python are immune to buffer overflow attacks, except for overflows in their interpreter. Vulnerabilities are more common in:

• C/C++: Lacks built-in buffer overflow protection.
• Assembly: Particularly vulnerable.
• Fortran: Similar vulnerabilities as C/C++.

Applications written in JavaScript or Perl are typically less vulnerable.

How to Prevent Buffer Overflows

Methods for Developers:

• Avoid standard library functions without bounds-checking (e.g., gets, scanf, strcpy).
• Use runtime-enforced bounds-checking.
• Build security measures into development code.
• Test code regularly to detect and fix errors.

Modern Operating System Protections:

1. Address Space Layout Randomization (ASLR): Randomizes address spaces, making attacks difficult.
2. Data Execution Prevention: Flags memory regions as executable or non-executable.
3. Structured Exception Handling Overwrite Protection (SEHOP): Prevents attackers from exploiting exception handling mechanisms.

Patching:

Quickly patch vulnerabilities and distribute updates to all users.

Buffer Overflow Attack Examples

Example 1: Stack-Based Overflow

char buf[BUFSIZE];
gets(buf);

The gets() function does not limit input size, making the code vulnerable.

Example 2: Using memcpy()

char buf[64], in[MAX_SIZE];
printf("Enter buffer contents:\n");
read(0, in, MAX_SIZE-1);
printf("Bytes to copy:\n");
scanf("%d", &bytes);
memcpy(buf, in, bytes);

Here, user input can exceed buffer size, causing an overflow.

Example 3: Function with Unchecked Data Properties

char *lccopy(const char *str) {
  char buf[BUFSIZE];
  char *p;
  strcpy(buf, str);
  for (p = buf; *p; p++) {
    if (isupper(*p)) {
      *p = tolower(*p);
    }
  }
  return strdup(buf);
}

The strcpy() function assumes input is smaller than BUFSIZE, which attackers can exploit.

Example 4: Complexity Leading to Errors

The following example shows vulnerabilities in the libPNG image decoder:

if (!(png_ptr->mode & PNG_HAVE_PLTE)) {
  png_warning(png_ptr, "Missing PLTE before tRNS");
} else if (length > (png_uint_32)png_ptr->num_palette) {
  png_warning(png_ptr, "Incorrect tRNS chunk length");
  png_crc_finish(png_ptr, length);
  return;
}
...
png_crc_read(png_ptr, readbuf, (png_size_t)length);

The blind length checks in the png_crc_read() call can lead to memory vulnerabilities.

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By understanding buffer overflow attacks, their consequences, and prevention techniques, developers can build more secure applications and mitigate risks effectively.