Buffer Overflows Explained

In the rapidly evolving landscape of cybersecurity, some vulnerabilities fade away while others remain persistent, critical threats. Buffer overflows belong to the latter category. Despite being one of the oldest known software vulnerabilities—famously exploited by the Morris Worm in 1988—they remain a top-tier threat in 2025.

Recently, the Australian Cyber Security Centre (ACSC) and CISA have issued joint guidance urging a shift toward "Secure by Design" principles, specifically calling out memory safety issues like buffer overflows as defects that must be eliminated.

For Australian businesses, understanding this vulnerability isn't just about code; it's about compliance, stability, and protecting your infrastructure from one of the most common attack vectors used by red teams and malicious actors alike.

What is a Buffer Overflow?

At its simplest, a buffer is a sequential section of memory allocated to hold data temporarily—think of it as a container or a bucket with a fixed size. A buffer overflow (often used interchangeably with buffer overrun) occurs when a program tries to put more data into that bucket than it can hold.

Imagine pouring 12 litres of water into a 10-litre bucket. The excess water doesn't disappear; it spills over onto the floor. In a computer system, that "spill" is data writing into adjacent memory spaces.

This overflow corruption is dangerous because computers store both user data (like your password) and control data (instructions on what the computer should do next) in the same memory structures. When a buffer overflows, it can overwrite this control data, causing the system to crash or, worse, allowing an attacker to take control.

Is it Buffer Overflow, Overload, or Overrun?

While you may see these terms used loosely, they have distinct nuances in a search context:

• Buffer Overflow: The technical term for the vulnerability where data exceeds the buffer boundary.

• Buffer Overrun: Synonymous with overflow, often referring to the action of the data moving past its limit.

• Buffer Overload: Often a layman’s term, but sometimes refers to Denial of Service (DoS) scenarios where buffers are filled to capacity to crash a service, even if they don't technically "overflow" into adjacent memory.

How the Attack Works: Stack vs. Heap

To effectively mitigate these risks, it is crucial to understand the areas of memory where these attacks occur. Attackers look for ways to manipulate user input to corrupt the application's logic.

Stack-Based Buffer Overflows

This is the most common type. The "stack" is a memory region used for static memory allocation (like local variables in a function). Crucially, the stack also stores the return pointer—a specific address that tells the computer where to go back to after it finishes executing a function.

In a classic "stack smashing" attack:

1. The application asks for input (e.g., a username) but fails to check the length.

2. The attacker sends a massive string of data that fills the buffer and overflows into the return pointer.

3. The attacker overwrites the return pointer with a new address that points to malicious executable code they have injected.

4. When the function finishes, the computer obediently jumps to the attacker's code instead of returning to normal operations.

Heap-Based Buffer Overflows

The "heap" is used for dynamic memory allocation (memory that changes size during runtime). Heap based attacks are generally more complex to execute than stack attacks because the heap structure is less predictable. However, they are just as dangerous. Exploiting the heap involves corrupting the data pointers that manage the memory itself, eventually leading to the attacker gaining the ability to overwrite arbitrary memory locations.

The Impact: Why It Matters

When memory corruption occurs, the consequences are severe. It is not just about a glitchy app; it is about total system compromise.

• Remote Code Execution (RCE): The "holy grail" for attackers. By overwriting the right pointers, an attacker can run their own commands with the privileges of the application.

• Privilege Escalation: If the vulnerable program is running as an administrator or "root," the attacker gains full control over the server.

• Denial of Service (DoS): Even if the attacker fails to gain control, the corrupted memory usually causes system crashes, taking critical business applications offline.

The Culprits: Vulnerable Languages vs. Memory Safe Languages

Not all programming languages are created equal. The vast majority of buffer overflows occur in C and C++.

Why? Because these languages offer manual memory management. They give developers high performance and direct access to hardware but require the developer to manually check bounds. If a developer uses a function like strcpy (string copy) without checking if the destination buffer is large enough, a vulnerability is born.

In contrast, modern memory safe languages like Rust, Go, Java, and Python have built in protection. They automatically manage memory allocation and bounds checking, making standard buffer overflows mathematically impossible in most scenarios. This is why the ASD and CISA are aggressively pushing for a migration to these languages for critical infrastructure.

Mitigation: How We Prevent Buffer Overflows

Fixing these issues requires a defense-in-depth approach, combining secure coding with modern operating system defenses.

1. Input Validation and Bounds Checking

The most effective fix is at the code level. Developers must ensure that every piece of user input is checked against the size of the buffer before it is written. Bounds checking ensures that you never try to put 12 bytes of data into an 8-byte container.

2. Automatic Protections (OS Level)

Modern operating systems (Windows, Linux, macOS) have introduced features to make exploitation harder:

• ASLR (Address Space Layout Randomization): Randomizes the locations of the stack, heap, and libraries in memory. This makes it hard for an attacker to know exactly where their malicious code is located.

• DEP (Data Execution Prevention) / NX (No-Execute): Marks certain areas of memory (like the stack) as non-executable. Even if an attacker injects code there, the processor refuses to run it.

• Stack Canaries: Small, random values placed before the return pointer. If a buffer overflow occurs, the "canary" is overwritten first. The system sees the dead canary and shuts down the process before the malicious code can run.

3. Regular Penetration Testing

Automated tools cannot find every logic flaw or complex heap vulnerability. Professional penetration testing and red teaming are essential to simulate how a motivated human attacker would attempt to exploit your specific environment. We look for the edge cases that automated scanners miss, ensuring your custom applications aren't the weak link in your security chain.

Buffer overflows may be "classic" vulnerabilities, but they are far from extinct. As long as legacy C/C++ codebases exist, attackers will continue to hunt for these memory corruption flaws.

By understanding the mechanics of stack smashing and heap manipulation, and by validating your applications through rigorous offensive security testing, you can ensure your organization remains resilient against these high-impact threats.

Ready to secure your applications? Don't wait for a breach to discover a memory flaw. Contact us today for a quote on our advanced Web Application Penetration Testing services.