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Address Space Layout Randomization (ASLR)

Address Space Layout Randomization (ASLR) is a memory protection mechanism that defends against buffer overflow attacks by randomizing where data is stored in RAM each time it is written. This prevents attackers from reliably targeting specific memory locations to execute malicious code.

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About this video

ASLR, or Address Space Layout Randomization, is a memory protection feature preconfigured on modern operating systems to defend against a class of attacks known as buffer overflow exploits. To understand what ASLR does, it helps to understand how memory works: RAM stores data being actively processed by the CPU, and every piece of that data is assigned a specific address so the system can locate and retrieve it. Applications reserve portions of memory called buffers to hold variable data such as user input, and those buffers are allocated a fixed size within memory regions known as the heap and the stack. A buffer overflow occurs when more data is written to a buffer than it was allocated to hold, causing the excess to spill over into adjacent memory. Attackers exploit this by deliberately overflowing a buffer with crafted input that includes malicious code, then overwriting a nearby return address so that when execution resumes, the system is redirected to run that injected code rather than legitimate instructions. The success of this attack depends entirely on the attacker knowing the exact memory address of the target buffer in order to calculate the correct payload. ASLR eliminates that certainty by randomizing the memory address assigned to buffers and other data structures each time they are used. Because the location changes unpredictably at runtime, an attacker cannot reliably determine what value to supply as the return address, which breaks the exploit before it can execute. This makes ASLR an effective mitigation against buffer overflow attacks and a foundational component of modern system-level security.

What you'll learn

Explain how Address Space Layout Randomization (ASLR) mitigates buffer overflow attacks by randomizing memory address locations at runtime

What's covered

ASLR Overview

Memory & Address Space
- RAM Processing Role
- Memory Addressing
- Buffers Store Variables
Buffer Overflow Attack
- Heap and Stack Buffers
- Injecting Code in Buffer
- Overwriting Return Address
- Attacker Needs Known Address
How ASLR Works
- Randomizes Buffer Location
- Return Address Unpredictable
- Mitigates Overflow Attacks

Key terms

Address Space Layout Randomization: ASLR is a memory protection technique that randomizes the base addresses of processes and libraries at load time to prevent attackers from reliably reaching exploit targets in memory.
Buffer: A region of memory used to temporarily store variable data during processing.
Buffer Overflow: A software vulnerability that occurs when a program writes more data to a fixed-size memory buffer than it can hold, overwriting adjacent memory. Attackers exploit buffer overflows to crash programs or execute arbitrary malicious code.
Exploit: A piece of software or technique that takes advantage of a vulnerability to gain unauthorized access or cause harm.
Vulnerability: A weakness in a system, application, or process that can be exploited by a threat actor.

Topics

Aslr Memory Protection Buffer Overflow Exploit Mitigation Operating Systems Application Security

Transcript

ASLR and Memory

Another thing that's preconfigured on your machines nowadays is this address space layout randomization, or ASLR.

ASLR has to do with memory — memory as in RAM on your computer. This right here is a stick of RAM, and we've got some data that's written inside of RAM. So essentially anything that gets processed on your machine will be sent to RAM, and then RAM will send it to the CPU. The CPU will process it, store it back into RAM, and then it will send it to wherever it needs to go.

Address Space

ASLR starts out with AS. The AS stands for address space. So what is address space? Well, we need to be able to write to this memory and be able to take information out of this memory, or extract information from this memory, and be able to manipulate and control this. Because of that, we need to address certain blocks of data within here. The way we do that is with addressing. So all of this is broken into small pieces here, and they're labeled with some sort of address, and we use that address to access that piece of memory.

Buffers and Buffer Overflow

We also have something called a buffer. A buffer is going to be a place that we're going to store information in. So for instance, let's say I type my name in here into some sort of application. It needs to store that somewhere. It needs to keep track — I've typed it in there for a reason. Since it needs to store it somewhere, it will store it within memory. Well, it stores it within what's called a buffer. A buffer is a piece of memory that we use to store variables in.

There is this thing called buffer overflow. We allot a certain size for these buffers, and if we add too much information to the buffer, then it overflows the buffer and writes over other pieces of information. So this is called buffer overflow.

We don't need to be concerned too much with heap or stack, but just know that there are parts that we write to as part of the heap and there are parts that we write to as part of the stack, and there are buffers in each one of those. The heap and the stack is going to be where this issue falls into play.

The Buffer Overflow Attack

Since a stack buffer overflow is more common, let's take a look at that side of it. Here's a buffer right here. This is where we're going to write information. It has some sort of address space — that is the address space location of where it starts — and then it's allocated a certain size to it. If you overwrite that, or if it overflows, it's going to write into these other fields here.

Now, this could be utilized for a certain type of attack. We call it a buffer overflow attack. So what does that look like? Well, normally, if I were to enter my name in here, Andrew, it would go into the buffer, it would be stored, and whenever it needs to access that, then it could access that and pull it. And then once it accesses that, it uses this return value to return back to whatever memory allocation it needs to return back to, whatever address space it needs to then go back and reference. That's the way to go back to the primary way that it's executing this code. You don't need to be too involved in that.

But what can happen here is that instead of putting my name Andrew in there, I could put some sort of code in this buffer instead. Now, normally it wouldn't execute this code and it wouldn't be problematic. I could put this code into this buffer, but it's just going to try to use it in some other way, and it's probably not going to make sense and it's going to error out. But what happens if I can overflow to write over this return address and return it back to this address? Now, instead of just grabbing that information and using that information, it's actually going to execute that information. And since it's code now that I slipped in there, it's going to execute that code and I can compromise the system.

Now, a key part in carrying out this attack is that I need to know where this address space is right here. Where's the beginning of the buffer? Because what I need to do is form whatever my input is going to be, so that it comes directly to this return address right here and it will return to that address space right there. So it needs to be very specific, what I type into here. This whole process really relies on this address.

What the Randomization Does

So now let's get back to this name, address space layout randomization, ASLR. What does it mean? Address space — here's the address space. We talked about that address space. We talked about the importance of this buffer and returning to that buffer. So what is the layout randomization? It means that every time I enter something into this value right here, it's actually going to store it in a different address location. It's going to take this buffer and move it somewhere else, and so this address is going to change. If this address keeps changing, then I can't figure out what to put in this return address in order for it to execute the code that I want to do. So what address space layout randomization does is it stops buffer overflow attacks from happening. It mitigates this issue.

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