Most programs use common sets of code to perform tasks, and ROP leverages this common code to perform a desired task. Sometimes, attackers set up execution of several sections of code across multiple libraries in a process known as ROP chaining.
Since the code the attacker needed was already present in instructional memory, there was no need to place it on the stack for execution. In an effort to stop ROP-based attacks , operating systems started to randomize the location of instructional memory to prevent attackers from knowing where desired code was stored. That randomization of instructional memory is called ASLR, which shuffles blocks of memory and makes it so that the location of a given object including code in memory is no longer a constant value.
An attack that works once may not work again, as the code the attacker tried to execute might no longer be there, causing unpredictable results. While effective, ASLR is constrained because, like NX, not every piece of instructional memory responds well to moving, so some code must opt out of the protection. Even for code that can handle ASLR, there are bypasses. The most common bypass leverages the limitation that the memory can only be randomized in blocks.
If there is a way to determine where a block of memory is, an attacker can calculate the location of the desired memory from the leaked value.
Unfortunately, since ASLR was not something that was baked into operating systems, they sometimes store the randomized location of something important in a known place, not unlike an employee choosing a good password but putting it on a Post-It note under their keyboard.
Again, just like NX, ASLR does not completely prevent an attack, but it does make attacks harder and less predictively successful. It would be nice to say that stack-based overflow attacks are gone due to the mitigation strategies in place, but that is simply not the case. Stack-based attacks might not be as common today, but they do exist. Due to the large size of operating system vendors, it is unlikely that a stack-based attack exists in Windows or Linux anymore, but smaller groups that pay less attention to security still release vulnerable code—and not every vulnerability can be mitigated by the operating system.
Brendan is a Senior Researcher on the Metasploit team and has been a team member since He works primarily with Metasploit Framework and Metasploit Payloads to write, vet, and land pull requests. Sesame open: Auditing password security with Metasploit 3. Read More. Help your new sweethearts call home to Metasploit. Quick Cookie Notification This site uses cookies, including for analytics, personalization, and advertising purposes. What are buffer overflow attacks? Deep dive on stack-based buffer overflow attacks Understanding stack-based overflow attacks involves at least a basic understanding of computer memory.
Taking a look at the hack We have looked at the stack, noticed that the buffers are located consecutively in memory, and talked about why gets is a bad function. Altering metadata Using stack overflow attacks against program metadata to affect code execution is not much different than the above example. In conclusion It would be nice to say that stack-based overflow attacks are gone due to the mitigation strategies in place, but that is simply not the case.
Popular Tags. Related Posts. Metasploit Wrap-Up Read More. Help your new sweethearts call home to Metasploit Read More. Metasploit 12 min. Metasploit Wrap-Up. Read Full Post. Exploits 12 min. Metasploit Weekly Wrapup 12 min. View All Posts. When we run it, a typical execution of this program with command-line parameter Mathilda would look like this:.
Now the code is compiled and the buf program was created, let's fire up gdb , the Linux command line debugger. In gdb , we can use the list command to display the code. Note that this works because we've compiled it with debug information.
The code will look familiar:. Usually you won't have the luxury of having the source-code available, which means you will have to dive right into the assembler code. The command disas func will show the assembler code for the method func :. Reading assembler is a different story and worthy of a blog post or 10 by itself. Here it is allocating bytes for the buffer 0x64 is in hexadecimal. Note again that the stack grows downward from high-memory to lower-memory addresses.
However, the buffer itself is filled from lower- to higher memory addresses. This means that if we would pass a name that is bigger than characters long, it would start overwriting the base pointer that's lower in the stack and higher up in the memory. Let's see what happens when we execute the buf -program with a name-parameter of characters long. The characters will be built up by times the letter 'A', 4 times a 'B', followed by 4 times the letter 'C'.
Later, when inspecting the program's memory, this distinction of the letters will make it easier to identify the memory segments that were overwritten. We're expecting the first A's to fill the buffer, the B's to overwrite the EBP and the C's to overwrite the return address. To those characters, the four B's 0x42 and the four C's 0x43 will be added, producing a string with a total length of bytes. From within gdb , the program can be executed using the run -command and when this string is passed to the program, it will produce the following result:.
Segmentation fault. This is an error the CPU produces when you something tries to access a part of the memory it should not be accessing. It didn't happen because a piece of memory was overwritten, it happened because the return address was overwritten with C's 0x There's nothing at address 0x and if there is, it does not belong to the program so it is not allowed to read it. This produces the segmentation fault. If we check out the registers by entering the command info registers in gdb, we confirm that the EBP and return address are overwritten:.
The EIP Extended Instruction Pointer contains the address of the next instruction to be executed, which now points to the faulty address. Now 0x is a faulty address. However, if this address would point to malicious code, we could have a problem. We've seen how to change the return address. In order to exploit the problem with the buffer we aim to change the return address to somewhere we would have some code that, when executed, could do something beneficial to us as an attacker; like launching a shell.
A shellcode is a small piece of code used as the payload in the exploitation of a software vulnerability. It is called "shellcode" because it typically starts a command shell from which the attacker can control the compromised machine, but any piece of code that performs a similar task can be called shellcode.
The art of crafting shellcode is a blog post of its own. Shellcodes depend on the operating system and CPU and are commonly written in assembler. Many samples of shellcode can be found on the Internet exploit-db and the piece we're using here is one that spawns a command shell.
Create a file called shellcode. Assemble it using nasm :. This produces a shellcode. Our goal is to get the faulty program buf to execute the shellcode. In order to do this, we will pass the shellcode as the command-line parameter so it will eventually end up in the buffer. We then overwrite the return address the C's in the previous example so it will point back to a memory address somewhere in the buffer.
This will make the program jump to the shellcode and execute that code instead of the regular program. Memory may move around a bit during execution of the program, so we do not exactly know on which address the shellcode will start in the buffer. The NOP-sled is a way to deal with this. Anywhere the return address lands in the NOP-sled, it's going to slide along the buffer until it hits the start of the shellcode. With a NOP-sled, it doesn't matter where the shellcode is in the buffer for the return address to hit it.
What we do know is that it will be somewhere in the buffer and its size will be 25 bytes. For instance, our code, which reads an IP address from a file, could be part of a function called readIpAddress , which reads an IP address from a file and parses it.
This function could be called by some other function, for example, readConfiguration. When readConfiguration calls readIpAddress , it passes a filename to it and then the readIpAddress function returns an IP address as an array of four bytes. During this function call, three different pieces of information are stored side-by-side in computer memory. For each program, the operating system maintains a region of memory which includes a part called the stack or call stack hence the name stack buffer overflow.
When a function is called, a fragment of the stack is allocated to it. This piece of the stack called a frame is used to:. So if a program has a buffer allocated in the stack frame and tries to insert more data than can fit there, user input data may spill over and overwrite the memory location where the return address is stored. If the problem was caused by random malformed user input data, the new return address most likely will not point to a memory location where any other program is stored, so the original program will simply crash.
However, if the data is carefully prepared, it may lead to unintended code execution. The second step is to place the address of this malicious data in the exact location where the return address should be. In effect, when the function reads the IP character string and places it into the destination buffer, the return address is replaced by the address of the malicious code. When the function ends, program execution jumps to malicious code.
Since the discovery of the stack buffer overflow attack technique, authors of operating systems Linux, Microsoft Windows, macOS, and others have been trying to find prevention techniques:. Some of these measures may also affect performance. Buffer overflow vulnerabilities exist in programming languages which, like C, trade security for efficiency and do not check memory access.
In higher-level programming languages e. In these languages, you simply cannot put excess data into the destination buffer. For example, try to compile and execute the following piece of Java code:. The Java compiler will not warn you, but the runtime Java virtual machine will detect the problem and instead of overwriting random memory, it will interrupt program execution.
However, even programmers who use high-level languages should know and care about buffer overflow attacks. Their programs are often executed within operating systems that are written in C or use runtime environments written in C, and this C code may be vulnerable to such attacks. In order to make it easier to distribute such an application, it may be packed into a single file archive — as a zip file, a tar file, or using a custom PHP format called phar.
A PHP extension called phar contains a class that you can use to work with such archives. With this class, you can parse an archive, list its files, extract the files, etc. Using this class is quite simple.
For example, to extract all files from an archive, use the following code:. For example, for an archive called myarchive. The reason why the authors implemented it this way is not important here — what is important is how they implemented it. Until , this operation was done using the following function see the old PHP source code :.
As you can see, this function creates a char array of called tmp.
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