Heap overflow ctf

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Heap overflow ctf

This is a writeup of the vuln3 service. Vuln3 is a service exposed to the Internet via xinetd or something similar. It accepts input from you writes it to the stack, parses it and performs actions based on it. It does this until you disconnect.

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The important parts of user input are dwords at offset 0x28 and 0x The dword at offset 0x50 is used only when you ask the service to allocate some memory for you, it is then used as the argument to malloc. The service supports four functions that you can control. Allocate a block of size 8 and put two function pointers inside it.

Allocate a block of a size specified by the dword at offset Call fgets to populate it. Call function pointers and free blocks. Check to make sure that we have allocated the block of size 8 with function pointers inside.

If we have, call the function pointers and free the block.

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Also check to see if we have allocated a block with an arbitrary size and free that too. Copy the contents as a string from the block of arbitrary size to the block of fixed size that was allocated at the beginning of the program.

The vulnerability here is a heap buffer overflow. What we need to do is manipulate the heap state such that the block of size 8 immediately follows the block allocated in the beginning of the program. Then we can allocate a large block to overflow the block with a static size. That overflow will corrupt the function pointers. We can then ask the application to call the function pointers thereby escaping intended control flow. Astute hackers will note that we do not control the stack however stack layout is predictable.

Right before the function is called the value on the top of the stack is a pointer to a buffer that we control. The pointer is pointing at the stack buffer and because the organizers are nice folks, the stack and heap are executable.

That means we have 0x28 bytes to work with for writing shellcode. I wrote a small trampoline that punts execution to the heap block that we have arbitrary control over. Allocate a large block with pointers to pop; ret to overwrite the function pointers in the small block.

heap overflow ctf

Copy the large block into the block of size 0x If it is large enough it will corrupt the small block. Next we create another large block that will contain our shellcode. Now we have no shellcode length constraints. Issue the command to call the pointers and free the block, naturally divert control before the free. You can find this exploit and many others in the CTF-Solutions repo on github.Stack overflow refers to the number of bytes written by the program to a variable in the stack that exceeds the number of bytes requested by the variable itself, thus causing the value of the variable in the stack adjacent to it to be changed.

This problem is a specific buffer overflow vulnerability, similar to heap overflow, bss segment overflow and other overflow methods. A stack overflow vulnerability can cause a program to crash, and in addition, an attacker can control the execution flow of the program. In addition, we are not difficult to find that the basic premise of stack overflow is. The most typical stack overflow exploit is to cover the return address of the program to the address controlled by the attacker.

Of course, you need to ensure that the segment where the address is located has executable permissions. Below, we give a simple example:. The main purpose of this program is to read a string and output it. We want to control the program to execute the success function. It can be seen that gets itself is a dangerous function.

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It never checks the length of the input string, but uses Enter to determine if the input is over, so it can easily cause the stack to overflow.

In the gcc compiler directive, -m32 refers to the generation of a bit program; -fno-stack-protector refers to the stack overflow protection not being turned on, that is, no canary is generated. In addition, in order to introduce the basic use of stack overflow more conveniently, it is also necessary to disable PIE Position Independent Executable to avoid the disruption of the load base address. Different gcc versions have different default configurations for PIE.

We can use the command gcc -v to view the default gcc switch status. If the --enable-default-pie parameter is present, it means that the PIE is enabled by default. You need to add the parameter -no-pie to the compile directive. Otherwise, the program will still load a fixed base address but not the base address of No PIE.

The specific options are. Similarly, you can configure the corresponding parameters. After confirming that the stack overflow and PIE protection are turned off, we use IDA to decompile the binary and view the vulnerable function. However, it should be noted that since in the computer's memory, each value is stored in bytes. In general, small-end storage is used, that is, the form of 0xB in memory is.

However, we can't input these characters directly in the terminal. So at this point we need to use a wave of pwntools about how to install and basic usage, please githubhere the code using pwntools is as follows:. By looking for dangerous functions, we quickly determine if the program is likely to have a stack overflow and, if so, where the stack overflows.

Common dangerous functions are as follows. A common method of operation is to turn on IDA and calculate the offset based on its given address. General variables will have the following index modes. CTF Wiki. In addition, we are not difficult to find that the basic premise of stack overflow is The program must write data to the stack. The size of the data written is not well controlled.Heap overflow means that the number of bytes written by the program into a heap block exceeds the number of bytes that can be used by the heap itself is the number of bytes that can be used instead of the number of bytes requested by the user, because the heap manager The number of bytes requested by the user is adjusted, which also causes the number of available bytes to be no less than the number of bytes requested by the userthus causing data overflow and covering to physically adjacent high The next heap of addresses.

For the attacker, the heap overflow vulnerability can make the program crash, and the attacker can control the execution flow of the program. A heap overflow is a specific buffer overflow and stack overflow, bss segment overflow, etc. However, unlike stack overflow, there is no return address on the heap that allows the attacker to directly control the execution flow, so we generally cannot control EIP directly through heap overflow.

In general, our strategy for using heap overflow is. The main purpose of this program is to call malloc to allocate memory on a heap, and then write a string to the heap. If the input string is too long, it will cause the area of the chunk to overflow and overwrite the top chunk.

In fact, puts internally calls malloc to allocate heap memory, which may not be covered by top chunk. Usually the heap is allocated by calling the glibc function malloc, which in some cases uses the calloc assignment.

The difference between calloc and malloc is that calloc is automatically emptied after allocation, which is fatal for the exploitation of certain information disclosure vulnerabilities. In addition to this, there is another type of allocation via realloc, which can function as both malloc and free. The operation of realloc is not as simple as it is literally, and its internal operations will be different depending on different situations. By looking for dangerous functions, we quickly determine if the program is likely to have a heap overflow and, if so, where the heap overflows.

A common misconception is that the malloc parameter is equal to the actual allocated heap size, but in fact the size allocated by ptmalloc is aligned. This length is typically twice the word length, such as a bit system with 8 bytes and a bit system with 16 bytes. However, for requests that are no longer than 2 times the word length, malloc will directly return the block of 2 times the word length, which is the smallest chunk.

For example, a bit system executing malloc 0 will return a block with a user area of 16 bytes.

NULLCON CTF Vuln3 - Heap Buffer Overflow

Go back and look at the previous sample code. Looking at the above code, the chunk size we applied for is 24 bytes. But when we compile it into a bit executable, the actual allocated memory will be 16 bytes instead of How does the byte space fit the next 24 bytes of content? Let's take a look at the conversion between the size of the memory requested by the user and the amount of memory actually allocated in glibc.

heap overflow ctf

And remove the 16 bytes of the chunk header. In fact, the number of bytes available to the user is Actually, ptmalloc allocates memory in double words as the basic unit.

heap overflow ctf

Taking bit system as an example, the allocated space is an integer multiple of 16, that is, the chunks applied by the user are byte aligned. CTF Wiki. It is not difficult to find that the basic premise of a heap overflow vulnerability is The program writes data to the heap.

The size of the data written is not well controlled. In general, our strategy for using heap overflow is Overwrite the contents of the next chunk physically adjacent to its.GitHub is home to over 50 million developers working together to host and review code, manage projects, and build software together. Work fast with our official CLI.

Learn more. If nothing happens, download GitHub Desktop and try again. If nothing happens, download Xcode and try again. If nothing happens, download the GitHub extension for Visual Studio and try again. This repo is for learning various heap exploitation techniques. We came up with the idea during a hack meeting, and have implemented the following techniques:. Consequently, these checks regularly break some of the techniques and require adjustments to bypass them if possible.

We address this issue by keeping multiple versions of the same technique for each Glibc-release that required an adjustment. Have a good example? Add it here! Try to inline the whole technique in a single. More info: mcheckmallopt. We use optional third-party analytics cookies to understand how you use GitHub.

You can always update your selection by clicking Cookie Preferences at the bottom of the page. For more information, see our Privacy Statement. We use essential cookies to perform essential website functions, e. We use analytics cookies to understand how you use our websites so we can make them better, e. Skip to content. A repository for learning various heap exploitation techniques. MIT License. Dismiss Join GitHub today GitHub is home to over 50 million developers working together to host and review code, manage projects, and build software together.

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Educational Heap Exploitation This repo is for learning various heap exploitation techniques. Heap Exploitation Tools There are some heap exploitation tools floating around.Much like a stack buffer overflowa heap overflow is a vulnerability where more data than can fit in the allocated buffer is read in. This could lead to heap metadata corruption, or corruption of other heap objects, which could in turn provide new attack surface.

Once free is called on an allocation, the allocator is free to re-allocate that chunk of memory in future calls to malloc if it so chooses. However if the program author isn't careful and uses the freed object later on, the contents may be corrupt or even attacker controlled.

This is called a use after free or UAF. In this example, we have a string structure with a length and a pointer to the actual string data.

We properly allocate, fill, and then free an instance of this structure. Then we make another allocation, fill it, and then improperly reference the freed string. This could be used to leak program data. Not only can the heap be exploited by the data in allocations, but exploits can also use the underlying mechanisms in mallocfreeetc.

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One hundred 14-yr-old children were observed over 1 yr to find out if caries incidence and caries progression could be predicted in a low prevalence child population by means of well-known caries related factors. The mean caries incidence was low (0. In only eight out of 35 children progressing lesions were demonstrated. Independent variables at baseline examination were caries prevalence, sucrose intake, fluoride exposure, oral hygiene, saliva secretion rate, and salivary concentrations of mutans streptococci and lactobacilli.

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