hacktricks/reversing-and-exploiting/linux-exploiting-basic-esp/stack-overflow/README.md

# Stack Overflow

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## What is a Stack Overflow

A **stack overflow** is a vulnerability that occurs when a program writes more data to the stack than it is allocated to hold. This excess data will **overwrite adjacent memory space**, leading to the corruption of valid data, control flow disruption, and potentially the execution of malicious code. This issue often arises due to the use of unsafe functions that do not perform bounds checking on input.

The main problem of this overwrite is that the **EIP** and **EBP** pointers to return to the previos function are **stored in the stack**. Therefore, an attacker will be able to overwrite those and **control the execution flow of the program**.

The vulnerability usually arises because a function **copies inside the stack more bytes than the amount allocated for it**, therefore being able to overwrite other parts of the stack.\
Some common functions vulnerable to this are: `strcpy`, `strcat`, `sprintf`, `gets`, `fgets`...

For example, the following functions could be vulnerable:

```c
void vulnerable() {
    char buffer[128];
    printf("Enter some text: ");
    gets(buffer); // This is where the vulnerability lies
    printf("You entered: %s\n", buffer);
}
```

### Finding Stack Overflows

The most common way to find stack overflows is to give a very big input of `A`s (e.g. `python3 -c 'print("A"*1000)'`) and expected a `Segmentation Fault` indicating that the **address `0x41414141` was tried to be accessed**.

Moreover, once you found that there is Stack Overflow vulnerability you will need to find the offset until it's possible to **overwrite the EIP pointer**, for this it's usually used a **De Bruijn sequence.** Which for a given alphabet of size _k_ and subsequences of length _n_ is a **cyclic sequence in which every possible subsequence of length **_**n**_** appears exactly once** as a contiguous subsequence.

This way, instead of needing to figure out which offset is overwriting the EIP by hand, it's possible to use as padding one of these sequences and then find the offset of the bytes that ended overwriting it.

It's possible to use **pwntools** for this:

```python
from pwn import *

# Generate a De Bruijn sequence of length 1000 with an alphabet size of 256 (byte values)
pattern = cyclic(1000)

# This is an example value that you'd have found in the EIP/IP register upon crash
eip_value = p32(0x6161616c)
offset = cyclic_find(eip_value)  # Finds the offset of the sequence in the De Bruijn pattern
print(f"The offset is: {offset}")
```

or **GEF**:

```bash
#Patterns
pattern create 200 #Generate length 200 pattern
pattern search "avaaawaa" #Search for the offset of that substring
pattern search $rsp #Search the offset given the content of $rsp
```

## Exploiting Stack Overflows

During an overflow (supposing the overflow size if big enough) you will be able to overwrite values of other variables inside the stack until reaching the EBP and EIP (or even more).\
The most common way to abuse this type of vulnerability is by **modifying the EIP pointer** so when the function ends the **control flow will be redirected wherever the user specified** in this pointer.

However, in other scenarios maybe just **overwriting some variables values in the stack** might be enough for the exploitation (like in easy CTF challenges).

### Ret2win

In this type of CTF challenges, there is a **function** **inside** the binary that is **never called** and that **you need to call in order to win**. For these challenges you just need to find the **offset to overwrite the EIP** and **find the address of the function** to call (usually [**ASLR**](../common-binary-protections/aslr.md) would be disabled) so when the vulnerable function returns, the hidden function will be called:

{% content-ref url="ret2win.md" %}
[ret2win.md](ret2win.md)
{% endcontent-ref %}

### Ret2Shellcode

## Types of protections

{% content-ref url="../common-binary-protections/" %}
[common-binary-protections](../common-binary-protections/)
{% endcontent-ref %}

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<summary><strong>Learn AWS hacking from zero to hero with</strong> <a href="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>

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* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks\_live)**.**
* **Share your hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.

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