hacktricks/reversing-and-exploiting/linux-exploiting-basic-esp/common-binary-protections-and-bypasses/stack-canaries/README.md

# Stack Canaries

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## **StackGuard and StackShield**

**StackGuard** inserts a special value known as a **canary** before the **EIP (Extended Instruction Pointer)**, specifically `0x000aff0d` (representing null, newline, EOF, carriage return) to protect against buffer overflows. However, functions like `recv()`, `memcpy()`, `read()`, and `bcopy()` remain vulnerable, and it does not protect the **EBP (Base Pointer)**.

**StackShield** takes a more sophisticated approach than StackGuard by maintaining a **Global Return Stack**, which stores all return addresses (**EIPs**). This setup ensures that any overflow does not cause harm, as it allows for a comparison between stored and actual return addresses to detect overflow occurrences. Additionally, StackShield can check the return address against a boundary value to detect if the **EIP** points outside the expected data space. However, this protection can be circumvented through techniques like Return-to-libc, ROP (Return-Oriented Programming), or ret2ret, indicating that StackShield also does not protect local variables.

## **Stack Smash Protector (ProPolice) `-fstack-protector`:**

This mechanism places a **canary** before the **EBP**, and reorganizes local variables to position buffers at higher memory addresses, preventing them from overwriting other variables. It also securely copies arguments passed on the stack above local variables and uses these copies as arguments. However, it does not protect arrays with fewer than 8 elements or buffers within a user's structure.

The **canary** is a random number derived from `/dev/urandom` or a default value of `0xff0a0000`. It is stored in **TLS (Thread Local Storage)**, allowing shared memory spaces across threads to have thread-specific global or static variables. These variables are initially copied from the parent process, and child processes can alter their data without affecting the parent or siblings. Nevertheless, if a **`fork()` is used without creating a new canary, all processes (parent and children) share the same canary**, making it vulnerable. On the **i386** architecture, the canary is stored at `gs:0x14`, and on **x86\_64**, at `fs:0x28`.

This local protection identifies functions with buffers vulnerable to attacks and injects code at the start of these functions to place the canary, and at the end to verify its integrity.

When a web server uses `fork()`, it enables a brute-force attack to guess the canary byte by byte. However, using `execve()` after `fork()` overwrites the memory space, negating the attack. `vfork()` allows the child process to execute without duplication until it attempts to write, at which point a duplicate is created, offering a different approach to process creation and memory handling.

### Lengths

In `x64` binaries, the canary cookie is an **`0x8`** byte qword. The **first seven bytes are random** and the last byte is a **null byte.**

In `x86` binaries, the canary cookie is a **`0x4`** byte dword. The f**irst three bytes are random** and the last byte is a **null byte.**

{% hint style="danger" %}
The least significant byte of both canaries is a null byte because it'll be the first in the stack coming from lower addresses and therefore **functions that read strings will stop before reading it**.
{% endhint %}

## Bypasses

**Leaking the canary** and then overwriting it (e.g. buffer overflow) with its own value.

* If the **canary is forked in child processes** it might be possible to **brute-force** it one byte at a time:

{% content-ref url="bf-forked-stack-canaries.md" %}
[bf-forked-stack-canaries.md](bf-forked-stack-canaries.md)
{% endcontent-ref %}

* If there is some interesting **leak or arbitrary read vulnerability** in the binary it might be possible to leak it:

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

* **Overwriting stack stored pointers**

The stack vulnerable to a stack overflow might **contain addresses to strings or functions that can be overwritten** in order to exploit the vulnerability without needing to reach the stack canary. Check:

{% content-ref url="../../stack-overflow/pointer-redirecting.md" %}
[pointer-redirecting.md](../../stack-overflow/pointer-redirecting.md)
{% endcontent-ref %}

* **Modifying both master and thread canary**

A buffer overflow in a threaded function protected with canary can be used to modify the master canary of the thread. As a result, the mitigation is useless because the check is used with two canaries that are the same (although modified).

* **Modify the GOT entry of `__stack_chk_fail`**

If the binary has Partial RELRO, then you can use an arbitrary write to modify the GOT entry of `__stack_chk_fail` to be a dummy function that does not block the program if the canary gets modified.

## References

* [https://guyinatuxedo.github.io/7.1-mitigation\_canary/index.html](https://guyinatuxedo.github.io/7.1-mitigation\_canary/index.html)
* [http://7rocky.github.io/en/ctf/htb-challenges/pwn/robot-factory/#canaries-and-threads](http://7rocky.github.io/en/ctf/htb-challenges/pwn/robot-factory/#canaries-and-threads)
  * 64 bits, no PIE, nx, modify thread and master canary.
* [https://7rocky.github.io/en/ctf/other/securinets-ctf/scrambler/](https://7rocky.github.io/en/ctf/other/securinets-ctf/scrambler/)
  * 64 bits, no PIE, nx, write-what-where primitive. Modify GOT entry of `__stack_chk_fail`.

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<summary>Support HackTricks</summary>

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