> For the complete documentation index, see [llms.txt](https://angelica.gitbook.io/hacktricks/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://angelica.gitbook.io/hacktricks/binary-exploitation/common-binary-protections-and-bypasses/stack-canaries/print-stack-canary.md).
# Print Stack Canary
{% hint style="success" %}
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{% endhint %}
## Enlarge printed stack
Imagine a situation where a **program vulnerable** to stack overflow can execute a **puts** function **pointing** to **part** of the **stack overflow**. The attacker knows that the **first byte of the canary is a null byte** (`\x00`) and the rest of the canary are **random** bytes. Then, the attacker may create an overflow that **overwrites the stack until just the first byte of the canary**.
Then, the attacker **calls the puts functionalit**y on the middle of the payload which will **print all the canary** (except from the first null byte).
With this info the attacker can **craft and send a new attack** knowing the canary (in the same program session).
Obviously, this tactic is very **restricted** as the attacker needs to be able to **print** the **content** of his **payload** to **exfiltrate** the **canary** and then be able to create a new payload (in the **same program session**) and **send** the **real buffer overflow**.
**CTF examples:**
* [**https://guyinatuxedo.github.io/08-bof\_dynamic/csawquals17\_svc/index.html**](https://guyinatuxedo.github.io/08-bof_dynamic/csawquals17_svc/index.html)
* 64 bit, ASLR enabled but no PIE, the first step is to fill an overflow until the byte 0x00 of the canary to then call puts and leak it. With the canary a ROP gadget is created to call puts to leak the address of puts from the GOT and the a ROP gadget to call `system('/bin/sh')`
* [**https://guyinatuxedo.github.io/14-ret\_2\_system/hxp18\_poorCanary/index.html**](https://guyinatuxedo.github.io/14-ret_2_system/hxp18_poorCanary/index.html)
* 32 bit, ARM, no relro, canary, nx, no pie. Overflow with a call to puts on it to leak the canary + ret2lib calling `system` with a ROP chain to pop r0 (arg `/bin/sh`) and pc (address of system)
## Arbitrary Read
With an **arbitrary read** like the one provided by format **strings** it might be possible to leak the canary. Check this example: [**https://ir0nstone.gitbook.io/notes/types/stack/canaries**](https://ir0nstone.gitbook.io/notes/types/stack/canaries) and you can read about abusing format strings to read arbitrary memory addresses in:
{% content-ref url="/pages/79TxZOXZKLGu7HXHHjFU" %}
[Format Strings](/hacktricks/binary-exploitation/format-strings.md)
{% endcontent-ref %}
*
* This challenge abuses in a very simple way a format string to read the canary from the stack
{% hint style="success" %}
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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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