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82 lines
5.1 KiB
Markdown
82 lines
5.1 KiB
Markdown
# Format Strings
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<details>
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<summary><strong>Learn AWS hacking from zero to hero with</strong> <a href="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>
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* Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
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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)
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* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
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* **Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** 🐦[**@carlospolopm**](https://twitter.com/hacktricks\_live)**.**
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* **Share your hacking tricks by submitting PRs to the** [**hacktricks repo**](https://github.com/carlospolop/hacktricks) **and** [**hacktricks-cloud repo**](https://github.com/carlospolop/hacktricks-cloud).
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</details>
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## Basic Information
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In C **`printf`** is function that can be used to **print** some string. The **first parameter** this function expects is the **raw text with the formatters**. The **following parameters** expected are the **values** to **substitute** the **formatters** from the raw text.
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The vulnerability appears when an **attacker text is put as the first argument** to this function. The attacker will be able to craft a **special input abusing** the **printf format** string capabilities to **write any data in any address**. Being able this way to **execute arbitrary code**.
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Formatters:
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```bash
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%08x —> 8 hex bytes
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%d —> Entire
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%u —> Unsigned
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%s —> String
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%n —> Number of written bytes
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%hn —> Occupies 2 bytes instead of 4
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<n>$X —> Direct access, Example: ("%3$d", var1, var2, var3) —> Access to var3
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```
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**`%n`** **writes** the **number of written bytes** in the **indicated address. Writing** as much **bytes** as the hex number we **need** to write is how you can **write any data**.
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```bash
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AAAA%.6000d%4\$n —> Write 6004 in the address indicated by the 4º param
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AAAA.%500\$08x —> Param at offset 500
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```
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### **Exploit Flow**
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As explained before this vulnerability allows to **write anything in any address (arbitrary write).**
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The goal is going to be to **overwrite** the **address** of a **function** in the **GOT** table that is going to be called later. Ideally we could set the **address to a shellcode** located in a executable section, but highly probable you won't be able to write a shellcode in a executable section.\
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So a different option is to **overwrite** a **function** that **receives** its **arguments** from the **user** and **point** it to the **`system`** **function**.
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To write the address, usually 2 steps are done: You **first writes 2Bytes** of the address and then the other 2. To do so **`$hn`** is used.
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**HOB** is called to the 2 higher bytes of the address\
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**LOB** is called to the 2 lower bytes of the address
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So, because of how format string works you need to **write first the smallest** of \[HOB, LOB] and then the other one.
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If HOB < LOB\
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`[address+2][address]%.[HOB-8]x%[offset]\$hn%.[LOB-HOB]x%[offset+1]`
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If HOB > LOB\
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`[address+2][address]%.[LOB-8]x%[offset+1]\$hn%.[HOB-LOB]x%[offset]`
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HOB LOB HOB\_shellcode-8 NºParam\_dir\_HOB LOB\_shell-HOB\_shell NºParam\_dir\_LOB
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\`python -c 'print "\x26\x97\x04\x08"+"\x24\x97\x04\x08"+ "%.49143x" + "%4$hn" + "%.15408x" + "%5$hn"'\`
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## Pwntools Template
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You can find a template to prepare a exploit for this kind of vulnerability in:
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{% content-ref url="format-strings-template.md" %}
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[format-strings-template.md](format-strings-template.md)
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{% endcontent-ref %}
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<details>
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<summary><strong>Learn AWS hacking from zero to hero with</strong> <a href="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>
|
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* Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
|
|
* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
|
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* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
|
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* **Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** 🐦[**@carlospolopm**](https://twitter.com/hacktricks\_live)**.**
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* **Share your hacking tricks by submitting PRs to the** [**hacktricks repo**](https://github.com/carlospolop/hacktricks) **and** [**hacktricks-cloud repo**](https://github.com/carlospolop/hacktricks-cloud).
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</details>
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