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87 lines
5.2 KiB
Markdown
87 lines
5.2 KiB
Markdown
# Large Bin Attack
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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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## Basic Information
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For more information about what is a large bin check this page:
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{% content-ref url="bins-and-memory-allocations.md" %}
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[bins-and-memory-allocations.md](bins-and-memory-allocations.md)
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{% endcontent-ref %}
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It's possible to find a great example in [**how2heap - large bin attack**](https://github.com/shellphish/how2heap/blob/master/glibc\_2.35/large\_bin\_attack.c).
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Basically here you can see how, in the latest "current" version of glibc (2.35), it's not checked: **`P->bk_nextsize`** allowing to modify an arbitrary address with the value of a large bin chunk if certain conditions are met.
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In that example you can find the following conditions:
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* A large chunk is allocated
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* A large chunk smaller than the first one but in the same index is allocated
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* Must be smalled so in the bin it must go first
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* (A chunk to prevent merging with the top chunk is created)
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* Then, the first large chunk is freed and a new chunk bigger than it is allocated -> Chunk1 goes to the large bin
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* Then, the second large chunk is freed
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* Now, the vulnerability: The attacker can modify `chunk1->bk_nextsize` to `[target-0x20]`
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* Then, a larger chunk than chunk 2 is allocated, so chunk2 is inserted in the large bin overwriting the address `chunk1->bk_nextsize->fd_nextsize` with the address of chunk2
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{% hint style="success" %}
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There are other potential scenarios, the thing is to add to the large bin a chunk that is **smaller** than a current X chunk in the bin, so it need to be inserted just before it in the bin, and we need to be able to modify X's **`bk_nextsize`** as thats where the address of the smaller chunk will be written to.
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{% endhint %}
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This is the relevant code from malloc. Comments have been added to understand better how the address was overwritten:
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{% code overflow="wrap" %}
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```c
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/* if smaller than smallest, bypass loop below */
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assert (chunk_main_arena (bck->bk));
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if ((unsigned long) (size) < (unsigned long) chunksize_nomask (bck->bk))
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{
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fwd = bck; // fwd = p1
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bck = bck->bk; // bck = p1->bk
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victim->fd_nextsize = fwd->fd; // p2->fd_nextsize = p1->fd (Note that p1->fd is p1 as it's the only chunk)
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victim->bk_nextsize = fwd->fd->bk_nextsize; // p2->bk_nextsize = p1->fd->bk_nextsize
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fwd->fd->bk_nextsize = victim->bk_nextsize->fd_nextsize = victim; // p1->fd->bk_nextsize->fd_nextsize = p2
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}
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```
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{% endcode %}
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This could be used to **overwrite the `global_max_fast` global variable** of libc to then exploit a fast bin attack with larger chunks.
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You can find another great explanation of this attack in [**guyinatuxedo**](https://guyinatuxedo.github.io/32-largebin\_attack/largebin\_explanation0/index.html).
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### Other examples
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* [**La casa de papel. HackOn CTF 2024**](https://7rocky.github.io/en/ctf/other/hackon-ctf/la-casa-de-papel/)
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* Large bin attack in the same situation as it appears in [**how2heap**](https://github.com/shellphish/how2heap/blob/master/glibc\_2.35/large\_bin\_attack.c).
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* The write primitive is more complex, because `global_max_fast` is useless here.
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* FSOP is needed to finish the exploit.
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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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