hacktricks/crypto-and-stego/hash-length-extension-attack.md

# Hash Length Extension Attack

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#### [WhiteIntel](https://whiteintel.io)

<figure><img src="../.gitbook/assets/image (1227).png" alt=""><figcaption></figcaption></figure>

[**WhiteIntel**](https://whiteintel.io) is a **dark-web** fueled search engine that offers **free** functionalities to check if a company or its customers have been **compromised** by **stealer malwares**.

Their primary goal of WhiteIntel is to combat account takeovers and ransomware attacks resulting from information-stealing malware.

You can check their website and try their engine for **free** at:

{% embed url="https://whiteintel.io" %}

***

## Summary of the attack

Imagine a server which is **signing** some **data** by **appending** a **secret** to some known clear text data and then hashing that data. If you know:

* **The length of the secret** (this can be also bruteforced from a given length range)
* **The clear text data**
* **The algorithm (and it's vulnerable to this attack)**
* **The padding is known**
  * Usually a default one is used, so if the other 3 requirements are met, this also is
  * The padding vary depending on the length of the secret+data, that's why the length of the secret is needed

Then, it's possible for an **attacker** to **append** **data** and **generate** a valid **signature** for the **previous data + appended data**.

### How?

Basically the vulnerable algorithms generate the hashes by firstly **hashing a block of data**, and then, **from** the **previously** created **hash** (state), they **add the next block of data** and **hash it**.

Then, imagine that the secret is "secret" and the data is "data", the MD5 of "secretdata" is 6036708eba0d11f6ef52ad44e8b74d5b.\
If an attacker wants to append the string "append" he can:

* Generate a MD5 of 64 "A"s
* Change the state of the previously initialized hash to 6036708eba0d11f6ef52ad44e8b74d5b
* Append the string "append"
* Finish the hash and the resulting hash will be a **valid one for "secret" + "data" + "padding" + "append"**

### **Tool**

{% embed url="https://github.com/iagox86/hash_extender" %}

### References

You can find this attack good explained in [https://blog.skullsecurity.org/2012/everything-you-need-to-know-about-hash-length-extension-attacks](https://blog.skullsecurity.org/2012/everything-you-need-to-know-about-hash-length-extension-attacks)

#### [WhiteIntel](https://whiteintel.io)

<figure><img src="../.gitbook/assets/image (1227).png" alt=""><figcaption></figcaption></figure>

[**WhiteIntel**](https://whiteintel.io) is a **dark-web** fueled search engine that offers **free** functionalities to check if a company or its customers have been **compromised** by **stealer malwares**.

Their primary goal of WhiteIntel is to combat account takeovers and ransomware attacks resulting from information-stealing malware.

You can check their website and try their engine for **free** at:

{% embed url="https://whiteintel.io" %}

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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)

<details>

<summary>Support HackTricks</summary>

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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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