# Hash Length Extension Attack
{% hint style="success" %}
Learn & practice AWS Hacking:[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)\
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{% endhint %}
#### [WhiteIntel](https://whiteintel.io)
[**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="" %}
***
## 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="" %}
### References
You can find this attack good explained in
#### [WhiteIntel](https://whiteintel.io)
[**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="" %}
{% hint style="success" %}
Learn & practice AWS Hacking:[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)\
Learn & practice GCP Hacking: [**HackTricks Training GCP Red Team Expert (GRTE)**](https://training.hacktricks.xyz/courses/grte)
Support HackTricks
* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
* **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)**.**
* **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.
{% endhint %}
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