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36 lines
1.7 KiB
Markdown
36 lines
1.7 KiB
Markdown
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# Hash Length Extension Attack
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## Summary of the attack
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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:
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* **The length of the secret** \(this can be also bruteforced from a given length range\)
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* **The clear text data**
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* **The algorithm \(and it's vulnerable to this attack\)**
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* **The padding is known**
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* Usually a default one is used, so if the other 3 requirements are met, this also is
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* The padding vary depending on the length of the secret+data, that's why the length of the secret is needed
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Then, it's possible for an **attacker** to **append** **data** and **generate** a valid **signature** for the **previos data + appended data**.
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### How?
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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**.
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Then, imagine that the secret is "secret" and the data is "data", the MD5 of "secretdata" is 6036708eba0d11f6ef52ad44e8b74d5b.
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If an attacker wants to append the string "append" he can:
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* Generate a MD5 of 64 "A"s
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* Change the state of the previously initialized hash to 6036708eba0d11f6ef52ad44e8b74d5b
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* Append the string "append"
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* Finish the hash and the resulting hash will be a **valid one for "secret" + "data" + "padding" + "append"**
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### **Tool**
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{% embed url="https://github.com/iagox86/hash\_extender" %}
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## References
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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)
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