Cracking JWT tokens: a tale of magic, Node.js and parallel computing - WebRebels Oslo, 5 June 2018

Learn how you can use some JavaScript/Node.js black magic to crack JWT tokens and impersonate other users or escalate privileges. Just add a pinch of ZeroMQ, a dose of parallel computing, a 4 leaf clover, mix everything applying some brute force and you'll get a powerful JWT cracking potion!

1. Cracking JWT tokensCracking JWT tokens
a tale of a tale of magicmagic, , Node.jsNode.js and and parallel computingparallel computing
Oslo - 5 JUN 2018
Luciano Mammino (Luciano Mammino ( ))@loige@loige
loige.link/jwt-crack-oslo
1

2. loige.link/jwt-crack-oslo
2

3. Luciano... who?Luciano... who?
Visit my castles:
-  (@loige)
-  (lmammino)
-
-  (loige.co)
Twitter
GitHub
Linkedin
Blog
Solution Architect at
with @mariocasciaro
with @andreaman87
with @ Podgeypoos79
3

4. Based on prior workBased on prior work
Chapters 10 & 11 in (book)
2-parts article on RisingStack:
   " "
Node.js design patterns
ZeroMQ & Node.js Tutorial - Cracking JWT Tokens
github.com/lmammino/jwt-cracker
github.com/lmammino/distributed-jwt-cracker
4

5. AgendaAgenda
What's JWTWhat's JWT
How it worksHow it works
Testing JWT tokensTesting JWT tokens
Brute-forcing a token!Brute-forcing a token!
5

6.   — RFC 7519— RFC 7519
 
is a compact, URL-safe means of representing claims to be
transferred between two parties. The claims in a JWT are
encoded as a JSON object that is used as the payload of a JSON
Web Signature (JWS) structure or as the plaintext of a JSON
Web Encryption (JWE) structure, enabling the claims to be
digitally signed or integrity protected with a Message
Authentication Code (MAC) and/or encrypted.
JSON Web Token (JWT)JSON Web Token (JWT)
6

7. eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJtZXeyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJtZX
NzYWdlIjoiaGVsbG8gcGVvcGxlIn0.II7XQbjvDCNzYWdlIjoiaGVsbG8gcGVvcGxlIn0.II7XQbjvDC
Hkt3UOh6weHY6tRcemT0gxRVmA6W6uZ8AHkt3UOh6weHY6tRcemT0gxRVmA6W6uZ8A
7

8. OKOK
Let's try to make itLet's try to make it
simpler...simpler...
8

9. JWT is...
An URL safe, stateless protocol
for transferring claims
9

10. 10

11. URL safe?
10

12. URL safe?
stateless?
10

13. URL safe?
stateless?
claims?
10

14. URL Safe...URL Safe...
It's a string that can be safely used as part of a URLIt's a string that can be safely used as part of a URL
(it doesn't contain URL separators like "(it doesn't contain URL separators like "==", "", "//", "", "##" or "" or "??")")
unicorntube.pl/?token= eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
11

15. Stateless?Stateless?
Token validity can be verified without having to interrogate aToken validity can be verified without having to interrogate a
third-party servicethird-party service
(Sometimes also defined as "self-contained")
12

16. What is a claim?What is a claim?
13

17. some information to transfersome information to transfer
identity identity (login session)(login session)
authorisation to perform actions authorisation to perform actions (api key)(api key)
ownership ownership (a ticket belongs to somebody)(a ticket belongs to somebody)
14

18. also...also...
validity constraintsvalidity constraints
token time constraints token time constraints (dont' use before/after)(dont' use before/after)
audience audience (a ticket only for a specific concert)(a ticket only for a specific concert)
issuer identity issuer identity (a ticket issued by a specific reseller)(a ticket issued by a specific reseller)
15

19. also...also...
protocol informationprotocol information
Type of tokenType of token
AlgorithmAlgorithm
16

20. In generalIn general
All the bits of information transferred with the tokenAll the bits of information transferred with the token
17

21. eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJtZXeyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJtZX
NzYWdlIjoiaGVsbG8gcGVvcGxlIn0.II7XQbjvDCNzYWdlIjoiaGVsbG8gcGVvcGxlIn0.II7XQbjvDC
Hkt3UOh6weHY6tRcemT0gxRVmA6W6uZ8AHkt3UOh6weHY6tRcemT0gxRVmA6W6uZ8A
18

22. eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..eyJtZXeyJtZX
NzYWdlIjoiaGVsbG8gcGVvcGxlIn0NzYWdlIjoiaGVsbG8gcGVvcGxlIn0..II7XQbjvDCII7XQbjvDC
Hkt3UOh6weHY6tRcemT0gxRVmA6W6uZ8AHkt3UOh6weHY6tRcemT0gxRVmA6W6uZ8A
3 parts3 parts
separated by "."separated by "."
19

23. 20

24. HEADERHEADER::
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpeyJhbGciOiJIUzI1NiIsInR5cCI6Ikp
XVCJ9XVCJ9
20

25. HEADERHEADER::
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpeyJhbGciOiJIUzI1NiIsInR5cCI6Ikp
XVCJ9XVCJ9
PAYLOADPAYLOAD::
eyJtZXNzYWdlIjoiaGVsbG8gcGVvceyJtZXNzYWdlIjoiaGVsbG8gcGVvc
GxlIn0GxlIn0
20

26. HEADERHEADER::
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpeyJhbGciOiJIUzI1NiIsInR5cCI6Ikp
XVCJ9XVCJ9
PAYLOADPAYLOAD::
eyJtZXNzYWdlIjoiaGVsbG8gcGVvceyJtZXNzYWdlIjoiaGVsbG8gcGVvc
GxlIn0GxlIn0
SIGNATURESIGNATURE::
II7XQbjvDCHkt3UOh6weHY6tRcemII7XQbjvDCHkt3UOh6weHY6tRcem
T0gxRVmA6W6uZ8AT0gxRVmA6W6uZ8A
20

27. HeaderHeader and and PayloadPayload are are
encoded encoded
let's decode them!let's decode them!
Base64UrlBase64Url
21

28. 22

29. HEADERHEADER::
22

30. HEADERHEADER::
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9
22

31. HEADERHEADER::
{"alg":"HS256","typ":"JWT"}{"alg":"HS256","typ":"JWT"}
22

32. HEADERHEADER::
PAYLOADPAYLOAD::
{"alg":"HS256","typ":"JWT"}{"alg":"HS256","typ":"JWT"}
22

33. HEADERHEADER::
PAYLOADPAYLOAD::
{"alg":"HS256","typ":"JWT"}{"alg":"HS256","typ":"JWT"}
eyJtZXNzYWdlIjoiaGVsbG8gcGVvceyJtZXNzYWdlIjoiaGVsbG8gcGVvc
GxlIn0GxlIn0
22

34. HEADERHEADER::
PAYLOADPAYLOAD::
{"alg":"HS256","typ":"JWT"}{"alg":"HS256","typ":"JWT"}
{"message":"hello people"}{"message":"hello people"}
22

35. HEADERHEADER::
The decoded info is JSON!The decoded info is JSON!
PAYLOADPAYLOAD::
{"alg":"HS256","typ":"JWT"}{"alg":"HS256","typ":"JWT"}
{"message":"hello people"}{"message":"hello people"}
22

36. HEADERHEADER::
{"alg":"HS256","typ":"JWT"}{"alg":"HS256","typ":"JWT"}
alg: alg: the kind of algorithm usedthe kind of algorithm used
"HS256" "HS256" HMACSHA256 Signature HMACSHA256 Signature (secret based hashing)(secret based hashing)
""RS256RS256" RSASHA256 Signature " RSASHA256 Signature (public/private key hashing)(public/private key hashing)
""nonenone" NO SIGNATURE!" NO SIGNATURE! (This is " (This is " ")")infamousinfamous
23

37. PAYLOADPAYLOAD::
{"message":"hello people"}{"message":"hello people"}
  
Payload can be anything thatPayload can be anything that
you can express in JSONyou can express in JSON
24

38. PAYLOADPAYLOAD::
"registered" (or standard) claims:"registered" (or standard) claims:
iss: issuer ID ("auth0")
sub: subject ID ("[email protected]")
aud: audience ID ("https://someapp.com")
exp: expiration time ("1510047437793")
nbf: not before ("1510046471284")
iat: issue time ("1510045471284")
25

39. PAYLOADPAYLOAD::
"registered" (or standard) claims:"registered" (or standard) claims:
{ 
  "iss": "auth0", 
  "sub": "[email protected]",  
  "aud": "https://someapp.com",  
  "exp": "1510047437793", 
  "nbf": "1510046471284", 
  "iat": "1510045471284" 
}
26

40. So far it's just metadata...So far it's just metadata...
What makes it safe?What makes it safe?
27

41. SIGNATURESIGNATURE::
II7XQbjvDCHkt3UOh6weHY6II7XQbjvDCHkt3UOh6weHY6
tRcemT0gxRVmA6W6uZ8AtRcemT0gxRVmA6W6uZ8A
  
A A Base64URLBase64URL encoded encoded cryptographiccryptographic
signaturesignature of the header and the payload of the header and the payload
28

42. With HS256With HS256
signature = HMACSHA256( 
  base64UrlEncode(header) + "." + 
    base64UrlEncode(payload), 
  secret 
)
header payload secret SIGNATURE+ + =
29

43. If a system knows the If a system knows the secretsecret
It can verify the authenticityIt can verify the authenticity
of the tokenof the token
With HS256With HS256
30

44. Let's create a token from scratchLet's create a token from scratch
runkit.com/lmammino/create-jwt-token
31

45. Playground for JWTPlayground for JWT
JWT.ioJWT.io
32

46. An exampleAn example
Session token
33

47. Classic implementationClassic implementation
cookie/session basedcookie/session based
34

48. 35

49.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
35
 
 
 
 
 
 
Sessions
Database
 
 
 
 
 
 

50.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
user:"luciano" 
pass:"mariobros"
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
35
 
 
 
 
 
 
Sessions
Database
 
 
 
 
 
 

51.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
2. generate session
id:"Y4sHySEPWAjc" 
user:"luciano"
user:"luciano" 
pass:"mariobros"
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
35
 
 
 
 
 
 
Sessions
Database
 
 
 
 
 
 
id:"Y4sHySEPWAjc" 
user:"luciano"

52.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
2. generate session
id:"Y4sHySEPWAjc" 
user:"luciano"
user:"luciano" 
pass:"mariobros"
3. session cookie
SID:"Y4sHySEPWAjc" 
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
35
 
 
 
 
 
 
Sessions
Database
 
 
 
 
 
 
id:"Y4sHySEPWAjc" 
user:"luciano"

53.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
2. generate session
id:"Y4sHySEPWAjc" 
user:"luciano"
user:"luciano" 
pass:"mariobros"
3. session cookie
SID:"Y4sHySEPWAjc" 
4. GET /profile
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
35
 
 
 
 
 
 
Sessions
Database
 
 
 
 
 
 
id:"Y4sHySEPWAjc" 
user:"luciano"SID:"Y4sHySEPWAjc" 

54.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
2. generate session
id:"Y4sHySEPWAjc" 
user:"luciano"
user:"luciano" 
pass:"mariobros"
3. session cookie
SID:"Y4sHySEPWAjc" 
4. GET /profile
5. query
id:"Y4sHySEPWAjc" 
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
35
 
 
 
 
 
 
Sessions
Database
 
 
 
 
 
 
id:"Y4sHySEPWAjc" 
user:"luciano"SID:"Y4sHySEPWAjc" 

55.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
2. generate session
id:"Y4sHySEPWAjc" 
user:"luciano"
user:"luciano" 
pass:"mariobros"
3. session cookie
SID:"Y4sHySEPWAjc" 
4. GET /profile
5. query
id:"Y4sHySEPWAjc" 
6. record
id:"Y4sHySEPWAjc" 
user:"luciano"
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
35
 
 
 
 
 
 
Sessions
Database
 
 
 
 
 
 
id:"Y4sHySEPWAjc" 
user:"luciano"SID:"Y4sHySEPWAjc" 

56.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
2. generate session
id:"Y4sHySEPWAjc" 
user:"luciano"
user:"luciano" 
pass:"mariobros"
3. session cookie
SID:"Y4sHySEPWAjc" 
4. GET /profile
5. query
id:"Y4sHySEPWAjc" 
6. record
id:"Y4sHySEPWAjc" 
user:"luciano"
7. (page)
<h1>hello luciano</h1>
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
35
 
 
 
 
 
 
Sessions
Database
 
 
 
 
 
 
id:"Y4sHySEPWAjc" 
user:"luciano"SID:"Y4sHySEPWAjc" 

57. JWT implementationJWT implementation
36

58. 37

59.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
37

60.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
user:"luciano" 
pass:"mariobros"
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
37

61.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
user:"luciano" 
pass:"mariobros"
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
Create Token for "luciano"
Add signature
2. create
JWT
37

62.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
3. JWT Token
{"sub":"luciano"}
user:"luciano" 
pass:"mariobros"
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJz
dWIiOiJsdWNpYW5vIn0.V92iQaqMrBUhkgEAyRaCY
7pezgH­Kls85DY8wHnFrk4
Create Token for "luciano"
Add signature
2. create
JWT
37

63.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
3. JWT Token
{"sub":"luciano"}
user:"luciano" 
pass:"mariobros"
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJz
dWIiOiJsdWNpYW5vIn0.V92iQaqMrBUhkgEAyRaCY
7pezgH­Kls85DY8wHnFrk4
4. GET /profile
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJz
dWIiOiJsdWNpYW5vIn0.V92iQaqMrBUhkgEAyRaCY
7pezgH­Kls85DY8wHnFrk4
Create Token for "luciano"
Add signature
2. create
JWT
37

64.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
3. JWT Token
{"sub":"luciano"}
user:"luciano" 
pass:"mariobros"
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJz
dWIiOiJsdWNpYW5vIn0.V92iQaqMrBUhkgEAyRaCY
7pezgH­Kls85DY8wHnFrk4
4. GET /profile
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJz
dWIiOiJsdWNpYW5vIn0.V92iQaqMrBUhkgEAyRaCY
7pezgH­Kls85DY8wHnFrk4
Token says this is "luciano"
Signature looks OK
5. verify
Create Token for "luciano"
Add signature
2. create
JWT
37

65.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
3. JWT Token
{"sub":"luciano"}
user:"luciano" 
pass:"mariobros"
6. (page)
<h1>hello luciano</h1>
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJz
dWIiOiJsdWNpYW5vIn0.V92iQaqMrBUhkgEAyRaCY
7pezgH­Kls85DY8wHnFrk4
4. GET /profile
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJz
dWIiOiJsdWNpYW5vIn0.V92iQaqMrBUhkgEAyRaCY
7pezgH­Kls85DY8wHnFrk4
Token says this is "luciano"
Signature looks OK
5. verify
Create Token for "luciano"
Add signature
2. create
JWT
37

66.  
 
 
 
 
 
Browser
 
 
 
 
 
 
 
1. POST /login
3. JWT Token
{"sub":"luciano"}
user:"luciano" 
pass:"mariobros"
6. (page)
<h1>hello luciano</h1>
 
 
 
 
 
 
Server
 
 
 
 
 
 
 
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJz
dWIiOiJsdWNpYW5vIn0.V92iQaqMrBUhkgEAyRaCY
7pezgH­Kls85DY8wHnFrk4
4. GET /profile
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJz
dWIiOiJsdWNpYW5vIn0.V92iQaqMrBUhkgEAyRaCY
7pezgH­Kls85DY8wHnFrk4
Token says this is "luciano"
Signature looks OK
5. verify
Create Token for "luciano"
Add signature
2. create
JWT
Note: Only the server
knows the secret
37

67. Cookie/sessionCookie/session
Needs a database to store the
session data
The database is queried for every
request to fetch the session
A session is identified only by a
randomly generated string
(session ID)
No data attached
Sessions can be invalidated at any
moment
JWTJWT
Doesn't need a session database
The session data is embedded in
the token
For every request the token
signature is verified
Attached metadata is readable
Sessions can't be invalidated, but
tokens might have an expiry flag
VSVS
38

68. Another great JWT use caseAnother great JWT use case
Creating Secure Password Reset LinksCreating Secure Password Reset Links
loige.link/jwt-pwd-reset
39

69. JWT LOOKS GREAT!JWT LOOKS GREAT!
But there are pitfalls...But there are pitfalls...
40

70. Data is public!Data is public!
41

71. Data is public!Data is public!
If you have a token,If you have a token,
you can easily read the claims!you can easily read the claims!
41

72. Data is public!Data is public!
If you have a token,If you have a token,
you can easily read the claims!you can easily read the claims!
You only have to Base64Url-decode the
token header and payload
and you have a readable JSON
41

73. There's no token database...There's no token database...
  
...if I can forge a token...if I can forge a token
nobody will know it's notnobody will know it's not
authentic!authentic!
42

74. DEMODEMO
JWT based web app
github.com/lmammino/sample-jwt-webapp
BUILT WITH
43

75. Given an HS256 signed JWTGiven an HS256 signed JWT
We can try to "We can try to "guessguess" the " the secretsecret!!
44

76. How difficult can it be?How difficult can it be?
45

77. Let's build a distributedLet's build a distributed
JWT token cracker!JWT token cracker!
 
npm.im/distributed-jwt-crackernpm.im/distributed-jwt-cracker
46

78. The idea...The idea...
47

79. The idea...The idea...
Take a valid JWT tokenTake a valid JWT token
47

80. The idea...The idea...
try to "guess" the secret and validate the token against ittry to "guess" the secret and validate the token against it
Take a valid JWT tokenTake a valid JWT token
47

81. The idea...The idea...
if the token is validated, then you found the if the token is validated, then you found the secretsecret!!
try to "guess" the secret and validate the token against ittry to "guess" the secret and validate the token against it
Take a valid JWT tokenTake a valid JWT token
47

82. The idea...The idea...
YOU CAN NOW YOU CAN NOW CREATE AND SIGNCREATE AND SIGN
ANY JWT TOKEN ANY JWT TOKEN FOR THISFOR THIS
APPLICATIONAPPLICATION!!
if the token is validated, then you found the if the token is validated, then you found the secretsecret!!
try to "guess" the secret and validate the token against ittry to "guess" the secret and validate the token against it
Take a valid JWT tokenTake a valid JWT token
47

83. Magic weaponsMagic weapons
48

84. Magic weaponsMagic weapons
Node.jsNode.js
48

85. Magic weaponsMagic weapons
Node.jsNode.jsZeroMQZeroMQ
48

86. Magic weaponsMagic weapons
Node.jsNode.js
modulemodule
jsonwebtokenjsonwebtoken
ZeroMQZeroMQ
48

87. ZeroMQZeroMQ
an open source embeddable an open source embeddable networkingnetworking
librarylibrary and a and a concurrency frameworkconcurrency framework
49

88. The brute force problemThe brute force problem
"virtually infinite" solutions space"virtually infinite" solutions space
all the strings (of any length) that can be generated within a given alphabet
(empty string), a, b, c, 1, aa, ab, ac, a1, ba, bb, bc, b1, ca, cb, cc, c1, 1a, 1b, 1c, 11, aaa,
aab, aac, aa1, aba, ...
50

89. bijection (int) ⇒ (string)bijection (int) ⇒ (string)
if we sort all the possible strings over an alphabet
 
Alphabet = [a,b]0 ⟶ (empty string) 
1 ⟶ a 
2 ⟶ b 
3 ⟶ aa 
4 ⟶ ab 
5 ⟶ ba 
6 ⟶ bb
7 ⟶ aaa
8 ⟶ aab
9 ⟶ aba
10 ⟶ abb 
11 ⟶ baa 
12 ⟶ bab 
13 ⟶ bba 
14 ⟶ bbb 
15 ⟶ aaaa 
16 ⟶ aaab
17 ⟶ aaba
18 ⟶ aabb
...
51

90. ArchitectureArchitecture
ServerServer ClientClient
Initialised with a valid JWT token
and an alphabet
coordinates the brute force
attempts among connected clients
knows how to verify a token against
a given secret
receives ranges of secrets to check
52

91. Networking patternsNetworking patterns
Router channels:
dispatch jobs
receive results
 
Pub/Sub channel:
termination
signal
53

92. Server stateServer state
the solution space can be sliced intothe solution space can be sliced into
chunkschunks of fixed length (batch size) of fixed length (batch size)
54

93. Server stateServer state
the solution space can be sliced intothe solution space can be sliced into
chunkschunks of fixed length (batch size) of fixed length (batch size)
0 3 6 9...
54

94. Server stateServer state
the solution space can be sliced intothe solution space can be sliced into
chunkschunks of fixed length (batch size) of fixed length (batch size)
0
batch 1
3 6 9...
54

95. Server stateServer state
the solution space can be sliced intothe solution space can be sliced into
chunkschunks of fixed length (batch size) of fixed length (batch size)
0
batch 1 batch 2
3 6 9...
54

96. Server stateServer state
the solution space can be sliced intothe solution space can be sliced into
chunkschunks of fixed length (batch size) of fixed length (batch size)
0
batch 1 batch 2 batch 3
3 6 9...
54

97. Server stateServer state
the solution space can be sliced intothe solution space can be sliced into
chunkschunks of fixed length (batch size) of fixed length (batch size)
0
...batch 1 batch 2 batch 3
3 6 9...
54

98. Initial server stateInitial server state
{ 
  "cursor": 0, 
  "clients": {} 
}
55

99. The first client connectsThe first client connects
{ 
  "cursor": 3, 
  "clients": { 
    "client1": [0,2] 
  } 
}
[0,2]
56

100. { 
  "cursor": 9, 
  "clients": { 
    "client1": [0,2],     
    "client2": [3,5],
    "client3": [6,8] 
  } 
}
Other clients connectOther clients connect
[0,2]
[3,5][6,8]
57

101. Client 2 finishes its jobClient 2 finishes its job
{ 
  "cursor": 12, 
  "clients": { 
    "client1": [0,2],     
    "client2": [9,11],
    "client3": [6,8] 
  } 
}
[0,2]
[9,11][6,8]
58

102. let cursor = 0
const clients = new Map()

const assignNextBatch = client => {
const from = cursor
const to = cursor + batchSize - 1
const batch = [from, to]
cursor = cursor + batchSize
client.currentBatch = batch
client.currentBatchStartedAt = new Date()

return batch
}

const addClient = channel => {
const id = channel.toString('hex')
const client = {id, channel, joinedAt: new Date()}
assignNextBatch(client)
clients.set(id, client)

return client
}
Server
59

103. Messages flowMessages flow
60

104. Messages flowMessages flow
 
 
 
JWT Cracker
Server
 
 
 
 
 
 
 
JWT Cracker
Client
 
 
 
 
60

105. Messages flowMessages flow
 
 
 
JWT Cracker
Server
 
 
 
 
 
 
 
JWT Cracker
Client
 
 
 
 
1. JOIN
60

106. Messages flowMessages flow
 
 
 
JWT Cracker
Server
 
 
 
 
 
 
 
JWT Cracker
Client
 
 
 
 
1. JOIN
2. START
{token, alphabet, firstBatch}
60

107. Messages flowMessages flow
 
 
 
JWT Cracker
Server
 
 
 
 
 
 
 
JWT Cracker
Client
 
 
 
 
1. JOIN
2. START
{token, alphabet, firstBatch}
3. NEXT
60

108. Messages flowMessages flow
 
 
 
JWT Cracker
Server
 
 
 
 
 
 
 
JWT Cracker
Client
 
 
 
 
1. JOIN
2. START
{token, alphabet, firstBatch}
3. NEXT
4. BATCH
{nextBatch}
60

109. Messages flowMessages flow
 
 
 
JWT Cracker
Server
 
 
 
 
 
 
 
JWT Cracker
Client
 
 
 
 
1. JOIN
2. START
{token, alphabet, firstBatch}
3. NEXT
4. BATCH
{nextBatch}
5. SUCCESS
{secret}
60

110. const router = (channel, rawMessage) => {
const msg = JSON.parse(rawMessage.toString())

switch (msg.type) {
case 'join': {
const client = addClient(channel)
const response = {
type: 'start',
id: client.id,
batch: client.currentBatch,
alphabet,
token
}
batchSocket.send([channel, JSON.stringify(response)])
break
}

case 'next': {
const batch = assignNextBatch(clients.get(channel.toString('hex')))
batchSocket.send([channel, JSON.stringify({type: 'batch', batch})])
break
}

case 'success': {
const secret = msg.secret
// publish exit signal and closes the app
signalSocket.send(['exit', JSON.stringify({secret, client: channel.toString('hex')})], 0, () => {
batchSocket.close()
signalSocket.close()
exit(0)
})

break
}
}
}
Server
61

111. let id, variations, token

const dealer = rawMessage => {
const msg = JSON.parse(rawMessage.toString())

const start = msg => {
id = msg.id
variations = generator(msg.alphabet)
token = msg.token
}

const batch = msg => {
processBatch(token, variations, msg.batch, (secret, index) => {
if (typeof secret === 'undefined') {
// request next batch
batchSocket.send(JSON.stringify({type: 'next'}))
} else {
// propagate success
batchSocket.send(JSON.stringify({type: 'success', secret, index}))
exit(0)
}
})
}

switch (msg.type) {
case 'start':
start(msg)
batch(msg)
break

case 'batch':
batch(msg)
break
}
}
Client
62

112. How a chunk is processedHow a chunk is processed
Given chunk [3,6] over alphabet "ab"
[3,6] ⇒
3 ⟶ aa 
4 ⟶ ab 
5 ⟶ ba 
6 ⟶ bb
⇠ check if one of the
strings is the secret
that validates the
current token
63

113. const jwt = require('jsonwebtoken')
const generator = require('indexed-string-variation').generator;
const variations = generator('someAlphabet')

const processChunk = (token, from, to) => {
let secret

for (let i = from; i < to; i++) {
try {
secret = variations(i)

jwt.verify(token, pwd, {
ignoreExpiration: true,
ignoreNotBefore: true
})

// finished, password found
return ({found: secret})
} catch (err) {} // password not found, keep looping
}

// finished, password not found
return null
}
Client
64

114. DemoDemo
65

115. Closing offClosing off
66

116. Is JWT safe to use?Is JWT safe to use?
67

117. DefinitelyDefinitely
YES!YES!
Heavily used by:
68

118. but...but...
69

119. Use a strong (≃long) Use a strong (≃long) secretsecret and keep it SAFE! and keep it SAFE!
Or, even better
Use RS256 (RSA public/private key pair) signatureUse RS256 (RSA public/private key pair) signature
Use it wisely!Use it wisely!
70

120. But, what if I createBut, what if I create
onlyonly
short lived tokensshort lived tokens......
71

121. JWT is STATELESS!JWT is STATELESS!
the expiry time is contained in the token...
if you can edit tokens, you can extend the expiry time as needed!
72

122. Should I be worried aboutShould I be worried about
brute forcebrute force??
73

123. Not reallyNot really
... As long as you know the basic rules... As long as you know the basic rules
(and the priorities) to defend yourself(and the priorities) to defend yourself
74

124. TLDR;TLDR;
JWT is a JWT is a cool & stateless™cool & stateless™ way to way to
transfer claims!transfer claims!
 
Choose the right Algorithm
With HS256, choose a good secret and keep it safe
Don't disclose sensitive information in the payload
Don't be too worried about brute force, but understand how it works!
75

125. Takk!
@loige@loige
https://loige.cohttps://loige.co
loige.link/jwt-crack-oslo
76

126. CreditsCredits
vector imagesvector images
designed by freepikdesigned by freepik
an      heartfelt thank you to:
@AlleviTommaso
@andreaman87
@cirpo
@katavic_d
@Podgeypoos79
@quasi_modal
77