Posts Tagged ‘security policies’

Rsync copy files with root privileges between servers with root superuser account disabled

Tuesday, December 3rd, 2019

 

rsync-copy-files-between-two-servers-with-root-privileges-with-root-superuser-account-disabled

Sometimes on servers that follow high security standards in companies following PCI Security (Payment Card Data Security) standards it is necessery to have a very weird configurations on servers,to be able to do trivial things such as syncing files between servers with root privileges in a weird manners.This is the case for example if due to security policies you have disabled root user logins via ssh server and you still need to synchronize files in directories such as lets say /etc , /usr/local/etc/ /var/ with root:root user and group belongings.

Disabling root user logins in sshd is controlled by a variable in /etc/ssh/sshd_config that on most default Linux OS
installations is switched on, e.g. 

grep -i permitrootlogin /etc/ssh/sshd_config
PermitRootLogin yes


Many corporations use Vulnerability Scanners such as Qualys are always having in their list of remote server scan for SSH Port 22 to turn have the PermitRootLogin stopped with:

 

PermitRootLogin no


In this article, I'll explain a scenario where we have synchronization between 2 or more servers Server A / Server B, whatever number of servers that have already turned off this value, but still need to
synchronize traditionally owned and allowed to write directories only by root superuser, here is 4 easy steps to acheive it.

 

1. Add rsyncuser to Source Server (Server A) and Destination (Server B)


a. Execute on Src Host:

 

groupadd rsyncuser
useradd -g 1000 -c 'Rsync user to sync files as root src_host' -d /home/rsyncuser -m rsyncuser

 

b. Execute on Dst Host:

 

groupadd rsyncuser
useradd -g 1000 -c 'Rsync user to sync files dst_host' -d /home/rsyncuser -m rsyncuser

 

2. Generate RSA SSH Key pair to be used for passwordless authentication


a. On Src Host
 

su – rsyncuser

ssh-keygen -t rsa -b 4096

 

b. Check .ssh/ generated key pairs and make sure the directory content look like.

 

[rsyncuser@src-host .ssh]$ cd ~/.ssh/;  ls -1

id_rsa
id_rsa.pub
known_hosts


 

3. Copy id_rsa.pub to Destination host server under authorized_keys

 

scp ~/.ssh/id_rsa.pub  rsyncuser@dst-host:~/.ssh/authorized_keys

 

Next fix permissions of authorized_keys file for rsyncuser as anyone who have access to that file (that exists as a user account) on the system
could steal the key and use it to run rsync commands and overwrite remotely files, like overwrite /etc/passwd /etc/shadow files with his custom crafted credentials
and hence hack you 🙂
 

Hence, On Destionation Host Server B fix permissions with:
 

su – rsyncuser; chmod 0600 ~/.ssh/authorized_keys
[rsyncuser@dst-host ~]$

 

For improved security here to restrict rsyncuser to be able to run only specific command such as very specific script instead of being able to run any command it is good to use little known command= option
once creating the authorized_keys

 

4. Test ssh passwordless authentication works correctly


For that Run as a normal ssh from rsyncuser

On Src Host

 

[rsyncuser@src-host ~]$ ssh rsyncuser@dst-host


Perhaps here is time that for those who, think enabling a passwordless authentication is not enough secure and prefer to authorize rsyncuser via a password red from a secured file take a look in my prior article how to login to remote server with password provided from command line as a script argument / Running same commands on many servers 

5. Enable rsync in sudoers to be able to execute as root superuser (copy files as root)

 


For this step you will need to have sudo package installed on the Linux server.

Then, Execute once logged in as root on Destionation Server (Server B)

 

[root@dst-host ~]# grep 'rsyncuser ALL' /etc/sudoers|wc -l || echo ‘rsyncuser ALL=NOPASSWD:/usr/bin/rsync’ >> /etc/sudoers
 

 

Note that using rsync with a ALL=NOPASSWD in /etc/sudoers could pose a high security risk for the system as anyone authorized to run as rsyncuser is able to overwrite and
respectivle nullify important files on Destionation Host Server B and hence easily mess the system, even shell script bugs could produce a mess, thus perhaps a better solution to the problem
to copy files with root privileges with the root account disabled is to rsync as normal user somewhere on Dst_host and use some kind of additional script running on Dst_host via lets say cron job and
will copy gently files on selective basis.

Perhaps, even a better solution would be if instead of granting ALL=NOPASSWD:/usr/bin/rsync in /etc/sudoers is to do ALL=NOPASSWD:/usr/local/bin/some_copy_script.sh
that will get triggered, once the files are copied with a regular rsyncuser acct.

 

6. Test rsync passwordless authentication copy with superuser works


Do some simple copy, lets say copy files on Encrypted tunnel configurations located under some directory in /etc/stunnel on Server A to /etc/stunnel on Server B

The general command to test is like so:
 

rsync -aPz -e 'ssh' '–rsync-path=sudo rsync' /var/log rsyncuser@$dst_host:/root/tmp/


This will copy /var/log files to /root/tmp, you will get a success messages for the copy and the files will be at destination folder if succesful.

 

On Src_Host run:

 

[rsyncuser@src-host ~]$ dst=FQDN-DST-HOST; user=rsyncuser; src_dir=/etc/stunnel; dst_dir=/root/tmp;  rsync -aP -e 'ssh' '–rsync-path=sudo rsync' $src_dir  $rsyncuser@$dst:$dst_dir;

 

7. Copying files with root credentials via script


The simlest file to use to copy a bunch of predefined files  is best to be handled by some shell script, the most simple version of it, could look something like this.
 

#!/bin/bash
# On server1 use something like this
# On server2 dst server
# add in /etc/sudoers
# rsyncuser ALL=NOPASSWD:/usr/bin/rsync

user='rsyncuser';

dst_dir="/root/tmp";
dst_host='$dst_host';
src[1]="/etc/hosts.deny";
src[2]="/etc/sysctl.conf";
src[3]="/etc/samhainrc";
src[4]="/etc/pki/tls/";
src[5]="/usr/local/bin/";

 

for i in $(echo ${src[@]}); do
rsync -aPvz –delete –dry-run -e 'ssh' '–rsync-path=sudo rsync' "$i" $rsyncuser@$dst_host:$dst_dir"$i";
done


In above script as you can see, we define a bunch of files that will be copied in bash array and then run a loop to take each of them and copy to testination dir.
A very sample version of the script rsync_with_superuser-while-root_account_prohibited.sh 
 

Conclusion


Lets do short overview on what we have done here. First Created rsyncuser on SRC Server A and DST Server B, set up the key pair on both copied the keys to make passwordless login possible,
set-up rsync to be able to write as root on Dst_Host / testing all the setup and pinpointing a small script that can be used as a backbone to develop something more complex
to sync backups or keep system configurations identicatial – for example if you have doubts that some user might by mistake change a config etc.
In short it was pointed the security downsides of using rsync NOPASSWD via /etc/sudoers and few ideas given that could be used to work on if you target even higher
PCI standards.

 

Scanning ports with netcat “nc” command on Linux and UNIX / Checking for firewall filtering between source and destination with nc

Friday, September 6th, 2019

scanning-ports-with-netcat-nc-command-on-Linux-and-UNIX-checking-for-firewall-filtering-between-source-destination-host-with-netcat

Netcat ( nc ) is one of that tools, that is well known in the hacker (script kiddie) communities, but little underestimated in the sysadmin world, due to the fact nmap (network mapper) – the network exploratoin and security auditing tool has become like the standard penetration testing TCP / UDP port tool
 

nc is feature-rich network debugging and investigation tool with tons of built-in capabilities for reading from and writing to network connections using TCP or UDP.

Its Plethora of features includes port listening, port scanning & Transferring files due to which it is often used by Hackers and PenTesters as Backdoor. Netcat was written by a guy we know as the Hobbit <hobbit@avian.org>.

For a start-up and middle sized companies if nmap is missing on server usually it is okay to install it without risking to open a huge security hole, however in Corporate world, due to security policies often nmap is not found on the servers but netcat (nc) is present on the servers so you have to learn, if you haven't so to use netcat for the usual IP range port scans, if you're so used to nmap.

There are different implementations of Netcat, whether historically netcat was UNIX (BSD) program with a latest release of March 1996. The Linux version of NC is GNU Netcat (official source here) and is POSIX compatible. The other netcat in Free Software OS-es is OpenBSD's netcat whose ported version is also used in FreeBSD. Mac OS X also comes with default prebundled netcat on its Mac OS X from OS X version (10.13) onwards, on older OS X-es it is installable via MacPorts package repo, even FreeDOS has a port of it called NTOOL.

The (Swiss Army Knife of Embedded Linux) busybox includes a default leightweight version of netcat and Solaris has the OpenBSD netcat version bundled.

A cryptography enabled version fork exists that supports that supports integrated transport encryption capabilities called Cryptcat.

The Nmap suite also has included rewritten version of GNU Netcat named Ncat, featuring new possibilities such as "Connection Brokering", TCP/UDP Redirection, SOCKS4 client and server support, ability to "Chain" Ncat processes, HTTP CONNECT proxying (and proxy chaining), SSL connect/listen support and IP address/connection filtering. Just like Nmap, Ncat is cross-platform.

In this small article I'll very briefly explain on basic netcat – known as the TCP Army knife tool port scanning for an IP range of UDP / TCP ports.

 

1. Scanning for TCP opened / filtered ports remote Linux / Windows server

 

Everyone knows scanning of a port is possible with a simple telnet request towards the host, e.g.:

telnet SMTP.EMAIL-HOST.COM 25

 

The most basic netcat use that does the same is achiavable with:

 

$ nc SMTP.EMAIL-HOST.COM 25
220 jeremiah ESMTP Exim 4.92 Thu, 05 Sep 2019 20:39:41 +0300


Beside scanning the remote port, using netcat interactively as pointing in above example, if connecting to HTTP Web services, you can request remote side to return a webpage by sending a false referer, source host and headers, this is also easy doable with curl / wget and lynx but doing it with netcat just like with telnet could be fun, here is for example how to request an INDEX page with spoofed HTTP headers.
 

nc Web-Host.COM 25
GET / HTTP/1.1
Host: spoofedhost.com
Referrer: mypage.com
User-Agent: my-spoofed-browser

 

2. Performing a standard HTTP request with netcat

 

To do so just pype the content with a standard bash integrated printf function with the included end of line (the unix one is \n but to be OS independent it is better to use r\n  – the end of line complition character for Windows.

 

printf "GET /index.html HTTP/1.0\r\nHost: www.pc-freak.net\r\n\r\n" | nc www.pc-freak.net 80

 

3. Scanning a range of opened / filtered UDP ports

 

To scan for lets say opened remote system services on the very common important ports opened from UDP port 25 till, 1195 – more specifically for:

  • UDP Bind Port 53
  • Time protocol Port (37)
  • TFTP (69)
  • Kerberos (88)
  • NTP 123
  • Netbios (137,138,139)
  • SNMP (161)
  • LDAP 389
  • Microsoft-DS (Samba 445)
  • Route BGP (52)
  • LDAPS (639)
  • openvpn (1194)

 

nc -vzu 192.168.0.1 25 1195

 

UDP tests will show opened, if no some kind of firewall blocking, the -z flag is given to scan only for remote listening daemons without sending any data to them.

 

4. Port Scanning TCP listening ports with Netcat

 

As prior said using netcat to scan for remote opened HTTP Web Server on port 80 an FTP on Port 23 or a Socks Proxy or MySQL Database on 3306 / PostgreSQL DB on TCP 5432 is very rare case scenario.

Below is example to scan a Local network situated IP for TCP open ports from port 1 till 7000.

 

# nc -v -n -z -w 5 192.168.1.2 1-7000

           nc: connect to host.example.com 80 (tcp) failed: Connection refused
           nc: connect to host.example.com 20 (tcp) failed: Connection refused
           Connection to host.example.com port [tcp/ssh] succeeded!
           nc: connect to host.example.com 23 (tcp) failed: Connection refused

 

Be informed that scanning with netcat is much more slower, than nmap, so specifying smaller range of ports is always a good idea to reduce annoying waiting …


The -w flag is used to set a timeout to remote connection, usually on a local network situated machines the timeout could be low -w 1 but for machines across different Data Centers (let say one in Berlin and one in Seattle), use as a minimum -w 5.

If you expect remote service to be responsive (as it should always be), it is a nice idea to use netcat with a low timeout (-w) value of 1 below is example:
 

netcat -v -z -n -w 1 scanned-hosts 1-1023

 

5. Port scanning range of IP addresses with netcat


If you have used Nmap you know scanning for a network range is as simple as running something like nmap -sP -P0 192.168.0.* (to scan from IP range 1-255 map -sP -P0 192.168.0.1-150 (to scan from local IPs ending in 1-150) or giving the network mask of the scanned network, e.g. nmap -sF 192.168.0.1/24 – for more examples please check my previous article Checking port security on Linux with nmap (examples).

But what if nmap is not there and want to check a bunch 10 Splunk servers (software for searching, monitoring, and analyzing machine-generated big data, via a Web-style interface.), with netcat to find, whether the default Splunk connection port 9997 is opened or not:

 

for i in `seq 1 10`; do nc -z -w 5 -vv splunk0$i.server-domain.com 9997; done

 

6. Checking whether UDP port traffic is allowed to destination server

 

Assuring you have access on Source traffic (service) Host A  and Host B (remote destination server where a daemon will be set-upped to listen on UDP port and no firewall in the middle Network router or no traffic control and filtering software HUB is preventing the sent UDP proto traffic, lets say an ntpd will be running on its standard 123 port there is done so:

– On host B (the remote machine which will be running ntpd and should be listening on port 123), run netcat to listen for connections

 

# nc -l -u -p 123
Listening on [0.0.0.0] (family 2, port 123)


Make sure there is no ntpd service actively running on the server, if so stop it with /etc/init.d/ntpd stop
and run above command. The command should run as superuser as UDP port 123 is from the so called low ports from 1-1024 and binding services on such requires root privileges.

– On Host A (UDP traffic send host

 

nc -uv remote-server-host 123

 

netcat-linux-udp-connection-succeeded

If the remote port is not reachable due to some kind of network filtering, you will get "connection refused".
An important note to make is on some newer Linux distributions netcat might be silently trying to connect by default using IPV6, bringing false positives of filtered ports due to that. Thus it is generally a good idea, to make sure you're connecting to IPV6

 

$ nc -uv -4 remote-server-host 123

 

Another note to make here is netcat's UDP connection takes 2-3 seconds, so make sure you wait at least 4-8 seconds for a very distant located hosts that are accessed over a multitude of routers.
 

7. Checking whether TCP port traffic allowed to DST remote server


To listen for TCP connections on a specified location (external Internet IP or hostname), it is analogous to listening for UDP connections.

Here is for example how to bind and listen for TCP connections on all available Interface IPs (localhost, eth0, eth1, eth2 etc.)
 

nc -lv 0.0.0.0 12345

 

Then on client host test the connection with

 

nc -vv 192.168.0.103 12345
Connection to 192.168.0.103 12345 port [tcp/*] succeeded!

 

8. Proxying traffic with netcat


Another famous hackers use of Netcat is its proxying possibility, to proxy anything towards a third party application with UNIX so any content returned be printed out on the listening nc spawned daemon like process.
For example one application is traffic SMTP (Mail traffic) with netcat, below is example of how to proxy traffic from Host B -> Host C (in that case the yandex current mail server mx.yandex.ru)

linux-srv:~# nc -l 12543 | nc mx.yandex.ru 25


Now go to Host A or any host that has TCP/IP protocol access to port 12543 on proxy-host Host B (linux-srv) and connect to it on 12543 with another netcat or telnet.

to make netcat keep connecting to yandex.ru MX (Mail Exchange) server you can run it in a small never ending bash shell while loop, like so:

 

linux-srv:~# while :; do nc -l 12543 | nc mx.yandex.ru 25; done


 Below are screenshots of a connection handshake between Host B (linux-srv) proxy host and Host A (the end client connecting) and Host C (mx.yandex.ru).

host-B-running-as-a-proxy-daemon-towards-Host-C-yandex-mail-exchange-server

 

Host B netcat as a (Proxy)

Host-A-Linux-client-connection-handshake-to-proxy-server-with-netcat
that is possible in combination of UNIX and named pipes (for more on Named pipes check my previous article simple linux logging with named pipes), here is how to run a single netcat version to proxy any traffic in a similar way as the good old tinyproxy.

On Proxy host create the pipe and pass the incoming traffic towards google.com and write back any output received back in the named pipe.
 

# mkfifo backpipe
# nc -l 8080 0<backpipe | nc www.google.com 80 1>backpipe

Other useful netcat proxy set-up is to simulate a network connectivity failures.

For instance, if server:port on TCP 1080 is the normal host application would connect to, you can to set up a forward proxy from port 2080 with

    nc -L server:1080 2080

then set-up and run the application to connect to localhost:2080 (nc proxy port)

    /path/to/application_bin –server=localhost –port=2080

Now application is connected to localhost:2080, which is forwarded to server:1080 through netcat. To simulate a network connectivity failure, just kill the netcat proxy and check the logs of application_bin.

Using netcat as a bind shell (make any local program / process listen and deliver via nc)

 

netcat can be used to make any local program that can receive input and send output to a server, this use is perhaps little known by the junior sysadmin, but a favourite use of l337 h4x0rs who use it to spawn shells on remote servers or to make connect back shell. The option to do so is -e

-e – option spawns the executable with its input and output redirected via network socket.

One of the most famous use of binding a local OS program to listen and receive / send content is by
making netcat as a bind server for local /bin/bash shell.

Here is how

nc -l -p 4321 -e /bin/sh


If necessery specify the bind hostname after -l. Then from any client connect to 4321 (and if it is opened) you will gain a shell with the user with which above netcat command was run. Note that many modern distribution versions such as Debian / Fedora / SuSE Linux's netcat binary is compiled without the -e option (this works only when compiled with -DGAPING_SECURITY_HOLE), removal in this distros is because option is potentially opening a security hole on the system.

If you're interested further on few of the methods how modern hackers bind new backdoor shell or connect back shell, check out Spawning real tty shells article.

 

For more complex things you might want to check also socat (SOcket CAT) – multipurpose relay for bidirectional data transfer under Linux.
socat is a great Linux Linux / UNIX TCP port forwarder tool similar holding the same spirit and functionality of netcat plus many, many more.
 

On some of the many other UNIX operating systems that are lacking netcat or nc / netcat commands can't be invoked a similar utilitiesthat should be checked for and used instead are:

ncat, pnetcat, socat, sock, socket, sbd

To use nmap's ncat to spawn a shell for example that allows up to 3 connections and listens for connects only from 192.168.0.0/24 network on port 8081:

ncat –exec "/bin/bash" –max-conns 3 –allow 192.168.0.0/24 -l 8081 –keep-open

 

9. Copying files over network with netcat


Another good hack often used by hackers to copy files between 2 servers Server1 and Server2 who doesn't have any kind of FTP / SCP / SFTP / SSH / SVN / GIT or any kind of Web copy support service – i.e. servers only used as a Database systems that are behind a paranoid sysadmin firewall is copying files between two servers with netcat.

On Server2 (the Machine on which you want to store the file)
 

nc -lp 2323 > files-archive-to-copy.tar.gz


On server1 (the Machine from where file is copied) run:
 

nc -w 5 server2.example.com 2323 < files-archive-to-copy.tar.gz

 

Note that the downside of such transfers with netcat is data transferred is unencrypted so any one with even a simple network sniffer or packet analyzier such as iptraf or tcpdump could capture the file, so make sure the file doesn't contain sensitive data such as passwords.

Copying partition images like that is perhaps best way to get disk images from a big server onto a NAS (when you can't plug the NAS into the server).
 

10. Copying piped archived directory files with netcat

 

On computer A:

export ARIBTRARY_PORT=3232
nc -l $ARBITRARY_PORT | tar vzxf –

On Computer B:

tar vzcf – files_or_directories | nc computer_a $ARBITRARY_PORT

 

11. Creating a one page webserver with netcat and ncat


As netcat could listen to port and print content of a file, it can be set-up with a bit of bash shell scripting to serve
as a one page webserver, or even combined with some perl scripting and bash to create a multi-serve page webserver if needed.

To make netact serve a page to any connected client run in a screen / tmux session following code:

 

while true; do nc -l -p 80 -q 1 < somepage.html; done

 

Another interesting fun example if you have installed ncat (is a small web server that connects current time on server on connect).
 

ncat -lkp 8080 –sh-exec 'echo -ne "HTTP/1.0 200 OK\r\n\r\nThe date is "; date;'

 

12. Cloning Hard disk partitions with netcat


rsync is a common tool used to clone hard disk partitions over network. However if rsync is not installed on a server and netcat is there you can use it instead, lets say we want to clone /dev/sdb
from Server1 to Server2 assuming (Server1 has a configured working Local or Internet connection).

 

On Server2 run:
 

nc -l -p 4321 | dd of=/dev/sdb

 

Following on Server2 to start the Partition / HDD cloning process run

 

dd if=/dev/sdb | nc 192.168.0.88 4321

 


Where 192.168.0.88 is the IP address listen configured on Server2 (in case you don't know it, check the listening IP to access with /sbin/ifconfig).

Next you have to wait for some short or long time depending on the partiiton or Hard drive, number of files / directories and allocated disk / partition size.

To clone /dev/sda (a main partiiton) from Server1 to Server2 first requirement is that it is not mounted, thus to have it unmounted on a system assuming you have physical access to the host, you can boot some LiveCD Linux distribution such as Knoppix Live CD on Server1, manually set-up networking with ifconfig or grab an IP via DHCP from the central DHCP server and repeat above example.


Happy netcating 🙂

Into great depression – What is like to live in the Balkans?

Wednesday, May 18th, 2011

I'm so depressed these days that I'm trying to write something decent here but everytime I try I do stop and delete all I have written and start from scratch again.
It's terrible, I believe everyone have this days and they're so dark that even the smallest ray of light is gone somewhere…

The causes for depression are multiple, I know we're entering into the season period and that could be a factor, but truly for a long time I haven't felt that bad and I really cannot find the true cause. It's like hunting the unexplained.
Being a citizen of a countries on the balkans brings a lot of questions which cannot get answered. Why we the Balkan and more specificly most of the Orthodox Christian countries are suffering so badly and economically in constant crisis and recession?
From a material perspective Bulgaria is one of the worst countries one can live in, we the people on the balkans are chronically depressed and it really seems like a downward spiral
We've been gone through so far, when I was a child we were teached in the spirit of communism and a believe in a hard material realities.

Communism has taught us we're all fleshly brothers and we should live in groups and stick to the group, now as the democracy come it's on the contraty, we're being constantly re-taught that we should leave behind the group kind of thinking and all built from communism destroy it all and build the new society… We're told by individualist nations like USA and Western europe that the only thing for the good of a person is to (get an absolute individualistic life and only exist for the greater goodness of each ones self as individuals..

As with everything the Balkans are notable for being a very unordered place. Living here is like living in chaos…
The social security policies here are not working, the jurisdiction is working on behalf of the rich, the police force is seriously disfunctional and easily bribable. Put next to all this shit a high levels of unemployment and a lot of unhappy depressed people crawling around the streets and you get the picture …
As a normal consequence most of the young people have entered a dark ways of alcoholism and hard-core nihillism.
There are high level of people who are oriented into the new dark realities of Metal or underground music.
Each philosophy that is being put in from the west is being adopted here and being multiplied million times and mostly the bad things are being adopted and less rarely the good ones…
It's so mixed up that nobody can explain why it is happening as it is here.
I really am trying hard to convince myself for a years now that it is worthy to live here but the more I live here in Bulgaria the more I see all is getting worser than getting for good.

I wonder for how long it will go this pointless way, we the balkan people are living in ruins literally.

The only light we still have is the Church, but very sadly most people has left behind the faith and prefer to follow the fake American dream than to obey to our old ways and traditions.

Globalisation has entered in the Balkans in a full-force and is destroying our ancient culture and traditions and building the fakeness of the coca-cola culture that most of the people prefer to adore nowdays …

Bulgaria's population is mostly based of old people and we're a dying nation, if a miracle doesn't happen then we definitely will be gone.