NETRESEC Network Security Blog - Tag : SMTPS

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Emotet C2 and Spam Traffic Video

This video covers a life cycle of an Emotet infection, including initial infection, command-and-control traffic, and spambot activity sending emails with malicious spreadsheet attachments to infect new victims.

The video was recorded in a Windows Sandbox in order to avoid accidentally infecting my Windows PC with malware.

Initial Infection

Palo Alto's Unit 42 sent out a tweet with screenshots and IOCs from an Emotet infection in early March. A follow-up tweet by Brad Duncan linked to a PCAP file containing network traffic from the infection on Malware-Traffic-Analysis.net.

Screenshot of original infection email from Unit 42

Image: Screenshot of original infection email from Unit 42

  • Attachment MD5: 825e8ea8a9936eb9459344b941df741a

Emotet Download

The PCAP from Malware-Traffic-Analysis.net shows that the Excel spreadsheet attachment caused the download of a DLL file classified as Emotet.

CapLoader download of Emotet DLL from diacrestgroup.com

Image: CapLoader transcript of Emotet download

  • DNS: diacrestgroup.com
  • MD5: 99f59e6f3fa993ba594a3d7077cc884d

Emotet Command-and-Control

Just seconds after the Emotet DLL download completes the victim machine starts communicating with an IP address classified as a botnet command-and-control server.

Emotet C2 sessions with JA3 51c64c77e60f3980eea90869b68c58a8 in CapLoader

Image: Emotet C2 sessions in CapLoader

  • C2 IP: 209.15.236.39
  • C2 IP: 147.139.134.226
  • C2 IP: 134.209.156.68
  • JA3: 51c64c77e60f3980eea90869b68c58a8
  • JA3S: ec74a5c51106f0419184d0dd08fb05bc
  • JA3S: fd4bc6cea4877646ccd62f0792ec0b62

Emotet Spambot

The victim PC eventually started sending out spam emails. The spam bot used TLS encryption when possible, either through SMTPS (implicit TLS) or with help of STARTTLS (explicit TLS).

Emotet spambot JA3 hash 37cdab6ff1bd1c195bacb776c5213bf2 in NetworkMiner Professional

Image: Emotet spambot JA3 hash in NetworkMiner Professional

  • SMTPS JA3: 37cdab6ff1bd1c195bacb776c5213bf2
  • STARTTLS JA3: 37cdab6ff1bd1c195bacb776c5213bf2

Transmitted Spam

Below is a spam email sent from the victim PC without TLS encryption. The attached zip file contains a malicious Excel spreadsheet, which is designed to infect new victims with Emotet.

Emotet spam email from PCAP

Image: Spam email extracted from Emotet PCAP with NetworkMiner

  • .zip Attachment MD5: 5df1c719f5458035f6be2a071ea831db
  • .xlsm Attachment MD5: 79cb3df6c0b7ed6431db76f990c68b5b

Network Forensics Training

If you want to learn additional techniques for analyzing network traffic, then take a look at our upcoming network forensic trainings.

Posted by Erik Hjelmvik on Monday, 09 May 2022 06:50:00 (UTC/GMT)

Tags: #Emotet#C2#video#pcap#JA3#JA3S#51c64c77e60f3980eea90869b68c58a8#SMTP#SMTPS#Windows Sandbox

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Installing a Fake Internet with INetSim and PolarProxy

INetSim + PolarProxy

This is a tutorial on how to set up an environment for dynamic malware analysis, which can be used to analyze otherwise encrypted HTTPS and SMTPS traffic without allowing the malware to connect to the Internet. Dynamic malware analysis (or behavioral analysis) is performed by observing the behavior of a malware while it is running. The victim machine, which executes the malware, is usually a virtual machine that can be rolled back to a clean state when the analysis is complete. The safest way to prevent the malware from infecting other machines, or doing other bad things like sending SPAM or taking part in DDoS attacks, is to run the victim machine in an offline environment. However, network traffic analysis of malware is a central part of dynamic malware analysis, which is is why a “fake Internet” is needed in most malware labs.

INetSim and PolarProxy

INetSim is a software suite that simulates common internet services like HTTP, DNS and SMTP, which useful when analyzing the network behavior of malware samples without connecting them to the Internet. INetSim also has basic support for TLS encrypted protocols, like HTTPS, SMTPS, POP3S and FTPS, but requires a pre-defined X.509-certificate to be loaded at startup. This can cause malware to terminate because the Common Names (CN) in the presented certificates don’t match the requested server names. The victim machine will actually get the exact same certificate regardless of which web site it visits. INetSim’s TLS encryption also inhibits analysis of the network traffic captured in the malware lab, such as C2 traffic or SPAM runs, because the application layer traffic is encrypted. PolarProxy can solve both these issues because it generates certificates on the fly, where the CN value is dynamically set to the requested host name, and saves the network traffic in decrypted form to PCAP files. It is therefore a good idea to replace the TLS services in INetSim with PolarProxy, which will be used as a TLS termination proxy that forwards the decrypted traffic to INetSim’s cleartext services.

Malware Lab Setup

Install Linux

The first step is to install a Linux VM, which will act as a fake Internet to the victim machine(s). I'm using Ubuntu Server 18.04.3 LTS in this tutorial, but you can use any 64-bit linux distro. I'm adding two network interfaces to the Linux VM, one interface with Internet access and one that connects to an isolated offline network to which the victim VM's will be connected. The offline interface is configured to use the static IP 192.168.53.19.

Important: Do not bridge, bond or enable IP forwarding between the two interfaces!

Network connection config Ubuntu Server 18.04

Install INetSim

INetSim is available in Ubuntu's repo, so it is possible to install it with "apt install inetsim". However, I recommend installing INetSim as described in the official documentation to get the latest packaged version of INetSim.

sudo -s

echo "deb http://www.inetsim.org/debian/ binary/" > /etc/apt/sources.list.d/inetsim.list

curl https://www.inetsim.org/inetsim-archive-signing-key.asc | apt-key add -

apt update

apt install inetsim

exit

INetSim listens on 127.0.0.1 by default, change this to INetSim's offline IP address by un-commenting and editing the service_bind_address variable in /etc/inetsim/inetsim.conf.

service_bind_address    192.168.53.19

Also configure INetSim's fake DNS server to resolve all domain names to the IP of INetSim with the dns_default_ip setting:

dns_default_ip    192.168.53.19

Finally, disable the "start_service https" and "start_service smtps" lines, because these services will be replaced with PolarProxy:

start_service dns
start_service http
#start_service https
start_service smtp
#start_service smtps

Restart the INetSim service after changing the config.

sudo systemctl restart inetsim.service

Verify that you can access INetSim's HTTP server with curl:

curl http://192.168.53.19

<html>
  <head>
    <title>INetSim default HTML page</title>
  </head>
  <body>
    <p></p>
    <p align="center">This is the default HTML page for INetSim HTTP server fake mode.</p>
    <p align="center">This file is an HTML document.</p>
  </body>
</html>

It looks like INetSim's web server can be accessed alright.

Install PolarProxy

Next step is to install PolarProxy as a systemd service (as instructed here):

sudo adduser --system --shell /bin/bash proxyuser

sudo mkdir /var/log/PolarProxy

sudo chown proxyuser:root /var/log/PolarProxy/

sudo chmod 0775 /var/log/PolarProxy/

sudo su - proxyuser

mkdir ~/PolarProxy

cd ~/PolarProxy/

curl https://www.netresec.com/?download=PolarProxy | tar -xzvf -

exit

sudo cp /home/proxyuser/PolarProxy/PolarProxy.service /etc/systemd/system/PolarProxy.service

We will need to modify the PolarProxy service config file a bit before we start it. Edit the ExecStart setting in /etc/systemd/system/PolarProxy.service to configure PolarProxy to terminate the TLS encryption for HTTPS and SMTPS (implicitly encrypted email submission). The HTTPS traffic should be redirected to INetSim's web server on tcp/80 and the SMTPS to tcp/25.

ExecStart=/home/proxyuser/PolarProxy/PolarProxy -v -p 10443,80,80 -p 10465,25,25 -x /var/log/PolarProxy/polarproxy.cer -f /var/log/PolarProxy/proxyflows.log -o /var/log/PolarProxy/ --certhttp 10080 --terminate --connect 192.168.53.19 --nosni nosni.inetsim.org

Here's a break-down of the arguments sent to PolarProxy through the ExecStart setting above:

  • -v : verbose output in syslog (not required)
  • -p 10443,80,80 : listen for TLS connections on tcp/10443, save decrypted traffic in PCAP as tcp/80, forward traffic to tcp/80
  • -p 10465,25,25 : listen for TLS connections on tcp/10465, save decrypted traffic in PCAP as tcp/25, forward traffic to tcp/25
  • -x /var/log/PolarProxy/polarproxy.cer : Save certificate to be imported to clients in /var/log/PolarProxy/polarproxy.cer (not required)
  • -f /var/log/PolarProxy/proxyflows.log : Log flow meta data in /var/log/PolarProxy/proxyflows.log (not required)
  • -o /var/log/PolarProxy/ : Save PCAP files with decrypted traffic in /var/log/PolarProxy/
  • --certhttp 10080 : Make the X.509 certificate available to clients over http on tcp/10080
  • --terminate : Run PolarProxy as a TLS termination proxy, i.e. data forwarded from the proxy is decrypted
  • --connect 192.168.53.19 : forward all connections to the IP of INetSim
  • --nosni nosni.inetsim.org : Accept incoming TLS connections without SNI, behave as if server name was "nosni.inetsim.org".

Finally, start the PolarProxy systemd service:

sudo systemctl enable PolarProxy.service

sudo systemctl start PolarProxy.service

Verify that you can reach INetSim through PolarProxy's TLS termination proxy using curl:

curl --insecure --connect-to example.com:443:192.168.53.19:10443 https://example.com

<html>
  <head>
    <title>INetSim default HTML page</title>
  </head>
  <body>
    <p></p>
    <p align="center">This is the default HTML page for INetSim HTTP server fake mode.</p>
    <p align="center">This file is an HTML document.</p>
  </body>
</html>

Yay, it is working! Do the same thing again, but also verify the certificate against PolarProxy's root CA this time. The root certificate is downloaded from PolarProxy via the HTTP service running on tcp/10080 and then converted from DER to PEM format using openssl, so that it can be used with curl's "--cacert" option.

curl http://192.168.53.19:10080/polarproxy.cer > polarproxy.cer

openssl x509 -inform DER -in polarproxy.cer -out polarproxy-pem.crt

curl --cacert polarproxy-pem.crt --connect-to example.com:443:192.168.53.19:10443 https://example.com

<html>
  <head>
    <title>INetSim default HTML page</title>
  </head>
  <body>
    <p></p>
    <p align="center">This is the default HTML page for INetSim HTTP server fake mode.</p>
    <p align="center">This file is an HTML document.</p>
  </body>
</html>

Yay #2!

Now let's set up routing to forward all HTTPS traffic to PolarProxy's service on tcp/10443 and SMTPS traffic to tcp/10465. I'm also adding a firewall rule to redirect ALL other incoming traffic to INetSim, regardless of which IP it is destined to, with the final REDIRECT rule. Make sure to replace "enp0s8" with the name of your interface.

sudo iptables -t nat -A PREROUTING -i enp0s8 -p tcp --dport 443 -j REDIRECT --to 10443

sudo iptables -t nat -A PREROUTING -i enp0s8 -p tcp --dport 465 -j REDIRECT --to 10465

sudo iptables -t nat -A PREROUTING -i enp0s8 -j REDIRECT

Verify that the iptables port redirection rule is working from another machine connected to the offline 192.168.53.0/24 network:

curl --insecure --resolve example.com:443:192.168.53.19 https://example.com

<html>
  <head>
    <title>INetSim default HTML page</title>
  </head>
  <body>
    <p></p>
    <p align="center">This is the default HTML page for INetSim HTTP server fake mode.</p>
    <p align="center">This file is an HTML document.</p>
  </body>
</html>

Yay #3!

curl --insecure --resolve example.com:465:192.168.53.19 smtps://example.com

214-Commands supported:
214- HELO MAIL RCPT DATA
214- RSET NOOP QUIT EXPN
214- HELP VRFY EHLO AUTH
214- ETRN STARTTLS
214 For more info use "HELP <topic>".

Yay #4!

It is now time to save the firewall rules, so that they will survive reboots.

sudo apt-get install iptables-persistent

Install the Victim Windows PC

Configure a static IP address on the victim Windows host by manually setting the IP address. Set the INetSim machine (192.168.53.19) as the default gateway and DNS server.

Windows IPv4 Properties

Download the X.509 root CA certificate from your PolarProxy installation here:
http://192.168.53.19:10080/polarproxy.cer

  1. Double-click on "polarproxy.cer"
  2. Click [Install Certificate...]
  3. Select 🔘 Local Machine and press [Next]
  4. Select 🔘 Place all certificates in the following store and press [Browse...]
  5. Choose "Trusted Root Certification Authorities" and press [OK], then [Next]
  6. Press [Finish]

You might also want to install the PolarProxy certificate in your browser. This is how you install it to Firefox:

  1. Options / Preferences
  2. Press [Privacy & Security]
  3. Scroll down to "Certificates" and press [View Certificates...]
  4. In the "Authorities" tab, press [Import...]
  5. Open "polarproxy.cer"
  6. ☑ Trust this CA to identify websites. (check the box)
  7. Press [OK]

Now, open a browser and try visiting some websites over HTTP or HTTPS. If you get the following message regardless of what domain you try to visit, then you've managed to set everything up correctly:

This is the default HTML page for INetSim HTTP server fake mode.

This file is an HTML document.

Accessing the Decrypted Traffic

PCAP files with decrypted HTTPS and SMTPS traffic are now available in /var/log/PolarProxy/

PolarProxy will start writing to a new capture file every 60 minutes. However, the captured packets are not written to disk instantly because PolarProxy uses buffered file writing in order to improve performance. You can restart the proxy service if you wish to flush the buffered packets to disk and have PolarProxy rotate to a new capture file.

sudo systemctl restart PolarProxy

I also recommend capturing all network traffic sent to INetSim with a sniffer like netsniff-ng. This way you’ll get PCAP files with traffic from INetSim’s cleartext services (like DNS and HTTP) as well.

PCAP or it didn’t happen!

Credits

I'd like to thank Thomas Hungenberg and Patrick Desnoyers for providing valuable feedback for this blog post!

Posted by Erik Hjelmvik on Monday, 09 December 2019 08:40:00 (UTC/GMT)

Tags: #PolarProxy#HTTPS#SMTPS#HTTP#SMTP#DNS#Malware#Sandbox#TLS#PCAP#proxy#tutorial#ASCII-art

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PolarProxy Released

I’m very proud to announce the release of PolarProxy today! PolarProxy is a transparent TLS proxy that decrypts and re-encrypts TLS traffic while also generating a PCAP file containing the decrypted traffic.

PolarProxy flow chart

PolarProxy enables you to do lots of things that have previously been impossible, or at least very complex, such as:

  • Analyzing HTTP/2 traffic without an SSLKEYLOGFILE
  • Viewing decrypted HTTPS traffic in real-time using Wireshark
    PolarProxy -p 10443,80,443 -w - | wireshark -i - -k
  • Replaying decrypted traffic to an internal or external interface using tcpreplay
    PolarProxy -p 10443,80,443 -w - | tcpreplay -i eth1 -
  • Forwarding of decrypted traffic to a NIDS (see tcpreplay command above)
  • Extracting DNS queries and replies from DNS-over-TLS (DoT) or DNS-over-HTTPS (DoH) traffic
    PolarProxy -p 853,53 -p 443,80 -w dns.pcap
  • Extracting email traffic from SMTPS, POP3S or IMAPS
    PolarProxy -p 465,25 -p 995,110 -p 993,143 -w emails.pcap

Here is an example PCAP file generated by PolarProxy:
https://www.netresec.com/files/polarproxy-demo.pcap

This capture files contains HTTP, WebSocket and HTTP/2 packets to Mozilla, Google and Twitter that would otherwise have been encrypted with TLS.

 HTTP/2 traffic from PolarProxy opened in Wireshark
Image: HTTP/2 traffic from PolarProxy opened in Wireshark

Now, head over to our PolarProxy page and try it for yourself (it’s free)!

Posted by Erik Hjelmvik on Friday, 21 June 2019 06:00:00 (UTC/GMT)

Tags: #PolarProxy#PCAP#NIDS#IDS#http2#HTTP/2#Wireshark#tcpreplay#DoH#SMTPS#IMAPS#TLS#SSL

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