Sniffing Decrypted TLS Traffic with Security Onion

Wouldn’t it be awesome to have a NIDS like
Snort,
Suricata or
Zeek
inspect HTTP requests leaving your network inside TLS encrypted HTTPS traffic?
Yeah, we think so too!
We have therefore created this guide on how to configure
Security Onion to sniff decrypted TLS traffic with help of PolarProxy.

Network drawing with Clients, SecurityOnion and the Internet

PolarProxy is a forward TLS proxy that decrypts incoming TLS traffic from clients, re-encrypts it and forwards it to the server.
One of the key features in PolarProxy is the ability to export the proxied traffic in decrypted form
using the PCAP format (a.k.a. libpcap/tcpdump format).
This makes it possible to read the decrypted traffic with external tools, without having to perform the decryption again.
It also enables packet analysis using tools that don’t have built-in TLS decryption support.

This guide outlines how to configure PolarProxy to intercept HTTPS traffic and send the decrypted HTTP traffic to an internal network interface,
where it can be sniffed by an IDS.


STEP 1 ☆ Install Ubuntu

Download and install the latest SecurityOnion ISO image, but don’t run the “Setup” just yet.

STEP 2 ☆ Add a Dummy Network Interface

Add a dummy network interface called “decrypted”, to which decrypted packets will be sent.


ip link add decrypted type dummy
ip link set decrypted arp off up

Add the commands above to /etc/rc.local before “exit 0” to have the network interface automatically configured after reboots.

dummy interface in rc.local

STEP 3 ☆ Install Updates

Install updates in Security Onion by running “sudo soup”.

STEP 4 ☆ Run the Security Onion Setup

Run the Security Onion setup utility by double-clicking the “Setup” desktop shortcut or executing “sudo sosetup” from a terminal.
Follow the setup steps in the Production Deployment documentation
and select “decrypted” as your sniffing interface.

Sniffing Interface Selection Window

Reboot and run Setup again to continue with the second phase of Security Onion’s setup.
Again, select “decrypted” as the interface to be monitored.

STEP 5 ☆ Install PolarProxy Service

Download and install PolarProxy:

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 -xzf –
exit

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

Edit /etc/systemd/system/PolarProxy.service and add
“–pcapoverip 57012” at the end of the ExecStart command.

--pcapoverip 57012 in PolarProxy.service

Start the PolarProxy systemd service:

sudo systemctl enable PolarProxy.service
sudo systemctl start PolarProxy.service

STEP 6 ☆ Install Tcpreplay Service

The decrypted traffic can now be accessed via PolarProxy’s PCAP-over-IP service on TCP 57012.
We can leverage tcpreplay and netcat to replay these packets to our dummy network interface in order to have them picked up by Security Onion.

nc localhost 57012 | tcpreplay -i decrypted -t –

However, it’s better to create a systemd service that does this automatically on bootup.
We therefore create a file called /etc/systemd/system/tcpreplay.service with the following contents:

[Unit] Description=Tcpreplay of decrypted traffic from PolarProxy
After=PolarProxy.service

[Service] Type=simple
ExecStart=/bin/sh -c ‘nc localhost 57012 | tcpreplay -i decrypted -t -‘
Restart=on-failure
RestartSec=3

[Install] WantedBy=multi-user.target

Start the tcpreplay systemd service:

sudo systemctl enable tcpreplay.service
sudo systemctl start tcpreplay.service

STEP 7 ☆ Add firewall rules

Security Onion only accepts incoming connections on TCP 22 by default,
we also need to allow connections to TCP port 10443 (proxy port),
and 10080 (root CA certificate download web server).
Add allow rules for these services to the Security Onion machine’s firewall:

sudo ufw allow in 10443/tcp
sudo ufw allow in 10080/tcp

Verify that the proxy is working by running this curl command on a PC connected to the same network as the Security Onion machine:

curl –insecure –connect-to www.netresec.com:443:[SecurityOnionIP]:10443 https://www.netresec.com/


Note: You can even perform this test from a Win10 PC, since curl is included with Windows 10 version 1803 and later.

Add the following lines at the top of /etc/ufw/before.rules
(before the *filter section) to redirect incoming packets on TCP 443 to PolarProxy on port 10443.

*nat
:PREROUTING ACCEPT [0:0] -A PREROUTING -i enp0s3 -p tcp –dport 443 -j REDIRECT –to 10443
COMMIT

Note: Replace “enp0s3” with the Security Onion interface to which clients will connect.

After saving before.rules, reload ufw to activate the port redirection:

sudo ufw reload

Verify that you can reach the proxy on TCP 443 with this command:

curl –insecure –resolve www.netresec.com:443:[SecurityOnionIP] https://www.netresec.com/

STEP 8 ☆ Redirect HTTPS traffic to PolarProxy

It’s now time to configure a client to run its HTTPS traffic through PolarProxy.
Download and install the PolarProxy X.509 root CA certificate from PolarProxy’s web service on TCP port 10080:

http://[SecurityOnionIP]:10080/polarproxy.cer

Install the certificate in the operating system and browser, as instructed in the PolarProxy documentation.

You also need to forward packets from the client machine to the Security Onion machine running PolarProxy.
This can be done either by configuring a local NAT rule on each monitored client (STEP 8.a)
or by configuring the default gateway’s firewall to forward HTTPS traffic from all clients to the proxy (STEP 8.b).

STEP 8.a ☆ Local NAT

Use this firewall rule on a Linux client to configure it to forward outgoing HTTPS traffic to the Security Onion machine:

sudo iptables -t nat -A OUTPUT -p tcp –dport 443 -j DNAT –to [SecurityOnionIP]

STEP 8.b ☆ Global NATNetwork drawing Firewall, PolarProxy, Clients

If the client isn’t running Linux, or if you wanna forward HTTPS traffic from a whole network to the proxy,
then apply the following iptables rules to the firewall in front of the client network.
See “Routing Option #2” in the PolarProxy documentation for more details.

  1. Add a forward rule on the gateway to allow forwarding traffic to our PolarProxy server:

    sudo iptables -A FORWARD -i eth1 -d [SecurityOnionIP] -p tcp –dport 10443 -m state –state NEW -j ACCEPT
  2. Add a DNAT rule to forward 443 traffic to PolarProxy on port 10443:

    sudo iptables -t nat -A PREROUTING -i eth1 -p tcp –dport 443 -j DNAT –to [SecurityOnionIP]:10443
  3. If the reverse traffic from PolarProxy to the client doesn’t pass the firewall (i.e. they are on the same LAN),
    then we must add this hide-nat rule to fool PolarProxy that we are coming from the firewall:

    sudo iptables -t nat -A POSTROUTING -o eth1 -d [SecurityOnionIP] -p tcp –dport 10443 -j MASQUERADE

For other network configurations, please see the various routing setups in the PolarProxy documentation.


STEP 9 ☆ Inspect traffic in SecurityOnion

Wait for the Elastic stack to initialize, so that the intercepted network traffic becomes available through the Kibana GUI.
You can check the status of the elastic initialization with “sudo so-elastic-status”.

You should now be able to inspect decrypted traffic in Security Onion
using Kibana, Squert, Sguil etc., just as if it was unencrypted HTTP.

Bro HTTP traffic in KibanaImage: Kibana showing HTTP traffic info from decrypted HTTPS sessions

MIME types in KibanaImage: MIME types in Kibana

NIDS alerts in KibanaImage: NIDS alerts from payload in decrypted traffic shown in Kibana

Snort alerts in SquertImage: Snort alerts from decrypted traffic shown in Squert

Security Considerations and Hardening

Security Onion nodes are normally configured to only allow access by SOC/CERT/CSIRT analysts,
but the setup described in this blog post requires that “normal” users on the client network can access the PolarProxy service running on the Security Onion node.
We therefore recommend installing PolarProxy on a dedicated
Security Onion Forward Node,
which is configured to only monitor traffic from the proxy.

We also recommend segmenting the client network from the analyst network,
for example by using separate network interfaces on the Security Onion machine or putting it in a DMZ.
Only the PolarProxy service (TCP 10080 and 10443) should be accessable from the client network.

PolarProxy could be used to pivot from the client network into the analyst network or to access the Apache webserver running on the Security Onion node.
For example, the following curl command can be used to access the local Apache server running on the Security Onion machine via PolarProxy:

curl –insecure –connect-to localhost:443:[SecurityOnionIP]:10443 https://localhost/

We therefore recommend adding firewall rules that prevent PolarProxy from accessing the analyst network as well as the local Apache server.

Hardening Steps:

  • Configure the Security Onion node as a Forward Node
  • Segment client network from analyst network
  • Add firewall rules to prevent PolarProxy from accessing services on the local machine and analyst network

For additional info on hardening, please see the
recommendations provided by Wes Lambert
on the Security-Onion mailing list.

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*** This is a Security Bloggers Network syndicated blog from NETRESEC Network Security Blog authored by Erik Hjelmvik. Read the original post at: http://www.netresec.com/?page=Blog&month=2020-01&post=Sniffing-Decrypted-TLS-Traffic-with-Security-Onion