NETRESEC Network Security Blog - Tag : IMAPS

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Reverse Proxy and TLS Termination

PolarProxy is primarily a TLS forward proxy, but it can also be used as a TLS termination proxy or reverse TLS proxy to intercept and decrypt incoming TLS traffic, such as HTTPS or IMAPS, before it is forwarded to a server. The proxied traffic can be accessed in decrypted form as a PCAP formatted data stream, which allows real-time analysis of the decrypted traffic by an IDS as well as post incident forensics with Wireshark.

PolarProxy version 0.8.15 and later can import an existing X.509 server certificate (aka leaf certificate or end-entity certificate) in order to perform the TLS decryption using a valid certificate signed by a trusted certificate authority. If no server certificate is provided, then PolarProxy falls back to generating server certificates on the fly and signing them with its own root CA certificate.

There are two principal ways to run PolarProxy as a reverse proxy, either as a TLS termination proxy or as a reverse proxy that decrypts and re-encrypts the traffic.

PolarProxy as a TLS Termination Proxy

TLS Termination Proxy

The TLS termination proxy mode is useful in order to offload the task of performing TLS encryption to PolarProxy instead of doing the decryption on the web server. This mode can also be used when the proxied services don’t support TLS encryption, such as legacy web servers or servers hosting other unencrypted services that you want to secure with TLS.

The following command sequence shows how to create a Let’s Encrypt SSL certificate, convert it to the PKCS#12 format, and load the server certificate into PolarProxy to terminate incoming HTTPS connections. In this setup PolarProxy decrypts the TLS traffic and relays the HTTP traffic to the web server on TCP port 80.

sudo certbot certonly --manual --preferred-challenges dns -d,

sudo openssl pkcs12 -export -out /etc/example.p12 -inkey /etc/letsencrypt/live/ -in /etc/letsencrypt/live/ --passout pass:PASSWORD

sudo mkdir /var/log/TlsTerminationProxy/

sudo ./PolarProxy --terminate --connect --nosni --servercert, -p 443,80,80 -o /var/log/TlsTerminationProxy/

Here’s a breakdown of the arguments sent to PolarProxy:

  • --terminate : Terminate incoming TLS sessions and forward proxied traffic in unencrypted form.
  • --connect : Forward all proxied traffic to instead of connecting to the host name provided in the SNI extension of the TLS ClientHello message.
  • --nosni : Treat incoming TLS sessions that don’t define a host name with the SNI extension as if they wanna to connect to “”.
  • --servercert, : Use the server certificate “/etc/example.p12” for incoming connections to “” and “”.
  • -p 443,80,80 : Listen on TCP port 443, save decrypted traffic in PCAP file as if it was directed to port 80, forward decrypted traffic to port 80.
  • -o /var/log/TlsTerminationProxy/ : Save decrypted traffic to hourly rotated PCAP files in “/var/log/TlsTerminationProxy/”.

PolarProxy is a generic TLS proxy that doesn’t care what application layer protocol the TLS tunnel carries. So if you want to terminate the TLS encryption of incoming IMAPS sessions as well, then simply append an additional argument saying “-p 993,143,143” to also forward decrypted IMAP sessions to This method can be used in order to wrap almost any TCP based protocol in a TLS tunnel, which can be useful for privacy reasons as well as to prevent network monitoring tools from detecting the actual application layer protocol.

PolarProxy as a Reverse TLS Proxy

Reverse TLS Proxy

There are setups for which it is preferable to also encrypt the internal sessions between PolarProxy and the final server. One such setup is when the server is hosting a web service with support for the HTTP/2 protocol, which in practice always uses TLS. Luckily PolarProxy is designed to decrypt and re-encrypt proxied traffic while also forwarding important TLS parameters, such as ALPN and SNI, between the internal and external TLS sessions.

To use TLS encryption on the inside as well as outside of PolarProxy, simply do as explained in the previous TLS termination section, but remove the “--terminate” argument and change the port argument to “-p 443,80,443” like this:

sudo ./PolarProxy --connect --nosni --servercert, -p 443,80,443 -o /var/log/ReverseTlsProxy/

PolarProxy will save the decrypted traffic as cleartext HTTP (or HTTP/2) to PCAP files in the “/var/log/ReverseTlsProxy/” directory.

Real-Time Analysis of Decrypted Traffic

Both the external (client-to-proxy) and internal (proxy-to-server) TCP sessions, in the reverse TLS proxy example above, are encrypted with TLS. This prevents passive network security monitoring tools, such as IDSs, DPI and DLP appliances, from analyzing the application layer data being sent and received. The PCAP files written to “/var/log/ReverseTlsProxy/” can be a valuable forensic asset when investigating an incident, but a real-time stream of the decrypted data is needed in order to swiftly detect and alert on potential security breaches and other incidents.

PolarProxy’s “--pcapoverip” option can be used to provide such a real-time stream of the decrypted data passing through the proxy. This data can easily be sent to a network interface using tcpreplay, as explained in our blog post “Sniffing Decrypted TLS Traffic with Security Onion”.

Security Considerations

The examples shown in this blog post all run PolarProxy with root privileges using sudo, which can be dangerous from a security perspective. PolarProxy is actually designed to be run without root privileges, but doing so prevents it from listening on a port below 1024. Luckily, this issue can easily be overcome with a simple port forwarding or redirect rule. The following iptables redirect rule can be used if PolarProxy is listening on TCP port 20443 and incoming HTTPS request are arriving to the eth0 interface of the proxy:

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 443 -j REDIRECT --to 20443

PolarProxy does not support loading settings from a config file. The password for the PKCS12 certificate will therefore need to be supplied on the command line, which can make it visible from a process listing. If this is a concern for you, then please consider using “hidepid” to hide processes from other users. You can find instructions on how to use hidepid in hardening guides for Debian, Arch, SUSE and most other Linux flavors.

Posted by Erik Hjelmvik on Thursday, 12 March 2020 15:45:00 (UTC/GMT)

Tags: #PolarProxy#TLS#SSL#PCAP#decrypt#HTTPS#HTTP#HTTP/2#http2#IMAPS#decrypt#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:

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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Chinese MITM attack on

An illustration from supplement to 'Le Petit Journal', 16th January 1898.

We were contacted by earlier today regarding a new Chinese man-in-the-middle (MITM) attack. This time the perpetrators decrypted traffic between Chinese users and Microsoft's IMAP mail server for As evidence provided us with a packet capture file, which we have analyzed.

Our conclusion is that this was a real attack on Microsoft's email service. Additionally, the attack is very similar to previous nationwide Chinese attacks on SSL encrypted traffic, such as the attack on Google a few months ago. Details such as email address, email password, email contents, email attachments and contacts may have been compromised in this attack. We do not know the scale of the attack, it could be anything from a fairly targeted attack to a nation wide attack in China. What we do know is that there are several users who have been subjected to the MITM attack and posted screenshots online.

Technical Analysis

Attacked IP Address: (
Attacked Protocol: SSL encryption of IMAPS (TCP 993)
Date of Attack: 2015-01-18
PCAP File:

In our technical analysis we first extracted the x509 certificates from the SSL traffic by loading the capture file into NetworkMinerCLI. We then parsed the extracted certificates with OpenSSL.

$ mono /opt/NetworkMinerProfessional_1-6-1/NetworkMinerCLI.exe -r Outlook_MITM_2015-01-18.pcapng
Closing file handles...
84 frames parsed in 0.888754 seconds.
$ ls AssembledFiles/\ -\ TCP\ 993/*.cer
AssembledFiles/ - TCP 993/[1].cer
AssembledFiles/ - TCP 993/[2].cer
AssembledFiles/ - TCP 993/
$ openssl x509 -inform DER -in AssembledFiles/\ -\ TCP\ 993/ -noout -issuer -subject -startdate -fingerprint
issuer= /CN=*
subject= /CN=*
notBefore=Jan 15 16:00:00 2015 GMT
SHA1 Fingerprint=75:F4:11:59:5F:E9:A2:1A:17:A4:96:7C:7B:66:6E:51:52:79:1A:32

When looking at the timestamps in the capture file we noticed that the SSL server's reply to the 'Client Hello' was very slow; response times varied between 14 and 20 seconds. Under normal circumstances the 'Server Hello' arrives within 0.3 seconds after the 'Client Hello' has been sent.

$ tshark -nr ./Outlook_MITM_2015-01-18.pcapng -Y 'ssl.handshake.type lt 3'
8 9.023876000 -> SSL 265 Client Hello
17 26.885504000 -> TLSv1 576 Server Hello, Certificate, Server Hello Done
45 101.747755000 -> SSL 265 Client Hello
49 116.258483000 -> TLSv1 576 Server Hello, Certificate, Server Hello Done
63 116.338420000 -> SSL 265 Client Hello
65 136.119127000 -> TLSv1 576 Server Hello, Certificate, Server Hello Done

This is slow SSL response is consistent with previous SSL MITM attacks conducted with support of the Great Firewall of China (GFW).

For more details on this attack, please see the Reuters story "After Gmail blocked in China, Microsoft's Outlook hacked" and GreatFire's own blog post "Outlook grim - Chinese authorities attack Microsoft".

Posted by Erik Hjelmvik on Monday, 19 January 2015 22:55:00 (UTC/GMT)


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