certmitm

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A tool for testing for certificate validation vulnerabilities of TLS connections made by a client device or an application.

Created by Aapo Oksman - https://github.com/AapoOksman/certmitm - MIT License

Published in DEF CON 31 on August 11 2023

YouTube: DEF CON 31 - certmitm: automatic exploitation of TLS certificate validation vulnerabilities - Aapo Oksman

DEF CON 31 certmitm Slides.pdf

DEF CON 31 certmitm Demo.mp4

Installation

certmitm has been tested on Debian 11 and Debian 12. It should work on Linux with Python 3.10 and up

Install required python packages with

pip install -r requirements.txt

Obtain real certificates such as a Let's Encrypt certificate and save them to real_certs

Usage

First you need to intercept TLS connections. The easies way for this is to configure the computer running certmitm to act as a router for other devices in the network.

For example:

1. Start a DHCP/DNS server

sudo ip addr add 10.0.0.1/24 dev eth0
sudo dnsmasq --no-daemon --interface eth0 --dhcp-range=10.0.0.100,10.0.0.200 --log-dhcp --log-queries --bind-interfaces -C /dev/null

2. Intercept TLS connections from the clients and redirect other connections to the internet through the WLAN interface

sudo iptables -A INPUT -i eth0 -j ACCEPT
sudo iptables -t nat -A PREROUTING -i eth0 -p tcp -m tcp --dport 443 -j REDIRECT --to-ports 9900
sudo iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE

3. Start certmitm

python3 certmitm.py --listen 9900 --workdir testing --verbose --show-data

4. Connect clients to the network and start applications. Note that you might need to retry running the applications a couple of times while the tests fail.

You should see any intercepted connections in the output of certmitm as CRITICAL. Additionally, if you run certmitm with the --verbose/-v flag, you will see also properly secured connections as INFO. Below you can see an example where connections to google.com are secured properly but connections to www.google.com can be intercepted.

test@certmitm$ python3 certmitm.py -v -l 9900
INFO - 10.0.0.140: 216.58.211.238:443:google.com for test self_signed = [SSL: TLSV1_ALERT_UNKNOWN_CA] tlsv1 alert unknown ca (_ssl.c:992)
INFO - 10.0.0.140: 216.58.211.238:443:google.com for test replaced_key = [SSL: TLSV1_ALERT_DECRYPT_ERROR] tlsv1 alert decrypt error (_ssl.c:992)
INFO - 10.0.0.140: 216.58.211.238:443:google.com for test real_cert_letsencrypt = Nothing received
INFO - 10.0.0.140: 216.58.211.238:443:google.com for test real_cert_CA_letsencrypt = [SSL: TLSV1_ALERT_UNKNOWN_CA] tlsv1 alert unknown ca (_ssl.c:992)
CRITICAL - 10.0.0.140: 216.58.209.164:443:www.google.com for test self_signed = data intercepted!
CRITICAL - 10.0.0.140: 216.58.209.164:443:www.google.com for test replaced_key = data intercepted!
CRITICAL - 10.0.0.140: 216.58.209.164:443:www.google.com for test real_cert_letsencrypt = data intercepted!
CRITICAL - 10.0.0.140: 216.58.209.164:443:www.google.com for test real_cert_CA_letsencrypt = data intercepted!

You can also recreate these secure and insecure connections with curl on a Linux machine connected to certmitm. You can also see that curl gives different errors for different certmitm tests.

test@victim:~$ curl https://google.com
curl: (60) SSL certificate problem: unable to get local issuer certificate
More details here: https://curl.se/docs/sslcerts.html

curl failed to verify the legitimacy of the server and therefore could not
establish a secure connection to it. To learn more about this situation and
how to fix it, please visit the web page mentioned above.
test@victim:~$ curl https://google.com
curl: (35) error:0407008A:rsa routines:RSA_padding_check_PKCS1_type_1:invalid padding
test@victim:~$ curl https://google.com
curl: (60) SSL: no alternative certificate subject name matches target host name 'google.com'
More details here: https://curl.se/docs/sslcerts.html

curl failed to verify the legitimacy of the server and therefore could not
establish a secure connection to it. To learn more about this situation and
how to fix it, please visit the web page mentioned above.
test@victim:~$ curl https://google.com
curl: (60) SSL certificate problem: unable to get local issuer certificate
More details here: https://curl.se/docs/sslcerts.html

curl failed to verify the legitimacy of the server and therefore could not
establish a secure connection to it. To learn more about this situation and
how to fix it, please visit the web page mentioned above.
test@victim:~$ curl https://google.com
<HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8">
<TITLE>301 Moved</TITLE></HEAD><BODY>
<H1>301 Moved</H1>
The document has moved
<A HREF="https://www.google.com/">here</A>.
</BODY></HTML>
test@victim:~$ curl --insecure https://www.google.com
curl: (52) Empty reply from server
test@victim:~$ curl --insecure https://www.google.com
curl: (52) Empty reply from server
test@victim:~$ curl --insecure https://www.google.com
curl: (52) Empty reply from server
test@victim:~$ curl --insecure https://www.google.com
curl: (52) Empty reply from server
test@victim:~$ curl --insecure https://www.google.com
<!doctype html><html itemscope="" itemtype="http://schema.org/WebPage" lang="fi"><head><meta content="text/html;

Hall of fame

List of publicly disclosed vulnerabilities found with certmitm. Open an issue if you have found a vulnerability with certmitm and want to be included.