About Certificates

Introduction

Mitmproxy can decrypt encrypted traffic on the fly, as long as the client trusts its built-in certificate authority. Usually this means that the mitmproxy CA certificates have to be installed on the client device.

Quick Setup

By far the easiest way to install the mitmproxy certificates is to use the built-in certificate installation app. To do this, just start mitmproxy and configure your target device with the correct proxy settings. Now start a browser on the device, and visit the magic domain mitm.it. You should see something like this:

_images/certinstall-webapp.png

Click on the relevant icon, follow the setup instructions for the platform you’re on and you are good to go.

Installing the mitmproxy CA certificate manually

Sometimes using the quick install app is not an option - Java or the iOS Simulator spring to mind - or you just need to do it manually for some other reason. Below is a list of pointers to manual certificate installation documentation for some common platforms.

The mitmproxy CA cert is located in ~/.mitmproxy after it has been generated at the first start of mitmproxy.

Windows (automated)

>>> certutil.exe -importpfx Root mitmproxy-ca-cert.p12

See also: https://technet.microsoft.com/en-us/library/cc732443.aspx

The mitmproxy certificate authority

The first time mitmproxy or mitmdump is run, the mitmproxy Certificate Authority (CA) is created in the config directory (~/.mitmproxy by default). This CA is used for on-the-fly generation of dummy certificates for each of the SSL sites that your client visits. Since your browser won’t trust the mitmproxy CA out of the box, you will see an SSL certificate warning every time you visit a new SSL domain through mitmproxy. When you are testing a single site through a browser, just accepting the bogus SSL cert manually is not too much trouble, but there are a many circumstances where you will want to configure your testing system or browser to trust the mitmproxy CA as a signing root authority. For security reasons, the mitmproxy CA is generated uniquely on the first start and is not shared between mitmproxy installations on different devices.

Certificate Pinning

Some applications employ Certificate Pinning to prevent man-in-the-middle attacks. This means that mitmproxy and mitmdump’s certificates will not be accepted by these applications without modifying them. It is recommended to use the Ignore Domains feature in order to prevent mitmproxy and mitmdump from intercepting traffic to these specific domains. If you want to intercept the pinned connections, you need to patch the application manually. For Android and (jailbroken) iOS devices, various tools exist to accomplish this.

CA and cert files

The files created by mitmproxy in the .mitmproxy directory are as follows:

mitmproxy-ca.pem The certificate and the private key in PEM format.
mitmproxy-ca-cert.pem The certificate in PEM format. Use this to distribute on most non-Windows platforms.
mitmproxy-ca-cert.p12 The certificate in PKCS12 format. For use on Windows.
mitmproxy-ca-cert.cer Same file as .pem, but with an extension expected by some Android devices.

Using a custom certificate

You can use your own (leaf) certificate by passing the --cert [domain=]path_to_certificate option to mitmproxy. Mitmproxy then uses the provided certificate for interception of the specified domain instead of generating a certificate signed by its own CA.

The certificate file is expected to be in the PEM format. You can include intermediary certificates right below your leaf certificate, so that your PEM file roughly looks like this:

-----BEGIN PRIVATE KEY-----
<private key>
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
<cert>
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
<intermediary cert (optional)>
-----END CERTIFICATE-----

For example, you can generate a certificate in this format using these instructions:

>>> openssl genrsa -out cert.key 2048
>>> openssl req -new -x509 -key cert.key -out cert.crt
    (Specify the mitm domain as Common Name, e.g. *.google.com)
>>> cat cert.key cert.crt > cert.pem

Now, you can run mitmproxy with the generated certificate:

For all domain names

>>>mitmproxy --cert *=cert.pem

For specific domain names

>>>mitmproxy --cert *.example.com=cert.pem

Note: *.example.com is for all the subdomains. You can also use www.example.com for a particular subdomain.

Using a custom certificate authority

By default, mitmproxy will use ~/.mitmproxy/mitmproxy-ca.pem as the certificate authority to generate certificates for all domains for which no custom certificate is provided (see above). You can use your own certificate authority by passing the --cadir DIRECTORY option to mitmproxy. Mitmproxy will then look for mitmproxy-ca.pem in the specified directory. If no such file exists, it will be generated automatically.

Using a client side certificate

You can use a client certificate by passing the --client-certs DIRECTORY|FILE option to mitmproxy. Using a directory allows certs to be selected based on hostname, while using a filename allows a single specific certificate to be used for all SSL connections. Certificate files must be in the PEM format and should contain both the unencrypted private key and the certificate.

Multiple certs by Hostname

If you’ve specified a directory to --client-certs, then the following behavior will be taken:

If you visit example.org, mitmproxy looks for a file named example.org.pem in the specified directory and uses this as the client cert.