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#  About Certificates

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

#  Quick Setup

By far the easiest way to install the mitmproxy CA certificate is to use the built-in certificate installation app. To do this, 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:

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

#  The mitmproxy certificate authority

The first time mitmproxy is run, it creates the keys for a certificate authority (CA) in the config directory (~/.mitmproxy by default). This CA is used for on-the-fly generation of dummy certificates for each visited website. Since your browser won’t trust the mitmproxy CA out of the box, you will either need to click through a TLS certificate warning on every domain, or install the CA certificate once so that it is trusted.

The following files are created:

Filename Contents
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.

For security reasons, the mitmproxy CA is generated uniquely on the first start and is not shared between mitmproxy installations on different devices. This makes sure that other mitmproxy users cannot intercept your traffic.

#  Installing the mitmproxy CA certificate manually

Sometimes using the quick install app is not an option and you need to install the CA manually. 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.

#  Upstream Certificate Sniffing

When mitmproxy receives a request to establish TLS (in the form of a ClientHello message), it puts the client on hold and first makes a connection to the upstream server to “sniff” the contents of its TLS certificate. The information gained – Common Name, Organization, Subject Alternative Names – is then used to generate a new interception certificate on-the-fly, signed by the mitmproxy CA. Mitmproxy then returns to the client and continues the handshake with the newly-forged certificate.

Upstream cert sniffing is on by default, and can optionally be disabled by turning the upstream_cert option off.

#  Certificate Pinning

Some applications employ Certificate Pinning to prevent man-in-the-middle attacks. This means that mitmproxy’s certificates will not be accepted by these applications without modifying them. If the contents of these connections are not important, it is recommended to use the ignore_hosts feature to prevent mitmproxy 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:

Please propose other useful tools using the “Edit on GitHub” button on the top right of this page.

#  Using a custom server certificate

You can use your own (leaf) certificate by passing the --certs [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
# (Specify the mitm domain as Common Name, e.g. \*.google.com)
openssl req -new -x509 -key cert.key -out cert.crt
cat cert.key cert.crt > cert.pem

Now, you can run mitmproxy with the generated certificate:

For all domain names

mitmproxy --certs *=cert.pem

For specific domain names

mitmproxy --certs *.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 --set confdir=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.

The mitmproxy-ca.pem certificate file has to look roughly like this:

-----BEGIN PRIVATE KEY-----
<private key>
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
<cert>
-----END CERTIFICATE-----

When looking at the certificate with openssl x509 -noout -text -in ~/.mitmproxy/mitmproxy-ca.pem it should have at least the following X509v3 extensions so mitmproxy can use it to generate certificates:

X509v3 extensions:
    X509v3 Key Usage: critical
        Certificate Sign
    X509v3 Basic Constraints: critical
        CA:TRUE

For example, when using OpenSSL, you can create a CA authority as follows:

openssl req -x509 -new -nodes -key ca.key -sha256 -out ca.crt -addext keyUsage=critical,keyCertSign
cat ca.key ca.crt > mitmproxy-ca.pem

#  Mutual TLS (mTLS) and client certificates

TLS is typically used in a way where the client verifies the server’s identity using the server’s certificate during the handshake, but the server does not verify the client’s identity using the TLS protocol. Instead, the client transmits cookies or other access tokens over the established secure channel to authenticate itself.

Mutual TLS (mTLS) is a mode where the server verifies the client’s identity not using cookies or access tokens, but using a certificate presented by the client during the TLS handshake. With mTLS, both client and server use a certificate to authenticate each other.

If a server wants to verify the clients identity using mTLS, it sends an additional CertificateRequest message to the client during the handshake. The client then provides its certificate and proves ownership of the private key with a matching signature. This part works just like server authentication, only the other way around.

#  mTLS between mitmproxy and upstream server

You can use a client certificate by passing the --set 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 TLS connections. Certificate files must be in the PEM format and should contain both the unencrypted private key and the certificate.

You can specify a directory to --set client_certs=DIRECTORY, in which case the matching certificate is looked up by filename. So, 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.

#  mTLS between client and mitmproxy

By default, mitmproxy does not send the CertificateRequest TLS handshake message to connecting clients. This is because it trips up some clients that do not expect a certificate request (most famously old Android versions). However, there are other clients – in particular in the MQTT / IoT environment – that do expect a certificate request and will otherwise fail the TLS handshake.

To instruct mitmproxy to request a client certificate from the connecting client, you can pass the --set request_client_cert=True option. This will generate a CertificateRequest TLS handshake message and (if successful) establish an mTLS connection. This option only requests a certificate from the client, it does not validate the presented identity in any way. For the purposes of testing and developing client and server software, this is typically not an issue. If you operate mitmproxy in an environment where untrusted clients might connect, you need to safeguard against them.

The request_client_cert option is typically paired with client_certs like so:

mitmproxy --set request_client_cert=True --set client_certs=client-cert.pem