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A man-in-the-middle proxy for iOS and OS X device push connections
Python Shell
branch: master

README.md

PushProxy

PushProxy is a man-in-the-middle proxy for iOS and OS X Push Notifications. It decodes the push protocol and outputs messages in a readable form. It also provides APIs for handling messages and sending push notifications directly to devices without sending them via Apple's infrastructure.

For a reference on the push protocol, see apple-push-protocol-ios5-lion.md. iOS4 and earlier used another version of the protocol, described in apple-push-protocol-ios4.md. This proxy only supports the iOS5 protocol.

I tested only using jailbroken iOS devices, but it may be possible to use a push certificate from a jailbroken device and use it to connect a non-jailbroken device to Apple's servers. At least some apps using push notifications will be confused if you do this, but I think this was a way for hacktivated iPhones to get a push certificate.

WARNING: PushProxy doesn't check device certificates properly, so don't put it on the open internet.

'Screenshot'

2013-02-17 21:54:15+0100 [#0] New connection from 192.168.0.120:61321
2013-02-17 21:54:15+0100 [#0] SSL handshake done: Device: B481816D-4650-49EA-977C-9FCBDEB30CB1
2013-02-17 21:54:15+0100 [#0] Connecting to push server: 17.172.232.59:5223
2013-02-17 21:54:15+0100 Starting factory <icl0ud.push.intercept.InterceptClientFactory instance at 0x147d5a8>
2013-02-17 21:54:15+0100 [#0] -> APSConnect presenceFlags: 00000002 state: 01
                                   push token: 4b5543c35b48429bbe770a8af11457f39374bd4e43e94adaa86bd979c50bdb05
2013-02-17 21:54:16+0100 [#0] <- APSConnectResponse 00 messageSize: 4096 unknown5: 0002
2013-02-17 21:54:16+0100 [#0] -> APSTopics for token <none>
                                   enabled topics: 
                                     com.apple.itunesstored
                                     com.apple.madrid
                                     com.apple.mobileme.fmip
                                     com.apple.gamed
                                     com.apple.ess
                                     com.me.keyvalueservice
                                     4357ca2451b7b787caea8a603e51a1f45feaeda4
                                     com.apple.mobileme.fmf1
                                     com.apple.mediastream.subscription.push
                                   disabled topics:
                                     com.apple.store.Jolly
2013-02-17 21:54:16+0100 [#0] -> APSKeepAlive 10min carrier: 31038 iPhone4,1/6.1.1/10B145
2013-02-17 21:54:16+0100 [#0] <- APSKeepAliveResponse
2013-02-17 22:10:04+0100 [#0] <- APSNotification 4357ca2451b7b787caea8a603e51a1f45feaeda4
                                   timestamp: 2013-02-17 22:10:16.642561 expiry: 1970-01-01 00:59:59
                                   messageId: 00000000                   storageFlags: 00
                                   unknown9:  None                       payload decoded (json)
                                   {u'aps': {u'alert': u'Chrome\u2014meeee/pushproxy \xb7 GitHub\nhttps://github.com/meeee/pushproxy',
                                             u'badge': 3,
                                             u'sound': u'elysium.caf'},
                                    u'urlhint': u'1234567'}
2013-02-17 22:10:06+0100 [#0] -> APSNotificationResponse message: 00000000 status: 00

Setup

Overview

Setup on iOS >= 6.0 and OS X >= 10.8 requires several steps:

  1. Install pushproxy including dependencies
  2. Create a CA and issue certificates
  3. Install CA on device
  4. Extract and copy device certificate
  5. Create configuration bag
  6. Redirect DNS

You can find instructions on how to redirect the push connection on iOS < 6.0 and OS X < 10.8 in setup-ios5-10.7.md.

Install pushproxy and dependencies

The proxy is written in Python, I recommend setting up a virtualenv and installing the requirements:

pip install -r src/requirements.txt

Setting up PushProxy requires redirecting push connections to your server and setting up X.509 certificates for SSL authentication.

The push protocol uses SSL for client and server authentication. You need to extract the device certificate and give it to the proxy so it can authenticate itself as device against Apple. You also need to make the device trust your proxy since you are impersonating Apple's servers.

Create CA and issue certificates

Your device has to trust two certificates, so the easiest way is to create a CA and issue the certificates. This way you only need to install one CA certificate on the device.

The first hostname you need to create a SSL server certificate for:

init-p01st.push.apple.com

You will need this certificate later to sign a configuration bag.

iOS <= 6, OS X <= 10.8

You can choose the hostname of the second certificate, the push hostname. When connecting to the push hostname (Apple's default is courier.push.apple.com), apsd prepends a random number to the hostname, perhaps for load balancing and/or fault tolerance (e.g. 22-courier.push.apple.com). You need to configure a DNS server which responds to these hostnames. You can use a wildcard subdomain to redirect all host names with different numbers to the proxy, like *.push.example.com. So in this case, you would create a certificate for:

courier.push.example.com

Store the generated certificate and private key in PEM encoding at the following path:

certs/courier.push.apple.com/server.pem

iOS >= 7, OS X >= 10.9

Beginning with OS X 10.9 and probably iOS 7, apsd does some sort of certificate pinning and checks the root certificate as well as the chain length and some attributes of the leaf certificate. OS X 10.10.1 contains pinned certificates, but the checks are ineffective and a certificate with the right attributes trusted via a chain with the root in the System keychain is enough. I haven't tested this on 10.9, but the checks might also be less effective there than I thought previously.

The certificate chain needs to have a length of 3, so you have to use a CA certificate as well as an additional intermediary CA certificate that signs the leaf.

Create the leaf certificate with the following attributes:

* Common Name: courier.push.apple.com
* Country Name: US
* State/Province Name: California
* Locality Name: Cupertino
* Organization Name: Apple Inc.
* Subject Alternative Name Extension
    * DNS Name: <the hostname you put into the bag>

apsd checks the leaf certificate for these attributes. As it also does normal SSL certificate validation, the certificate must be valid for the host it connects to. This host is the name you put in the configuration bag. Luckily, we can use the Subject Alternative Name extension to make the certificate also valid for our hostname.

Store the generated certificate, the intermediary CA certificate and the private key in PEM encoding at the following path:

certs/courier.push.apple.com/server.pem

Install CA on device

You can install the CA certificate on iOS devices via Safari or iPhone Configuration Utility.

On OS X you can use keychain access to install the certificate, make sure to install it in the System keychain, not your login keychain. Mark it as trusted, Keychain Access should then display it as 'marked as trusted for all users'.

If you can't mark the CA as trusted for all users, you might have to remove the certificate from your login keychain, so you only have it in the system keychain.

Patch apsd (possibly optional)

OS X 10.9 and iOS 7 had some sort of certificate pinning implemented, but this seems to be no longer the case in OS X 10.10.1. I haven't checked for anything else, but you might try without patching first.

This patch replaces the pinned root certificate in the apsd binary with a chosen one having the same or a shorter length than the original certificate.

You can run the script with the following command. When running the script, think about making a backup and make sure to restore permissions afterwards.

setup/osx/patch_apsd.py <path to apsd> <new root CA cert> <new intermediate CA cert> <code signing identity>

<path to apsd>: Path to the apsd binary. Usually stored in `/System/Library/PrivateFrameworks/ApplePushService.framework/apsd`. You might need to copy it/change permissions to patch as a user.
<new root ca cert>: Path to a root certificate in DER form to replace the original root certificate by Entrust. This certificate must be no longer than 1120 bytes (length of the original certificate, file size, not key length). Shorter is ok, the rest will be zero-padded and the certificate size will be adjusted in the code.
<new intermediate CA cert> same as for new root cert, only for the intermediate cert
<code signing identity>: Name of a code signing certificate understood by the `codesign` utility, make sure your machine trusts this cert (root)

Make sure to do this before extracting the device certificate. Once you replaced the apsd binary, the keychain will not allow the apsd daemon to use the existing keychain any more and returns the following or a similar error:

The operation couldn’t be completed. (OSStatus error -25293.)

When you restart/run apsd afterwards (kill or launchctl), after a few failing attempts to access the keychain, apsd will request a new push certificate, which you can then extract as describe in the following section.

Extract and copy device certificate

Extract iOS Certificates

First, download the nimble tool, extract PushFix.zip and place nimble into setup/ios. The following script will copy the tool to your iOS device, run it and copy the extracted certificates back to your computer. It assumes you have SSH running on your device. I recommend setting up key-based authentication, otherwise you will be typing your password a few times.

Make sure you are in the pushproxy root directory, otherwise the script will fail.

cd pushproxy
setup/ios/extract-and-convert-certs.sh root@<device hostname>

You can find the extracted certificates in certs/device. Both public and private key are in one PEM-file.

Alternative to nimble (Keychain-Dumper)

On some newer devices, nimble might refuse to start with a message like Illegal instruction: 4.

Thanks to angelovAlex for having the idea and documenting how to use Keychain-Dumper, here's an alternative using a fork of Keychain-Dumper:

To extract certificate from iOS device you need to download and compile Keychain-Dumper (modified version is here: https://github.com/reinitialized/Keychain-Dumper) and a binary should be signed. There's an instruction of how to sign the binary using self-signed certificate in Keychain-Dumper's README. Upload this binary to device and run it with -k:

./KeychainDumper_signed -k

Find the key where label is APSClientIdentity and copy the key

Key
---
Entitlement Group: com.apple.apsd
Label: APSClientIdentity
Application Label: <XXXX>
Key Class: Private
Permanent Key: True
Key Size: 1024
Effective Key Size: 1024
For Encryption: False
For Decryption: True
For Key Derivation: True
For Signatures: True
For Signature Verification: False
For Key Wrapping: False
For Key Unwrapping: True
-----BEGIN RSA PRIVATE KEY-----
XXXX
-----END RSA PRIVATE KEY-----

Then run this tool with -i and copy certificate

./KeychainDumper_signed -i
Identity
--------
Certificate
-----------
Summary: XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXX                   // IDENTIFIER ! ! !
Entitlement Group: com.apple.apsd
Label: APSClientIdentity
Serial Number: XXXX
Subject Key ID: XXXX
Subject Key Hash: XXXX
-----BEGIN CERTIFICATE-----
XXXX
-----END CERTIFICATE-----

Put key and certificate in one file and rename it to the IDENTIFIER.pem and put this file in certs/device.

Extract OS X Certificates

Note: If you want to connect at least one device via a patched push daemon, you need to patch the push daemon on OS X first.

OS X stores the certificates in a keychain in /Library/Keychains, either in applepushserviced.keychain or in apsd.keychain.

This step extracts the push private key and certificate from the keychain. It stores them in certs/device/<UUID>.pem

setup/osx/extract_certificate.py -f

You can remove the -f parameter to get key and certificate on stdout instead of writing them to a file.

Create Configuration Bag

Since iOS 6 and OS X 10.8, apsd loads a signed configuration bag. This bag contains the push domain to connect to among a number of less interesting parameters.

Apple's original bag can be found here: http://init-p01st.push.apple.com/bag (Download)

Run the following command to create a bag for your own domain:

setup/bag.py <push domain> <signing certificate> > bag

push domain: The domain apsd should connect to, e.g. courier.push.example.com. apsd then actually connects to this domain prepended with a random number between 1 and 50, e.g. 22-courier.push.example.com.

signing certificate: the previously created server certificate for init-p01st.push.apple.com. The script signs the bag using this certificate and includes it, so apsd can verify the signature.

You can either upload this bag to your own webserver or run the setup/bag.py command with a -s switch at the end. It starts a webserver on port 80, so you have to run the command as root. The webserver requires flask, which can be installed via pip install flask.

Redirect DNS

You can choose whatever method you want to redirect DNS, pushproxy includes a script to generate an /etc/hosts file. You can run it using the following command:

setup/generate-hosts-file.py <webserver ip> > hosts

webserver ip: IP of your webserver that serves the configuration bag. apsd uses init-p01st.push.apple.com as HTTP host, so you can use a vhost for that domain if you want.

When apsd fails to load the configuration bag, it uses the old iOS 5/OS X 10.7 method as fallback. Thus the generate-hosts-file command redirects all these hosts to 127.0.0.1 to ensure apsd only connects to pushproxy.

You obviously need to copy the generated hosts file to the device.

Running

cd src
./runpush.sh

This should be enough for most cases, if you want it to run as daemon and write output to a logfile in data/, create an empty file in src:

touch production

If you want to change some configuration, just edit `pushserver.py.

API

Send notifications

PushProxy offers a Twisted Perspective Broker API for sending push notification directly to connected devices. It has the following signature:

remote_sendNotification(self, pushToken, topic, payload)

It doesn't implement the store part of the store-and-forward architecture Apple's push notifiction system implements, so notifications sent via this API for will be lost for offline devices.

See src/icl0ud/push/notification_sender.py for the implementation.

Message handler

You can subclass icl0ud.push.dispatch.BaseHandler, look at dispatch.py, pushtoken_handler.py and notification_sender.py in src/icl0ud/push.

Handlers can be configured in src/pushserver.py

Debugging

Apple provides a document on debugging push connections. Especially useful are their instructions on how to enable debug logging on iOS and OS X. You can find the download link to the configuration file for iOS in the upper right corner of the page.

The document is a bit outdated. If you want to enable debug logging on newer OS X versions like 10.8.2, you have to replace applepushserviced with apsd in the defaults and killall commands.

Troubleshooting certificate trust problems

  • The push server certificate must always have a common name (CN) and the other attributes as specified above, even when the host in the bag is another one. The host in the bag has to be in the Subject Alternative Name extension. Since OS X 10.10 or possibly earlier, apsd compares the cert's CN against the hostname it connects to, in addition to an explicit check against courier.push.apple.com and courier.sandbox.push.apple.com.

  • If the logs don't give you enough information and you only see an error message like the following, you can use dtrace for debugging the verification in detail.

    apsd[xx]: Failed to evaluate trust: No error. (0), result=5; retrying with revocation checking optional
    apsd[xx]: Failed to evaluate trust: No error. (0), result=5; retrying with system roots
    apsd[xx]: Failed to evaluate trust: No error. (0), result=5
    

Limitations

Device certificates are not checked

The intermediate certificate (subject=/C=US/O=Apple Inc./OU=Apple iPhone/CN=Apple iPhone Device CA) between Apple's root CA and the push device certificates expired on Apr 16 22:54:46 2014 GMT and I couldn't find a replacement. For a tool that is probably not usually used on the open internet, I thought implementing a workaround was too much effort, so I disabled device certificate checking.

PushProxy only checks there's a device certificate in in certs/device/<common name>.pem and uses it to connect to Apple servers. Notice that the device certificate common names are transmitted in plain text, so they can't be considered secret. A connection with a fake certificate would fail later when pushproxy tries to connect to an Apple server, which hopefully does proper certificate validation. Nevertheless, an attacker could make PushProxy connect to Apple servers with a lot of fake certificates from your server, which you might not want. There might also be other implications I haven't thought about.

Contributors

  • Michael Frister
  • Martin Kreichgauer

Thanks

  • Matt Johnston for writing extractkeychain that is used for deriving the master key and decrypting keys from the OS X keychain
  • Vladimir "Farcaller" Pouzanov for writing python-bplist which is included and helps extracting the push private key from the OS X keychain
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