+---
+title: "Reverse Engineering Mobile Apps Part Two"
+date: 2024-05-15T23:21:00+03:00
+description: ""
+tags: []
+type: blog
+draft: true
+---
+
+## Note to the git snooper - I moved these here due to revalations made later, I will deep-dive these topics at a later date.
+
+## The smoke-test
+
+So we have a potential API endpoint, let's put it to the test. We're not going to recompile anything yet or do any byte-patching,
+let's try a quick smoke-test.
+
+**UPDATE 15/05/2024: It turns out this old app version uses HTTP only, instead of HTTPS. You will only need to perform the AdAway instruction here.
+As this information is still vastly useful for reverse engineering most apps, I will be leaving this section intact.**
+
+Ensure your phone is rooted and you have a variant of Xposed Framework installed (I used LSPosed).
+We will need to unarm the SSL pinning present in most apps, including this one, I used [io.github.tehcneko.sslunpinning](https://github.com/Xposed-Modules-Repo/io.github.tehcneko.sslunpinning) module.
+(NOTE: Users without root might want to skip to end of article where I showcase unpinning the app manually)
+
+Next, install [AdAway app from F-Droid](https://f-droid.org/packages/org.adaway/) so we can setup a redirection on any network we are on.
+Inside AdAway, add a redirection rule for the address we just found and point it to an IP address in your LAN that will run the API server.
+
+Generate a self-signed certificate authority and a certificate signed by it and run a webserver with both HTTP and HTTPS on the API server machine.
+```
+# Create an ext file containing the Subject Alternative Name (SAN)
+# DNS.1 should correspond to the API endpoint of the app.
+# NOTE! If you are changing the API endpoint to a public domain, you can just use a public cert, no need for any of this.
+cat > auxbrain.ext << EOF
+authorityKeyIdentifier=keyid,issuer
+basicConstraints=CA:FALSE
+keyUsage = digitalSignature, nonRepudiation, keyEncipherment, dataEncipherment
+subjectAltName = @alt_names
+
+[alt_names]
+DNS.1 = www.auxbrain.com
+EOF
+
+# Create your own Certificate Authority
+openssl genrsa -des3 -out myCA.key 2048
+openssl req -x509 -new -nodes -key myCA.key -sha256 -days 1825 -out myCA.pem
+# Create a CSR and lets have the new CA sign it
+openssl genrsa -out auxbrain.key 2048
+openssl req -new -key auxbrain.key -out auxbrain.csr -nodes
+openssl x509 -req -in auxbrain.csr -CA myCA.pem -CAkey myCA.key -CAcreateserial -out auxbrain.crt -days 825 -sha256 -extfile auxbrain.ext
+cat auxbrain.crt myCA.pem > auxbrain.pem
+# You now have:
+# myCA.pem - the public certificate of your root CA
+# auxbrain.key - the private key for your webserver
+# auxbrain.pem - the public cert for your webserver.
+```
+
+Use the generated `auxbrain.pem` and `auxbrain.key` files for your webserver SSL/TLS configuration. For nginx, append following values to your server directive:
+```
+listen 443 ssl;
+ssl_certificate /path/to/auxbrain.pem;
+ssl_certificate_key /path/to/auxbrain.key;
+ssl_session_cache shared:SSL:1m;
+ssl_session_timeout 5m;
+ssl_ciphers HIGH:!aNULL:!MD5;
+ssl_prefer_server_ciphers on;
+```
+
+Import the self-signed CA (myCA.pem) to your phone's truststore (Check under your phone's Security/Encryption settings). Once all of that is done, run the app for first time.
+
+
+## Rootless SSL Unpinning + Endpoint URL patching
+Let's make the app not require a VPN or root privileges - let's make user CAs work and the endpoint URL something we control on the public net.
+Start off by pulling the following repository
+```
+git clone https://github.com/ilya-kozyr/android-ssl-pinning-bypass.git
+python3 -m venv .venv
+source .venv/bin/activate
+pip install -r requirements.txt
+cp /path/to/your/apk .
+python3 apk-rebuild.py egginc.apk --pause
+```
+
+**NOTE!** IF you do not intend to patch the API endpoint and just want to proceed with AdAway redirecting traffic, you can stop here and press ENTER!
+Proceed only if you own a domain in your control (that is equal or less in length to www.auxbrain.com) and want to use the app without a VPN/redirection.
+
+Open a new terminal window, the script will wait for us to perform modifications, enter the created folder `egginc.apk-decompiled` and `lib`.
+
+We have two folders here now, `arm64-v8a` and `armeabi-v7a`, just as we saw when we pulled the .so file out of the apk earlier. Let's tackle
+the 64-bit build first.
+
+For arm64 build it was really simple to perform bytepatch on the said endpoint. We already know it's supposed to look as `G?www.auxbrain.com` - let's probe the .so library a bit.
+```
+$> hexdump -C libegginc.so | grep "ww.auxbrain.co" -A2 -B2
+00b02b40 cd cc 4c 3f 00 00 00 00 00 00 00 00 00 00 80 3f |..L?...........?|
+00b02b50 00 00 00 00 00 00 00 00 00 00 00 00 14 ae 47 3f |..............G?|
+00b02b60 77 77 77 2e 61 75 78 62 72 61 69 6e 2e 63 6f 6d |www.auxbrain.com|
+00b02b70 00 48 54 54 50 20 52 45 51 3a 20 25 64 00 64 61 |.HTTP REQ: %d.da|
+00b02b80 74 61 3d 00 65 69 2f 66 69 72 73 74 5f 63 6f 6e |ta=.ei/first_con
+```
+
+We seem to have nothing blocking our way, let's create hex representations of `G?www.auxbrain.com` and a target domain of equal length, for example `G?eggs.based.quest`.
+
+(Note: You can choose a shorter name as well, if you null-terminate the extra bytes as padding)
+```
+$> echo "G?www.auxbrain.com" | hexdump -ve '1/1 "%.2X"'
+473F7777772E617578627261696E2E636F6D0A
+$> echo "G?eggs.based.quest" | hexdump -ve '1/1 "%.2X"'
+473F656767732E62617365642E71756573740A
+```
+
+Remove the trailing `0A` from end of both hex strings and now proceed as follows:
+```
+# Place the source in first bracket of sed and the new URL at second bracket.
+hexdump -ve '1/1 "%.2X"' libegginc.so | sed "s/473F7777772E617578627261696E2E636F6D/473F656767732E62617365642E7175657374/g" | xxd -r -p > patched.so
+```
+
+Huzzah! We now have a patched linked-library for the arm64 build. Let's also patch the 32-bit version.
+```
+$> hexdump -C libegginc.so | grep "ww.auxbrain.co" -A2 -B2
+0087b770 69 67 68 5f 74 6f 6f 5f 6d 61 6e 79 5f 70 78 00 |igh_too_many_px.|
+0087b780 74 61 62 6c 65 74 5f 68 64 70 69 00 00 00 00 00 |tablet_hdpi.....|
+0087b790 77 77 77 2e 61 75 78 62 72 61 69 6e 2e 63 6f 6d |www.auxbrain.com|
+0087b7a0 00 00 00 00 00 00 00 00 65 69 2f 66 69 72 73 74 |........ei/first|
+0087b7b0 5f 63 6f 6e 74 61 63 74 00 00 00 00 00 00 00 00 |_contact........|
+```
+This one lacks the `G?` prefix on API endpoint, but we still have null terminators we can rely on. Let's replace the `473F` from our previous strings with `0000`.
+```
+# Place the source in first bracket of sed and the new URL at second bracket.
+hexdump -ve '1/1 "%.2X"' libegginc.so | sed "s/00007777772E617578627261696E2E636F6D/0000656767732E62617365642E7175657374/g" | xxd -r -p > patched.so
+```
+
+Replace both of the libegginc.so files with the patched.so files. Move back to main terminal window and press ENTER.
+
+We now have a patched and debug signed apk for the game that isn't SSL pinned and contains a custom API endpoint we control without a VPN.
+
projects / web-hugo.git / commitdiff