LDNWebPerf December 2017 - Andy Davies

Inspecting iOS App Traffic with JavaScript

A session presented by Andy Davies
Tuesday, 5th December 2017

Skill level: Intermediate / Advanced

What do you do when you've been asked to look at an app that's slow but you can't get the source code?

Andy will show you what he did when he was faced with this situation and yes he really did use JavaScript!

Transcript

1. Inspecting iOS App Traffic with JavaScript
   @AndyDavies
   https://www.flickr.com/photos/marc-flores/8367323660
   

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2. WHY?
   

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3. DEMO
   

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4. http://www.delaat.net/rp/2015-2016/p52/report.pdf
   

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5. Inject JavaScript into an App!
   https://www.frida.re/
   App
   JavaScript VM
   Script
   Host
   Injects script
   Receives Messages
   

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6. https://opensource.apple.com/source/coreTLS/coreTLS-83.20.8/lib/tls1Callouts.c.auto.html
   /*
   * The TLS pseudorandom function, defined in RFC2246, section 5.
   * This takes as its input a secret block, a label, and a seed, and produces
   * a caller-specified length of pseudorandom data.
   *
   * Optimization TBD: make label optional, avoid malloc and two copies if it's
   * not there, so callers can take advantage of fixed-size seeds.
   */
   // Note: This is exported as SPI.
   int tls_handshake_internal_prf(
   tls_handshake_t ctx,
   const void *vsecret,
   size_t secretLen,
   const void *label, // optional, NULL implies that seed contains
   // the label
   size_t labelLen,
   const void *seed,
   size_t seedLen,
   void *vout, // mallocd by caller, length >= outLen
   size_t outLen)
   {
   int serr = errSSLInternal;
   …
   Master Secret
   Client & Server
   Randoms
   

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7. var hexChar = ["0", "1", "2", "3", "4", "5", "6", "7","8", "9", "A", "B", "C", "D", "E", "F"];
   function byteToHex(byte) {
   return hexChar[(byte >> 4) & 0x0f] + hexChar[byte & 0x0f];
   }
   var f = Module.findExportByName("libsystem_coretls.dylib", "tls_handshake_internal_prf");
   Interceptor.attach(f, {onEnter: function (args) {
   var secretLength = parseInt(args[2], 16);
   var seedLength = parseInt(args[6], 16);
   if(secretLength == 48 && (seedLength == 64 || seedLength == 77)) {
   var secretAddr = new NativePointer(args[1])
   var secretBytes = new Uint8Array(Memory.readByteArray(secretAddr, secretLength));
   var secret = "";
   for(var i = 0; i < secretLength; i++) {
   secret += byteToHex(secretBytes[i]);
   }
   Find function
   Hook function
   Extract master secret
   

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8. var seedLength = parseInt(args[6], 16);
   var seedAddr = new NativePointer(args[5]);
   var seedBytes = new Uint8Array(Memory.readByteArray(seedAddr, seedLength));
   var clientRandom = "";
   var serverRandom = "";
   if(seedLength == 64) {
   for(i = 0; i < 32; i++) {
   clientRandom += byteToHex(seedBytes[i]);
   }
   for( ; i < 64; i++) {
   serverRandom += byteToHex(seedBytes[i]);
   }
   }
   else if(seedLength == 77) { // key expansion
   var offset = 13;
   for(i = offset; i < 32 + offset; i++) {
   serverRandom += byteToHex(seedBytes[i]);
   }
   for( ; i < 64 + offset; i++) {
   clientRandom += byteToHex(seedBytes[i]);
   }
   }
   Extract client and
   server randoms
   

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9. if(clientRandom !== "") {
   send("CLIENT_RANDOM "+ clientRandom + " " + secret);
   }
   }
   }
   });
   Send it to the host
   

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10. DEMO
    

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11. Areas that need more work
    ★ TLS Session Resumption
    ★ Safari
    ★ iOS 11
    

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12. So what did I learn?
    ★ Just like on the web… sometimes we forget to
    Compress JSON responses
    Reuse connections
    Optimise images
    And a whole bunch of other things
    

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13. https://www.flickr.com/photos/willypayne/3116394973
    Thank You
    &
    Happy Christmas!
    

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