Loading iOS binaries

May 19, 2014

There are several posts explaining the process to decrypt an iOS app, this is not new, but no one explained the instruction to do it with r2.

We have no aim in promoting piracy or cracking, but that’s the only way to analyze applications from the AppleStore.

Retrieving information

First of all you need a jailbroken device, in Cydia add the cydia.radare.org repository, in order to get the latest radare2 package from there. Once installed, ssh into the device and type the following commands to retrieve the encryption information stored in the MACH0 header.

$ export APP=/var/mobile/Applications/*/Target.app/Target
$ rabin2 -k 'info/*' $APP
uuid.0=034324c306b633909dbd0f229ceae7c9
crypto=true
cryptid=0x1
cryptoff=0x4000
cryptsize=0x6e4000

At the moment, only few information are stored in SDB, but this will change during next months. You may also access this information from the r2 shell this way:

$ r2 $APP
[0x00004320]> k bin/cur/info/*

Let’s create a simple r2 script to add two flags to store cryptoff and *cryptsiz*e variables that we may use later for dumping and patching.

$ cat > bin.r2 <<EOF
f cryptoff=0x4000
f cryptsize=0x6e4000
EOF

Dumping the memory from the debugger can be done with the following commands:

$ r2 -i bin.r2 -d $APP
[0x2be78028]> dm
sys 0x00000000 - 0x00001000 s --- --- 00 share/priv/not-reserved
sys 0x0002d000 - 0x00031000 s r-x r-x 01 copy/priv/not-reserved
sys 0x00031000 - 0x00715000 s r-x r-x 02 copy/priv/not-reserved
sys 0x00715000 - 0x00889000 s rw- rw- 03 copy/priv/not-reserved
sys 0x00889000 - 0x0088d000 s rw- rw- 04 copy/priv/not-reserved
sys 0x0088d000 - 0x00909000 s r-- r-- 05 copy/priv/not-reserved
sys 0x27cd7000 - 0x27cd8000 s --- rwx 06 copy/priv/not-reserved
sys 0x27cd8000 - 0x27dd7000 s rw- rwx 07 copy/priv/not-reserved
sys 0x2be77000 * 0x2be98000 s r-x r-x 08 copy/priv/not-reserved
sys 0x2be98000 - 0x2be99000 s rw- rw- 09 copy/priv/not-reserved
sys 0x2be99000 - 0x2bec1000 s rw- rw- 0a copy/priv/not-reserved
sys 0x2bec1000 - 0x2becf000 s r-- r-- 0b copy/priv/not-reserved
sys 0x2c000000 - 0x3c000000 s r-- rwx 0c share/priv/reserved
sys 0x3c000000 - 0x40000000 s r-- rwx 0d share/priv/reserved
[0x2be78028]> s `dm~r-x:0[1]`
[0x00040000]> s+cryptoff
[0x00044000]> wt dump.bin cryptsize
dumped 0x6e4000 bytes

Patching the binary

Once we get the dump we will want to patch the program. We will ignore the RBin information (-n) and enable the write mode option (-w). This will load the file in ‘raw’ and we will patch with the decrypted block.

$ cp -f $APP bin
$ r2 -i bin.r2 -nw bin
[0x00000000]> wf dump.bin @ cryptoff

We will have to patch the binary to mark it as unencrypted. Setting cryptid to 0.

[0x00000000]> /v cryptsize
[0x00000000]> s hit0_0+4
[0x00000a08]> wx 00

Loading Objective-C metadata

Most iOS apps are written in ObjC, and all the symbolic information are stored there, this is very useful to understand the app workflow and design.

At the moment, the way to load ObjC information into radare2 is by using the objc.pl script which makes uses of class-dump.

$ ~/radare2/doc/objc.pl bin > bin.objc.r2

Or directly from the r2 shell

[0x00004000]> .!./objc.pl $FILE

Extracting sub-bins:

As a final note, just say that you may find iOS apps in MACH0 or FATMACH0 formats, in the last case we will be interested in extracting and analyzing only the one that fits best to our needs.

For example, inside a FATMACH0 iOS app you can find ARMv6, ARMv7 and ARM64 executables. The tool rabin2 can be used to list all sub-binaries contained inside a FATMACH0 file using the -A flag:

$ rabin2 -A bin

Also we can use the -x flag to extract them all.

$ rabin2 -x bin

Or we may need to specify which sub-bin we want to load by using the r2 -a <arch> -b <bits> flags when loading a FATMACH0.