RESEARCH | November 19, 2015

Breaking into and Reverse Engineering iOS Photo Vaults

By Michael Allen

Every so often we hear stories of people losing their mobile phones, often with sensitive photos on them. Additionally, people may lend their phones to friends only to have those friends start going through their photos. For whatever reason, a lot of people store risqué pictures on their devices. Why they feel the need to do that is left for another discussion. This behavior has fueled a desire to protect photos on mobile devices.

One popular option are photo vault applications. These applications claim to protect your photos, videos, etc. In general, they create albums within their application containers and limit access with a passcode, pattern, or, in the case of newer devices, TouchID.

I decided to take a look at some options for the iOS platform. I did a quick Google search for “best photo vaults iOS” and got the following results:

Figure 1: Search results for best iOS photo vaults

I downloaded the free applications, as this is what most typical users would do. This blog post discusses my findings.

Lab Setup

Jailbroken iPhone 4S (7.1.2)
BurpSuite Pro
Hex editor of choice
Cycript

Applications Reviewed

Private Photo Vault
Photo+Video Vault Keep Safe(My Media)
KeepSafe

I will cover the common techniques I used during each iOS application assessment. Additionally, I will take a deeper look at the Private Photo Vault application and get into some actual reverse engineering. The techniques discussed in the reverse engineering section can be applied to the other applications (left as an exercise for the reader).

Note: Unless otherwise stated, all commands are issued from Mac OS after having ssh’d into the jailbroken mobile device.

Private Photo Vault

The first application I looked at was Private Photo Vault. As it turns out, there have been several write-ups about the security, or lack thereof, of this application (see the references section for additional details). Because those write ups were published some time ago, I wanted to see if the developers had corrected the issues. As it turns out, the app was still vulnerable to some of the issues at the time of writing this post.

Bypassing the Lock Screen

When the app launches, the user is presented with the following screen:

Figure 2: Private Photo Vault Lock Screen

Bypassing this lock screen was trivial. The first step is to identify the Photo Vault process and then attach to it with cycript. For those not familiar with cycript, the following is taken from the cycript.org site:

“Cycript allows developers to explore and modify running applications on either iOS or Mac OS X using a hybrid of Objective-C++ and JavaScript syntax through an interactive console that features syntax highlighting and tab completion.”

To identify the Photo Vault process we issue the following commands:

ps –e | grep “Photo”

And then attach to it as shown:

cycript –p PhotoVault

Figure 3: Attaching to PhotoVault process with cycript

We then print the current view hierarchy using recursiveDescription by issuing the following command:

[[UIApp keyWindow] recursiveDescription]

Figure 4: Current View Hierarchy with recursiveDescription

Reviewing the current view hierarchy, we see a number of references to PasswordBox. Let’s see if we can alter what is being displayed. To do that we can issue the following command:

[#viewAddress setHidden:YES].

In this case we are testing the first PasswordBox at 0x15f687f0.

Figure 5: Hiding the first password input box

After executing the command we note that the first PasswordBox disappeared:

Figure 6: First password box hidden

Great, we can alter what is being displayed. Having obtained the current view, we now need to determine the controller. This can be accomplished with nextResponder as shown below. See references for details on nextResponder and the whole MVC model.

In a nutshell we keep calling nextResponder on the view until we hit a controller. In our case, we start with [#0x15f687f0 nextResponder] which returns <UIView: 0x15f68790> and then we call nextResponder on that like [#0x15f68790 nextResponder]. Eventually we hit a controller as shown:

Figure 7: Determining the View Controller

The WhiteLockScreenViewController controller seems to be responsible for the view. Earlier I dumped the class information with class-dump-z (not shown here). Examining the class revealed an interesting method called dismissLockScreen. Could it be that easy? As it turned out, it was. Simply calling the method as shown below was enough to bypass the lock screen:

Figure 8: Bypassing the lock screen with dismissLockScreen

Insecure Storage

During the setup I entered a passcode of 1337 to “protect” my photos. The next step, therefore, was to determine how the application was storing this passcode. Browsing the application directory revealed that the passcode was being stored in plaintext in the com.enchantedcloud.photovault.plistfile in /var/mobile/Applications/A025EF5F-ED84-4D82-A23D-BBCFE183F539/Library/Preferences.

As a side note I wonder what would be the effect of changing “Request Pin = 0”? It may be worth investigating further.

Figure 9: Passcode stored in plaintext

Not only is the PIN stored in plaintext, but the user has the option to password protect their albums. This again is stored in plaintext in the Albums.plist plist file.

Figure 10: Album password stored in plaintext

No Encryption

As if this wasn’t bad enough, the stored photos were not encrypted. This has already been pointed out elsewhere. Surprisingly enough, it remains the case as at the time of this writing. The pictures are stored in the /0 directory:

Figure 11: Location of unencrypted photos

Figure 12: Unencrypted view of stored photo

Reverse Engineering Private Photo Vault

Ok, so we have seen how to bypass the lock screen using cycript. However, let’s say that you wanted to go “under the hood “in order to achieve the same goal.

You could achieve this using:

IDA Pro/Hopper
LLDB
Usbmuxd
Debugserver

Environment Setup

Attach the mobile device to your host machine and launch tcprelay (usbmuxd directory) using the following syntax:

tcprelay.py -t <remote-port-to-forward><local-port-to-listen>

Figure 13: Launching tcprelay for debugging via USB

Tcprelay allows us to debug the application over USB as opposed to wifi, which can often be slow.

SSH into the device, start the debugserver, and attach to the PhotoVault process. Issue the following command:

debugserver *:8080 –a “PhoyoVault”

Figure 14: Launching debugserver on the mobile device

Launch lldb from your host machine, and connect to the listening debug server on the mobile device by issuing the following command from within lldb:

process connect connect://localhost:8080

Figure 15: Connecting to debugserver with lldb

Decrypt Binary

Now that the environment is configured, we can start debugging. Decrypt the binary and open it in IDA. I used the following command to decrypt the binary:

DYLD_INSERT_LIBRARIES=dumpdecrypted_armv7.dylib /var/mobile/Applications/A025EF5F-ED84-4D82-A23D-BBCFE183F539/PhotoVault.app/PhotoVault mach -o decryption dumper

Figure 16:Decrypting the PhotoVault binary

At this point I usually check to see if the binary is a FAT binary (i.e. it supports many devices). The command for that is: otool –hV PhotoVault.decrypted

Figure 17: Determining if the binary is a FAT binary

If it is indeed a FAT binary, I strip the architecture for my device. Given I am running an iPhone 4S, I ran the following command:

lipo -thin armv7 -output PhotoVault-v7 PhotoVault.decrypted.

The PhotoVault-v7 binary is what we will analyze in IDA or Hopper (see the reference section for instructions on configuring lldb and debugserver).

Earlier we established that WhiteLockScreenViewController was the controller responsible for the lock screen. This will become important in a bit. If we enter an incorrect password, the application prints “Wrong Passcode – Try Again”. Our first step, therefore, is to find occurrences of this in IDA.

Figure 18: Searching for occurrences of “Wrong Passcode”

We see several occurrences, but any reference to WhiteLockScreenViewController should be of interest to us. Let’s investigate this more. After a brief examination we discover the following:

Figure 19: Examining WhiteLockScreenViewController customKeyboardButtonPressed

At 0013A102, the lockScreen:isPasswordCorrect method is called, and at 0013A114 the return value is checked. At 0013A118 a decision is made on how to proceed. If we entered an incorrect passcode, the application will branch to loc_13A146 and display “Wrong Passcode” as shown in the snippet below.

Figure 20: Wrong Passcode branch we need to avoid

Obviously, we want the application to branch to 0013A11A, because in that branch we see references to a selRef_lockScreen_didEnterCorrectPassword_ method.

Figure 21: Branch we are interested in taking

Let’s set breakpoint at 0013A110, right before the check for the return value is done. Before we can do that, we need to account for ASLR, so we must first determine the ASLR Offset on the device.

Figure 22: Breakpoint will set at 0013A110

To accomplish this we issue the following command from within our lldb session:

image list –o -f

The image is loaded at 0xef000 so the address we need to break on is the 0x000ef000 + 0013A110.

Figure 23: Determining ASLR offset

We set a breakpoint at 0x229110 with the following command: br s -a 0x229110.

From this point breakpoints will be calculated as ASLR Offset + IDA Offset.

Figure 24: Setting the breakpoint

With our breakpoint set, we go back to the device and enter an arbitrary passcode. The breakpoint is hit and we print the value of register r8 with p $r8.

Figure 25: Examining the value in register r8

Given the value of zero, the TST.W R8, #0xFF instructions will see us taking the branch that prints “Wrong Passcode”. We need to change this value so that we take the other branch. To do that we issue the following command: register write r8 1

Figure 26: Updating the r8 register to 1

The command sets the value of register r8 to 1. We are almost there. But before we continue, let’s examine the branch we have taken. In that branch we see a call to lockScreen_didEnterCorrectPassword. So again, we search IDA for that function and the results show it references the PasswordManager class.

Figure 27: Searching IDA for occurences of lockScreen_didEnterCorrectPassword

We next examine that function where we notice several things happening:

Figure 28: Examining lockScreen_didEnterCorrectPassword

In short, in this block the application reads the PIN stored on the device and compares it to the value we entered at 0012D498. Of course this check will fail, because we entered an incorrect PIN. Looking further at this function revealed a call to the familiar dismissLockScreen. Recall this is the function we called in cycript earlier.

Figure 29: Branch with dismissLockScreen

It seems then that we need to take this branch. To do that, we change the value in r0 so that we get our desired result when the TST instruction is called. This ends up tricking the application into thinking we entered the correct PIN. We set a breakpoint at 0012D49C (i.e. the point at which we test for the return value from isEqualToString). Recall we are still in the lockScreen_didEnterCorrectPassword method of the PasswordManager class.

Figure 30: Point at which the PIN is verified

We already know the check will fail, since we entered an incorrect PIN, so we update the value in r0 to 1.

Figure 31: Updating r0 register to 1 to call dismissLockScreen

When we do that and continue execution, the dismissLockScreen method is called and the lock screen disappears, granting us access to the photos. So to recap the first patch allows us to call lockScreen_didEnterCorrectPassword. And then the second patch allows us to call dismissLockScreen, all with an arbitrary pin.

Let’s now look at the other applications.

My Media

Insecure Webserver

This was a very popular application in the app store. However, it suffered from some of the same vulnerabilities highlighted previously. What is interesting about this application is that it starts a web server on port 5555, which essentially allows users to manage their albums.

Figure 32: Web server started on port 5555

Browsing to the address results in the following interface:

Figure 33: My Media Wifi Manager

The first thing I noticed was that there was no authentication. Anybody on the same network could potentially gain access to the user’s albums. Looking a little deeper revealed that the application was vulnerable to common web application vulnerabilities. An example of this is stored cross-site scripting (XSS) in the Album name parameter:

Figure 34: Stored XSS in Album name

Insecure Storage

Again, the photos were not encrypted (verified using the hex editor from earlier). Photos and videos were stored in the following location:

/var/mobile/Applications/61273D04-3925-41EF-BD63-C2B0BC128F70/Library/XFFile/Decoy/1/Picture/Album1

Figure 35: Insecure storage of photos

On the device, this looks like the following:

Figure 36:Albums and photos on device

One of the features of these password-protected photo vaults is the ability to setup decoys. If the user chooses to create a decoy, then when the decoy password is entered it takes them to a “fake” album. I created a decoy in this application and found that the photos were also not encrypted (not that I expected them to be). I created a decoy album and added a photo. The decoy photos were stored in the following location:

/var/mobile/Applications/B73BB177-CEB7-4576-BDFC-2408A0369D42/Library/XFFile/Decoy/2/Picture/Decoy Album

The application stored user credentials in an unencrypted sqlite database. The passcode required to access the application was 1234 and was tied to the administrative account as shown. This was the passcode I chose when I was configuring the application. I also created a decoy user. These credentials were stored plaintext in the users table.

Figure 37: Extracting credentials with sqlite

KeepSafe

Bypassing the Lock Screen

Again bypassing the lock screen was trivial.

Figure 38: KeepSafe LockScreen

The lock screen could be bypassed by calling the showMainStoryboard method from the KeepSafe.AppDelegateclass. Again we attach to the process with cycript and this time we get the instance methods:

Examining the instance methods reveals the following methods:

And calling the showAccountStoryboard or showMainStoryboard methods as shown bypasses the lock screen:

Figure 39: Bypassing the lock screen

The application also allows the user to password protect each album. Each album is synced to the cloud.

Figure 40: KeepSafe cloud storage

I discovered that the password for the albums was returned in plaintext from the server as shown:

Figure 41: Album password return in plaintext from the server

An attacker could therefore not only bypass the lockscreen but he could obtain the passcode for password protected albums as well.

Conclusion

Ok so let’s do a quick recap on what we were able to accomplish. We used:

cycript to bypass the lock screens
sqlite to extract sensitive information from the application databases
plutil to read plist files and access sensitive information
BurpSuite Pro to intercept traffic from the application
IDA Pro to reverse the binary and achieve results similar to cycript

The scary part of this is that, on average, it took less than 30 minutes to gain access to the photos and user credentials for each application. The only exception was the use of IDA, and as we highlighted, we only did that to introduce you to and get you comfortable reading ARM assembly. In other words, it’s possible for an attacker to access your private photos in minutes.

In all cases it was trivial to bypass the lock screen protection. In short I found:

No jailbreak detection routines
Insecure storage of credentials
Photos stored unencrypted
Lock screens are easy to bypass
Common web application vulnerabilities

Let me hasten to say that this research does not speak to ALL photo vault applications. I just grabbed what seemed to be the most popular and had a quick look. That said, I wouldn’t be surprised if others had similar issues, as developers often make incorrect assumptions (believing users will never have access to the file system).

What are some of the risks? First and foremost, if you are running these apps on a jailbroken device, it is trivial to gain access to them. And in the case of My Media, even if you are not on a jailbroken device, anyone could connect to the media server it starts up and access your data.

More importantly though is the fact that your data is not encrypted. A malicious app (some of which have popped up recently) could gain access to your data.

The next time you download one of these apps, keep in mind it may not be doing what you think it is doing. Better yet, don’t store “private” photos on your device in the first place.

References:

RESEARCH | September 15, 2015

The iOS Get out of Jail Free Card

By Michael Allen

If you have ever been part of a Red Team engagement, you will be familiar with the “Get out of Jail Free Card”. In a nutshell, it’s a signed document giving you permission to perform the activity you were caught doing. In some instances, it’s the difference between walking away and spending the night in a jail cell. You may be saying, “Ok, but what does a Get out of Jail Free Card have to do with iOS applications?”

Well, iOS mobile application assessments usually occur on jailbroken devices, and application developers often implement measures that seek to thwart this activity. The tester often has to come up with clever ways of bypassing detection and breaking free from this restriction, a.k.a. “getting out of jail”. This blog post will walk you through the steps required to identify and bypass frequently recommended detection routines. It is intended for persons who are just getting started in reverse engineering mobile platforms. This is not for the advanced user.

Environment Setup

§ Jailbroken iPhone 4S (iOS 7.xx)

§ Xcode 6.4 (command-line tools)

§ IDA Pro or Hopper

§ Mac OS X

Intro to ARM Architecture

Before we get started, let’s cover some very basic groundwork. iOS applications are compiled to native code for the ARM architecture running on your mobile device. The ARM architecture defines sixteen 32-bit general-purpose registers, numbered from R0-R15. The first 12 are for general-purpose usage, and the last three have special meaning. R13 is denoted as the stack pointer (SP), R14 the link register (LR), and R15 the program counter (PC). The link register normally holds the return address during a function call. R0-R3 hold arguments passed to functions with R0 storing the return value. For the purposes of this post, the most important takeaway is that register R0 holds the return value from a function call. See the references section for additional details on the ARM architecture.

Detecting the Jailbreak

When a device is jailbroken, a number of artifacts are often left behind. Typical jailbreak detection routines usually involve checking for those artifacts before allowing access to the application. Some of the checks you will often see include checking for:

Known file paths
Use of non-default ports such as port 22(OpenSSH), which is often used to connect to and administer the device
Symbolic links to various directories (e.g. /Applications, etc.)
Integrity of the sandbox (i.e. a call to fork() should return a negative value in a properly functioning sandbox)

In addition to the above, developers will often seek to prevent us from debugging the process with the use of PT_ATTACH_DENY, which prevents the use of the ptrace() system call (a call used in the debugging of iOS applications). The point is, there are a multitude of ways developers try to thwart our efforts as pen testers. That discussion, however, is beyond the scope of this post. You are encouraged to check out the resources included in the references section. Of the resources listed, The Mobile Application Hackers Handbook does a great job covering the topic.

Simple PoC

We begin with bypassing routines that check for known file paths. This approach will lay the foundation for bypassing the other checks later on. To demonstrate this, I wrote a very simple PoC that checks for the existence of some these files. In the event one is found, the program prints out a message stating the device is jailbroken. In a real world scenario, the application may perform a number of actions that include, but are not limited to, preventing you from accessing the application entirely or restricting access to parts of the application.

Figure 1: Jailbreak detection PoC

If you have a jailbroken device and would like to test this for yourself, you can use clang – the Clang C, C++, and Objective-C compiler to compile it from your Mac OS host. Refer to the man page for details on the following command.

clang -framework Foundation -arch armv7 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/ jailbreak.c -o jailbreak -miphoneos-version-min=5.0

Once compiled, simply copy the binary to your jailbroken device and execute it from the command line. On a side note, Objective-C is a strict superset of C, and in most instances you will see jailbreak detections implemented in C/C++. A sample run of the program reveals that the device is jailbroken:

Figure 2: PoC execution showing device is jailbroken

One of the first steps during an assessment is static analysis on the binary. Let’s begin by viewing the symbols with the following command nm jailbreak

Figure 3: Static analysis – extracting symbol information

Luckily for us, symbols have not been stripped, and based on the above, the _isJailBrokenmethod looks like the function responsible for determining the state of the device.

Figure 4: Examining the binary in IDA

The isJailBrokenmethod is called at 0000BE22, after which the value in R0 is compared to 0. Recall from earlier that the return value from a function call is stored in the R0 register. Let’s confirm this in gdb. We will set a break point at 0000BE28the BEQ (branch if eq) instruction and examine the contents of the R0 register.

Figure 5: Examining the contents of the R0 register

As expected the value at R0 is 1, and the program takes the branch that prints the message “Device is JAILBROKEN”. Our goal then is to take the other branch. Let’s examine the isJailBroken function.

Figure 6: isJailBroken function analysis

At 0000BEA0, the stat function is called, and at 0000BEA8, R0 is compared to 0. If the value is not zero, meaning the path hasn’t been found, the program processes the next file path in the jbFiles[] array shown in the PoC earlier on. However, if the path was found, we see R0 is set to 1 before the program returns and eventually exits.

Figure 7: Determining where R0 register is set

This above snippet corresponds to the following section in our PoC:

Figure 8: R0 gets set to 1 if artifact found

So if we update this, and instead of moving 1 in R0 we move a 0, we should be able to bypass the jailbreak detection check and thus get of jail. Let’s make this modification and run our binary again.

Figure 9: Updating R0 register to 0

Linking this to our PoC, it is the equivalent of doing:

Figure 10: Effect of setting R0 to 0

In other words, the isJailBroken function will always return 0. Let’s set another breakpoint after the comparison like we did before and examine the value in R0. If we are right, then according to the following snippet, we should branch to loc_BE3C and defeat the check.

Figure 11: PoC snippet

As expected the value is now 0, and when we continue program execution we get our desired result.

Figure 12: Bypassing the detection

But what if the binary has been stripped?

Earlier we said that we were lucky because the binary was not stripped, so we had the symbol information. Suppose however, that the developer decided to make our job a bit more difficult by stripping the binary (in our case we used strip <binaryname>). In this case, our approach would be a bit different. Let’s first look at the stripped binary using the nm tool:

Figure 13: Stripped binary

And then with gdb on the jailbroken mobile device:

Figure 14: Examining the stripped binary in gdb

It should be immediately clear that we now have a lot less information to work with. In the case of gdb, we now see “No symbol table is loaded”. Where has our isJailBrokensymbol gone? Let’s push ahead with our analysis by running strings on the binary.

Figure 15: Extracting strings from the binary

Ok, so we seem to be getting closer as we can see some familiar messages. Let’s head back to IDA once again:

Figure 16: Disassembled stripped binary

This certainly looks different from what we saw when the symbols were included. Nonetheless, let’s find the references to the strings we saw earlier. You can use the strings view in IDA and then use Ctrl+X to find all references to where they are used.

Figure 17: Locating references to the status message

Navigating to the highlighted location, we again see the familiar CMP R0, #0:

Figure 18: Locating CMP R0, #0

And if we go to the highlighted sub_BE54, we end up in our isJailBroken function. From that point on, it’s a repeat of what we already discussed.

Ok, but the functions are now inline

Another practice that is often recommended is to inline your functions. This causes the compiler to embed the full body of the function, as opposed to making a function call. With this in mind, our modified isJailBroken function declaration now looks like this:

Figure 19: Declaring functions inline

Before we continue, let’s remind ourselves of what the disassembled binary looked like prior to this change:

Figure 20: Binary before inline function

Now, let’s examine the modified binary:

Figure 21: Modified binary with function now inline

The _isJailBrokenmethod has now been inlined, and we no longer see the call(bl) instruction at 0000BE22 as before. Note that the original function body is still stored in the binary:

Figure 22: Function body still stored despite being inline

To prevent this, some very astute developers will go a step further and make the function static thereby removing the function body. The new function declaration will now be:

Figure 23: Static inline function

Again let’s look at this in IDA.

Figure 24: Disassembled inline binary

Instead of R0 being set from a function call as we saw previously, it is set from a value read from the stack using the LDRcommand at 0000BE9C. On closer examination we see the R0 register being set at 0000BE8Aand 0000BE98 and then stored on the stack:

Figure 25: Disassembled inline binary

At this point it’s the same process as before, we just need to move a 0 into R0at location 0000BE8A. And the rest is history.

Let’s block debugging then

We hinted at this earlier, when we said that the ptrace() system call is used when debugging an application. Let’s examine one implementation often used by developers:

Figure 26: ptrace function declaration

Figure 27: ptrace implementation

See the references section for additional details and source of the above code snippet. If we run our modified binary on the device and try to debug it as we have been doing with gdb, we are presented with the following:

Figure 28: Unable to use gdb

We were stopped in our tracks. Also, keep in mind that the function was inlined and the binary stripped of symbols. Examining it in IDA reveals the following:

Figure 29: Disassembled binary with call to ptrace_ptr

Pay special attention to the highlighted area. Recall from earlier, we said function arguments are in registers R0-R3. The ptrace_ptr call takes four arguments the first being PT_DENY_ATTACH with a value of 31. The /usr/include/sys/ptrace.h header file reveals the list of possible values:

Figure 30: Snippet ptrace documentation

So what happens if we pass a value that is outside of the expected values? The parameter is set at 0000BDF2 and later passed as the parameter to ptrace_ptrat 0000BDFC. We see a value of 1F, which translates, to 31 in decimal. Lets update this to 0x7F.

Figure 31: Updating PT_DENY_ATTACH to random value

We copy the modified binary back to our device and have our bypass.

Figure 32: Bypassing PT_DENY_ATTACH

Another oft recommended technique for determining if a debugger is attached is the use of the sysctl()function. Now it doesn’t explicitly prevent a debugger from being attached, but it does return enough information for you to determine whether you are debugging the application. The code is normally a variation of the following:

Figure 33: Using sysctl()

When we run this from gdb, we get:

Figure 34: Output from running with sysctl()

Let’s pop this in IDA. Again the binary has been stripped and the checkDebugger function inlined.

Figure 35: Call to sysctl()

At 0000BE36we see the sysctl() function call and at 0000BE3A we see the comparison of register R0 to -1. If the call was not successful, then at 0000BE40 the program copies 1 to R0 before returning. That logic corresponds to:

Figure 36: Code snippet showing call to sysctl()

The fun begins when sysctl() was successful. We see the following snippet:

Figure 37: A look at the ternary operator

This corresponds to the following code snippet:

Figure 38: Ternary operator in our source code

When the application is being debugged/traced, the kernel sets the P_TRACED flag for the process where P_TRACED is defined in /usr/include/sys/proc.h as:

Figure 39: P_TRACED definition in /usr/include/sys/proc.h

So at 0000BE4Awe see the bitwise AND against this value. In effect, the loc_BE46 block corresponds to the above ternary operator in the return statement. What we want then is for this call to return 0.

If we change the instruction at 0000BE46 to a MOVS R0, 0 instead:

Figure 40: Patching the binary

When we run the program again we get

Figure 41: Successful bypass

Now, you may be asking, how did we know to update that specific instruction? Well, as we said, the loc_BE46 block corresponds to the use of the ternary operator in our code. Now don’t allow the compiler’s representation of the operator to confuse you. The ternary operator says if the process is being traced return 1 otherwise return 0. At 0000BE46 R0 is set to 1, and R0is also set at 0000BE5C.[EW1] However, in the latter case, the register is set in a conditional block. That block gets hit when the check returns 0. In other words, the process is not being traced. Let’s look at this in gdb. We will set a breakpoint at 0000BE5E, the point at which R0gets stored on the stack at [sp,#0x20].

Figure 42: Inspecting the R0 register

As you can see, R0has a value of 1, and this was set at 0000BE46 as discussed earlier. This value is then written to the stack and later accessed at 0000BE60 to determine if the process is being traced. We see the comparison against 0 at 0000BE62, and if it’s true, we take the path that shows we bypassed the debug check.

Figure 43: Reversing the binary

So, if we set a breakpoint at 0000BE60 and look what value was read from the stack, we see the following:

Figure 44: Examining the value in R0 that will be checked to determine if process is being debugged

This value(0x00000001) is the 1 that was copied to R0 earlier on. Hence, updating this to 0 helps achieve our goal.

Bringing down the curtains

Before we go, let’s revisit our very first example:

Figure 45: Revisiting first example

Recall we modified the isJailBroken function by setting register R0 to 0. However, there is a much simpler way to achieve our objective, and some of you would have already picked it up. The instruction at 0000BE28 is BEQ (branch if eq), so all we really need to do is change this to an unconditional jump to loc_BE3C. After the modification we end up with:

Figure 46: Updated conditional jump to unconditional jump

And we are done, however, we had to take the long scenic route first.

Conclusion

As we demonstrated, each time the developer added a new measure, we were able to bypass it. This however does not mean that the measures were completely ineffective. It just means that developers should implement these and other measures to guard against this type of activity. There is no silver bullet.

From a pen tester’s stand point, it comes down to time and effort. No matter how complex the function, at some point it has to return a value, and it’s a matter of finding that value and changing it to suit our needs.

Detecting jailbreaks will continue to be an interesting topic. Remember, however, that an application running at a lower privilege can be tampered with by one that is at a higher privilege. A jailbroken device can run code in kernel-mode and can therefore supply false information to the application about the state of the device.

Happy hacking.

References:

http://www.amazon.com/The-Mobile-Application-Hackers-Handbook/dp/1118958500
http://www.amazon.com/Practical-Reverse-Engineering-Reversing-Obfuscation/dp/1118787315
http://www.amazon.com/Hacking-Securing-iOS-Applications-Hijacking/dp/1449318746
https://www.owasp.org/index.php/IOS_Application_Security_Testing_Cheat_Sheet
https://www.theiphonewiki.com/wiki/Bugging_Debuggers
http://www.opensource.apple.com/source/xnu/xnu-792.13.8/bsd/sys/ptrace.h