Monday, June 6, 2011

protection by checking for debuggers

one way to figure out what an app is doing is to use a debugger so you can step through line by line. apktool makes it possible to debug apps to which you do not have the source, and you also have to setup a few other things covered in lesson 1 of the way of the android crack tutorials.

some apps try to protect against this and there are two techniques of doing so. the first is to check the android manifest to see if the app is set to debuggable. the java code would look something like this:

boolean isDebuggable = (0 != (getApplcationInfo().flags &= ApplicationInfo.FLAG_DEBUGGABLE));
if ( isDebuggable )
  invalidLicense(); // you get the idea 

in smali, specifically from the testtarget app distributed with antilvl, it looks like:
# get app info object
invoke-virtual {p0}, Lcom/lohan/testtarget/Main;->getApplicationInfo()Landroid/content/pm/ApplicationInfo;
move-result-object v1

# get flags mask
iget v2, v1, Landroid/content/pm/ApplicationInfo;->flags:I

# 0x2 is ApplicationInfo.FLAG_DEBUGGABLE
and-int/lit8 v2, v2, 0x2

iput v2, v1, Landroid/content/pm/ApplicationInfo;->flags:I

# if v2 is 0x0 (the debuggable flag is not set), goto :cond_0
if-eqz v2, :cond_0

the second method is to check if a debugger is currently connected. the java looks like:
boolean debugConn = Debug.isDebuggerConnected();

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