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Updated appid_calc.pl

January 8, 2016 in appid_calc.pl, Forensic Analysis, Software Releases

I was recently contacted by Terry O. who spotted and reported a bug in appid_calc.pl which I released back in 2013. After analysis I realized that I made a stupid mistake in the way the parameters were passed to the functions responsible for APPID calculation. Oops.

I have updated the script yesterday and hopefully it should work now :)

You can download its latest version here.

Thanks to Terry for providing the feedback. This is much appreciated.

IDAPython – making strings decompiler-friendly

December 21, 2015 in Malware Analysis, Reversing, Software Releases

Update

As pointed out by 0stracon there is an option in Hexrays that actually enables it to print all strings. Go to Hex-Rays Decompiler Analysis Options and untick ‘Print only constant string literals’.

To make it permanent, enable it in hexrays.cfg:

#define HO_CONST_STRINGS   0x0040   // Only print string literals if they reside
                                    // in read-only memory (e.g. .rodata segment).
                                    // When off, all strings are printed as literals.
                                    // You can override decompiler's decision by
                                    // adding 'const' or 'volatile' to the
                                    // string variable's type declaration
HEXOPTIONS               = 0x....   // Combination of HO_... bits

I was not aware of this option and reinvented the wheel :)

Old post

One of the features of IDA is its ability to recognize strings. This is a great feature, especially useful when you combine it with a power of HexRays decompiler – together they can produce a very nice pseudocode.

There is only one annoying bit there: if strings are recognized and defined inside a writable segment, they will not be presented by the decompiler as strings, but as variable names referring to strings.

Let’s have a look at the example.

In the below example (Dexter sample) IDA recognizes the string “UpdateMutex:”

strings_1When we now switch to the decompiler view, we will see that the decompiler changes it to s__Updatemutex:

strings_1a

(the ‘s__’ prefix comes from the string prefix I typically use i.e. ‘s->’ which decompiler ‘flattens’ to ‘s__’). The s__Updatemutex refers to a string as shown below i.e. “UpdateMutex:” :

strings_2Obviously, a  decompiled code that refers to the actual string is much more readable – see the same piece of code as shown above where data is referred to by actual strings:

strings_2aIn order to make the decompiler use these actual strings (not the reference) we have two options:

  • Make the segment where the string is recognized read-only (by disabling ‘Write’ in segment properties):

strings_3Unfortunately, this will confuse the decompiler and will make the output not trustworthy (it is often truncated). You will also receive a friendly reminder that you are doing something stupid 😉 a.k.a. a red card from the decompiler’s authors:

strings_3a

  • The second option is to use a ‘proper’ method of fixing the issue by telling the IDA that the string is a read-only a.k.a. constant i.e. you can change the type of the string from existing one to the one prefixed with a keyword ‘const’:

strings_4Since most of the time strings are static it is handy to convert all the strings in IDA to read-only i.e. retyping all of them using the ‘const’ trick.

This is exactly what the strings_to_const.py script is intended to do.

It enumerates all segments, finds all strings recognized by IDA (note the comment about the prefix I use, you may need to adapt it to your needs), and then converts them to read-only.

The result?

See below – before and after:

strings_before_after