Code Execution via surgical callback overwrites (e.g. DNS memory functions)

June 12, 2019 in Code Injection, Sysmon

Today I looked at Sysmon v10 and its support for logging DNS queries. It’s a pretty cool feature that intercepts all the DNS requests on a monitored host, and if possible, maps them to the process name making that request. It is a nice addition to Sysmon’s already awesome logging capabilities.

Just for fun, I created a simple POC that used DnsQuery_A API to send a multiline DNS query, because I wanted to see how Sysmon will react to it. It was obviously a non-sensical exercise, but it’s fun to see we can modify the layout of Event Logs by introducing some unexpected, redundant data:


I decided to look at the DnsQuery_A function in IDA as well. I was curious if/what characters it accepts & if there is any limit to the buffer it can process. This was a quick & dirty attempt to see if I could send a query that Sysmon would truncate in a similar fashion as I described in this post.

While digging into the code I noticed an interesting way dnsapi.dll is allocating memory. Instead of a fixed (inline) function it relies on a couple of callbacks. One of them is a memory allocation routine. When the library needs memory, it calls the function, and if it is not set, it relies on its own internal routines.

This immediately caught my attention. If we can find the address of this callback inside a remote process we can use it to execute code next time DNS library asks for memory.

This is the memory allocation function used by DnsQuery_* functions (32-bit):

Under normal circumstances finding callback pointers in a remote process memory is quite hard and noisy (lots of ReadProcessMemory calls, possible disassembling). Unless of course there is an interface we can use to surgically target some specific callback (e.g. using documented windows messages, or SetProp function). As far as I can tell there is no such interface in our case.

I found a surrogate solution that we can try to exploit though.

When I looked at references to the callback function (which I named fnMemAlloc on the listing above) I discovered a exported function called DnsApiHeapReset. It takes 3 arguments and each of them is … a callback replacement:

I quickly analyzed each callback’s role and they just are 3 basic/core memory allocation/reallocation/release primitives.


If we can locate the address of dnsapi.dll in a remote process (easy), find the address of exported DnsApiHeapReset function (easy), then with a basic parsing of its code we can discover the address of each callback (also easy). Then, with a single, surgical WriteProcessMemory call we can modify any of them.

This is not a new code injection trick. It’s just one way to execute code without engaging remote threads, APCs, windows hooks, side-loading, process hollowing, patching API code (e.g. NtClose), etc..

There are of course tons of other callbacks like this, but finding their exact location without any point of reference is hard. Or… not really. Just think of all the Windows Procedures – all of them are callbacks.

Enter Sandbox part 25: How to get into argument

June 11, 2019 in File Formats ZOO, Malware Analysis, Sandboxing

When you begin your programming career one of the first lessons focuses on reading command line arguments. It is very trivial, but when you start coding more and in new languages you will quickly discover that it’s actually less than trivial and a bit of a mess.

Programming languages use many different ways to access the command line arguments, e.g.:

  • argv
  • wargv
  • args
  • $argv
  • @ARGV
  • arg
  • sys.argv
  • ParamStr
  • Command$
  • WScript.Arguments
  • etc.

I can’t count how many times I googled proper name/syntax for these over the years – ad hoc programming in different languages makes it quite difficult to remember. Also, some programming languages start indexing of arguments from 0, some from 1.

A way to access these parameters also differs. Sometimes you have it available as a string, an array, sometimes you need to call a function to retrieve specific items for you, and in some cases you need to write your own parser or tokenizer.

And finally, some frameworks require certain (standard) approach to passing arguments so that a (standard) parsing routine can extract them properly. Then there are quirks – paths with spaces, extra spaces, ANSI, Unicode characters, and you have two buffers available for parsing – a path to actual executable, and its command line. And the first is not always a full path, or is a path expressed in a different way than expected.

It gets even more complicated when you start reversing. This time it’s not only programming languages per se, but also the binaries they produce and these differ depending on architecture, OS, compiler’s flavor, version, optimization settings. It is all very messy.

Grepping a repo of import function names I came up with this short list of APIs & external, or internal symbols/variables:

  • CommandArgs
  • CommandLineToArgvW
  • GetCommandLineA
  • GetCommandLineW
  • g_shell_parse_argv
  • osl_getCommandArg
  • osl_getCommandArgCount
  • rb_argv
  • StringToArgv
  • _acmdln
  • _wcmdln
  • __argc
  • __argv
  • __p__acmdln
  • __p__wcmdln
  • __p___argc
  • __p___argv
  • __p___wargv
  • __wargv

Why would we need these?

Many programs require command line arguments to run. Sandboxes that can’t recognize these will fail to produce an accurate report. Not only some malware is using this trick on purpose, there are also tones of good programs that end up in sandbox repositories and never get properly analyzed (e.g. compiled work from students of IT, or native OS binaries)

Sandboxes that recognize programming frameworks & the way they parse command line arguments are in a better position to analyze such samples. This is because there is at least a theoretical possibility of heuristic determination if a sample require command arguments, or, if it accepts any. At the very least, they should hint that in their reports.

There are some command line arguments that are universal and can be guessed e.g. /? or /h. Others require a lot of reversing since program’s logic is often hidden under many layers of code and nested calls.

What kind of heuristics we can come up with?

For instance, if an API called immediately after GetCommandLine is ExitProcess then the chances are this program requires command line arguments.

If we can determine location and internal layout of WinMain or main functions and then also of an argc variable (using e.g. signatures, hooking, or emulation, or by monitoring stack), we can attempt to trace the access to this variable. When access is detected we can try to analyze code that is using the variable’s value. If our sample exits almost immediately after this comparison the program most likely is requiring command line arguments.

Other possibilities could involve:

  • monitoring of dedicated parsing routines, e.g. getopt function, but also many inline functions that are embedded in popular frameworks
  • string detection for popular arguments, e.g. /s, -embedding
  • string detection for help information, e.g.: usage:
  • detection of installer type, version (they usually accept some command line arguments that are predefined)
  • fuzzy comparison against known files (if we know sample X required command line arguments, chances are that a similar file will too)
  • ‘reverse proof’ of no CLI requirement
    • if it calls GUI functions then less likely to wait for arguments (but may still accept them)
    • if it is an installer, then we typically know how to handle it (e.g. using clickers)
    • if it is a driver – no command line arguments
    • if it is a DLL, most likely no command line processing (BUT some of the exported functions do rely on command line arguments!)
  • etc.

Overall this is a non-trivial task and there are very poor chances of offering a generic solution here, but it is a good idea to at least flag the file for manual analysis. Either in-house or in a report for client.