PCDC 2016 RE Challenge Solutions - Part 1

When we sit down and plan the next year's version of PCDC, the goal is to challenge competitors with new and specific learning objectives. This year, our goal was to push more reverse engineering and incident response type challenges for the competitors to complete as part of their injects. For this, I created 4 different reverse engineering challenges. The rules were simple: Use the tools provided to you or freely available for download to analyze the challenges and get the flag. Flags were in the format 'FLAG{}' with a custom string for each challenge in-between the '{}' characters. In order to get the points for the challenge, teams must submit the input to the challenge and the correct flag.

Since PCDC is run for high schools, colleges, and professionals, the challenges were created with increasing levels of difficulty. Additionally, the competition only runs for about 8 hours and these challenges are not the only tasks the Blue Teams must complete. With that in mind, here is Part 1 of the RE solutions.

Challenge 1 - strungout

The first challenge is called 'strungout'. Let's run it and see what it does.

Ok. Looks like we need to figure out the password. Let's just guess something and see what happens.



So password wasn't the password.

In an attempt to provide some insight to the challenges, each challenge had a name that hinted at its solution. This first challenge was meant to be a gentle introduction to reverse engineering. In order to reverse engineer any target, the goal is to gather as much information about that target as you can. The defacto go to is to extract some form of human readable text, aka, strings. As it happens, the Microsoft SysInternals suite has a tool called strings that does exactly what we want. If we run strings on the challenge, we can see the human readable strings embedded in the executable.



And if we scroll down...



We see the 2 strings we saw before, "Sorry. Can haz password??" and "Ouch. Try again!". Interestingly, we see a different string that looks suspicious! "I_Love_PCDC_RE_Challenges!!!". Let's see what happens if we try this for a password.



So the answer to the first RE challenge would have been:
Password: I_Love_PCDC_RE_Challenges!!!
Flag: FLAG{playing_with_strings_iz_easy!!!}

Challenge 2 - maths

This next challenge increases the difficulty a little bit. The goal was to introduce new students to debugging and x86 assembly. For the competition, we made sure that there were copies of Immunity Debugger installed on Blue Team computers already. Additionally, the free version of IDA Pro was installed. For this write-up I'm going to be using IDA to just show off the control flow graph of a program. This helps layout a program into a diagram that makes understanding how different parts of the program get executed. The remaining details will use Immunity debugger just because it is more easily available for people to use. 

So opening up maths in IDA will produce a diagram similar to this:

What we see here is it looks like a number is being read into the program, some math is happening on it, and if it's right, we fall a specific part of the program, otherwise we get the message "Knew you couldn't guess it!!". Let's see if this is true.

Ok. Let's open this program up in Immunity debugger and figure out what's going on.

So things look a little different in Immunity, but that's OK. What we want to do first is look at the executable modules of this program. This can be done by clicking the 'e' in the top bar of the previous picture or by pressing <Alt + e>. This should bring up the following screen:

From here, right click on the list and choose the option "Analyze all modules" to invoke some of Immunity's built in analysis tools. Once the analysis completes, double click on the line with maths to get taken to the code we want to look at. You should see a picture like the one below.

At first this doesn't look too interesting, but scroll down and we will see some details that should look familiar.

Now. Before we go any further, let's talk about what we are actually looking at. The large window with the multicolored text is showing us the output of a program called a disassembler. What a disassembler does is take raw bytes in a program and print them out as an easier to understand and analyze form called assembly. Modern x86 assembly is incredibly complicated and is comprised of over 1,000 instructions! For these challenges, though, we won't need to know anywhere near that many. The understand this assembly, the left most column is the address in the program that the assembly sits at, the next column shows the raw bytes of the assembly, the third column shows the human readable mnemonics of the assembly, and for some lines, there is a 4th column generated by Immunity that shows some comment that may provide you with more information. The window to the right of the disassembly shows the register values of the CPU while you're debugging the program. This is helpful to keep track of values as they move from the CPU to memory and back again. Below the disassembly and register windows are the listing for the program's .data section and stack. We don't need to go into too much detail about each of them, but the .data section is on the left and it shows data built into the program such as the strings. The bottom right window is the stack that is used by the program for various memory operations.

OK. We can see the messages that we saw in IDA and our tests before in our disassembly window. Let's review what we know so far:

1. The program wants us to guess its secret number
2. The name of the program is called maths
3. If we get it wrong, the program will print out "Knew you couldn't guess it!!"

Now let's look at the assembly. We know that we need to guess a magic number. In x86 assembly, values are compared with the CMP instruction. If we look at the assembly at address 0x00401365, we see the instruction CMP DWORD PTR SS:[EBP-4],DEADBEEF. Now, this looks a little scary, but the CMP instruction performs comparison checks. This is just comparing some variable with the hexadecimal value 0xDEADBEEF. Since we see the string "Knew you couldn't guess it!!" right below the CMP at address 0x00401375, this seems like it may be be a good candidate for our password. Let's try!

Hmmm...didn't work. Remember. The name of the program is actually a clue how to solve it. If we look up a couple of instruction from the CMP, we can see some instructions that sound like math operations operating on the same variable that's used in the CMP DWORD PTR SS:[EBP-4],DEADBEEF instruction. Those math instructions are SUB, ADD, and IMUL. Working backwards from the CMP instruction, we can see that there is math being performed on the variable before it gets compared to 0xDEADBEEF. Here is a summary of what those instructions are:

1. Read 'magic number' from user
2. Multiply (IMUL) 0x0a to the 'magic' number at 0x00401345
3. Add (ADD) 0x0a to the 'magic number' at 0x0040134E
4. Subtract (SUB) 0x01 from the 'magic number' at 0x0040135F

So if we do the opposite of this, we should be able to get our original input:

(((0xdeadbeef + 0x01) - 0x0a) / 0x0a) = 0x16449317

Let's try that!

Still not right! What did we miss! Well, we did miss something. There is another instruction that operates on the variable where our input is stored. This instruction is SHL at 0x00401357. This instruction does a binary shift left which effectively multiplies the number by 2 for the size of the shift. In this case, there is a shift left of 1, so the value is doubled. So to get our answer, we need another divide by 2. Here is the new equation:

((((0xdeadbeef + 0x01) / 2) - 0x0a) / 0x0a) = 0xB22498B

Ok. Let's try this one.

Again it didn't work! Why not?! Well, the answer is found by looking at how the number is actually read in. Look at 0x00401334, we can see in the comments section ASCII "%d" and the next line has a CALL to MSVCR90.scanf_s. This means that the routine reading in the user input is looking for the input to be in decimal form. So converting 0xB22498B to decimal is 186796427. Finally, let's try this.

There we go! So the answer to this one is:

Password: 186796427

Flag: FLAG{deadbeef_is_best_beef!!!}

Alternatives:

Now, this may seem a little complicated for people just entering the RE world. It would be much easier if we had the original source code for this program to actually figure out what's going on. Unfortunately, the process from going a binary to source code is incredibly difficult. A decompiler will never be able to give you the original source. Too much information is lost. There are decompilers out there that can give you something resembling the source, however. Let's look at a few.

RetDec

Found here, RetDec is an online trial of a decompiler. Here is the function we are interested in once decompiled by RetDec:

// Address range: 0x401310 - 0x40138a
int32_t function_401310(char * a1) {
int32_t v1;
g2 = &v1;
printf("I bet you can't guess my secret number...\n\n");
printf("Enter your guess --> ");
scanf_s();
if (20 * g3 == -0x21524124) {
// 0x40136e
function_401240();
// branch -> 0x401383
} else {
// 0x401375
printf("\nKnew you couldn't guess it!!\n");
// branch -> 0x401383
}
// 0x401383
exit(1);
// UNREACHABLE
}

IDA Pro - Hexrays

IDA Pro also has a decompiler plugin, but it is very expensive. Here is some of its output:

//----- (00401310) -------------------------------------------------------

int __cdecl __noreturn main(int argc, const char **argv, const char **envp)

{

  int v3; // ecx@0

  int v4; // [sp+0h] [bp-4h]@1

  v4 = v3;

  printf(aIBetYouCanTGue);

  printf(aEnterYourGuess);

  scanf_s(aD, &v4);

  v4 = 2 * (10 * v4 + 10) - 1;

  if ( v4 == -559038737 )

    sub_401240();

  else

    printf(aKnewYouCouldnT);

  exit(1);

}

Snowman

Another free decompiler is Snowman. It is integrated into the also free debugger, x64dbg. Here is the function decompiled by Snowman:

int32_t g4020a4 = 0x73cd20c1;
int32_t g402098 = 0x73cd2719;
void fun_401240();

int32_t g40209c = 0x73cc2455;

void fun_401311(int32_t ecx, int32_t a2) {
     g4020a4("I bet you can't guess my secret number...\n\n");
     g4020a4("Enter your guess --> ");
     g402098("%d", reinterpret_cast<int32_t>(__zero_stack_offset()) - 4);
     if ((ecx * 10 + 10 << 1) - 1 != 0xdeadbeef) {
          g4020a4("\nKnew you couldn't guess it!!\n");
     } else {
          fun_401240();
    }
    g40209c(1);
    goto a2;
}

As you can see, the outputs from these decompilers are pretty different and this is just for a relatively simple function. None of which, look like the original code:

// Main function

int main(int argc, char *argv[]) {

// Integer to store user input

unsigned int user_input;

// Print info and read input

printf("I bet you can't guess my secret number...\n\n");

printf("Enter your guess --> ");

scanf_s("%d", &user_input);

// Obfuscate, answer == 186796427

user_input *= 10;

user_input += 10;

user_input *= 2;

user_input -= 1;

// Check for valid input

if (user_input == 0xdeadbeef) {

get_flag();

}

else {

printf("\nKnew you couldn't guess it!!\n");

}

// Exit

exit(1);

}

But with these different outputs, it was easier to see the mathematical comparison and check for the magic number. One thing you'll notice is that each decompiler treated the final number, 0xDEADBEEF, differently. This is because type information is lost at this lower level and each decompiler uses different algorithms and heuristics to recover this information. The numbers produced by the decompilers are all technically correct, you just have to interpret it in the appropriate way.

Conclusion

These challenges were meant to get competitors thinking about different areas of cyber security. Hopefully we were successful. In my next post, I'll detail the answers for the remaining 2 RE challenges, exclusive and hashbrowns. If you have any questions or want more clarity on how to solve either of these two challenges, please don't hesitate to ask!

Palmetto Cyber Defense Competition - 2015

Last month my fellow employees and I wrapped up our third annual Palmetto Cyber Defense Competition (PCDC). Inspired by the Collegiate Cyber Defense Competitions, PCDC has been a computer security competition for high schools and colleges in South Carolina. I've written about the event itself in the past PCDC-2014 so I won't go into detail about what the event actually is. Those details can be found at the official PCDC site. Instead I wanted to focus on a couple of the things that make PCDC unique, lessons learned from putting on a computer security competition, and where we are going in the future. For those of you looking for a detailed Red Team write up, that can be found here. I took the time after the competition to create an in-depth analysis of the Red Team's process for those looking to learn from their mistakes at this year's competition. The rest of this blog post is a few of my thoughts from an organizer's perspective and not from the perspective of the Red Team lead.

First off, PCDC is unique in that for the first two years, both high schools and colleges competed. Each group had their own dedicated competition day which meant that the PCDC competition was actually run twice. This means that after the first day, all the machines must get reset, re-imaged, and configured slightly different for the next day. This year, we had a third day added; a professional day. For the first time, PCDC was to be run three times. The schedule of events was as follows: first day was high school, second day was college, and the third day was for professionals. For the professional day, we had groups representing a mixture of government and private industry. From the government side teams were comprised of members from the 24th Air Force and U.S. Cyber Command while the industry teams had members from Scientific Research Corporation (SRC), and SPARC. All in all, we had 4 government teams and 4 teams from industry.

Our goal from the beginning was to design the competition network to be believable and realistic. Since it was not known to us during the planning and design phase of the competition that there would be a professional day, this year's theme was based around an online game development company. Each of the Blue Teams would be responsible for making sure their development company continued to function through the course of the day and deliver the latest version of their game to their user base through audience accessible tablets connected to each Blue Team via a dedicated wireless connection. One of the things that we quickly realized was that our ambitions greatly eclipsed the amount of time we had available to create the network. Remember, a decent portion of our time goes into infrastructure development so that the competition can be rerun the next day. To add to that pressure, we do not have control over the facility in which the competition is hosted. As a result, or preparation time from the end of one day the beginning of the next is usually around 3 hours.

To put it simply, there was a lot of different attributes that we wanted to include into this year's competition, but we ran out of time. One of the biggest things we feel these types of competition lack when trying to simulate real world networks is realistic user activity. This year we attempted to remedy that by developing simulated users. We got all the code developed and tested for the user simulators, but due to a hardware failure, we were unable to deploy them to the competition network this year. In the interest of education and sharing, I have opened sourced the code for the user simulators on GitHub. We'd really like to hear back from anyone that is doing something similar.

A few of us have been involved in multiple CCDCs and PCDCs and every year, we make the comment that the scoring system needs to be altered. Although this was in the works before this year's competition even took place, we haven't had time to finalize what fixing the scoring means. At this point, I think we have a much better idea of how we are going to fix the scoring. To highlight why scoring is such an issue, I want to talk about how winning Blue Teams typically approach this competition. Within the first few minutes, the strategy includes removing all unnecessary accounts, changing all the default passwords, and for some, unplugging their entire network while they continue to harden. Now, from a strategic perspective with the goal to win a game in mind, I can't argue with this approach. The issue that I do have, however, is that this leaves the networks in a pretty unrealistic state.

Each Blue Team is given an information packet at the beginning of the competition. In that packet includes the names of the accounts that the automated scoring engine will use to log into their systems and perform service checks to make sure the Blue Teams still have their services up and running. Once these account names have been identified, the Blue Teams will delete every other account off the workstation or server. This means you could have 3 or 4 domain joined Windows workstations with zero user accounts and only the scoring engine account. It is important to note, that the Red Team is not allowed to leverage the scoring engine account to gain access to the Blue Team's networks. It's also not realistic. A computer security competition should force the students and competitors to perform real security tasks with the presence of real users. Now, since this is a competition and getting that many unbiased volunteer users is unrealistic, we need simulated users.

Other areas where improvements to scoring need to be made is in the way the scoring engine actually evaluates successful checks. Up to this point, a common service to check for is a functioning MySQL database. Typically the scoring engine will login to the MySQL database, make a query, and check for a specific key-value pair or for the presence of a specific table. This simply isn't good enough. For a real company, the database needs to have constant transactions generated by realistic activity. Right now, Blue Teams get away with making a backup of the database in the beginning of the day and just restoring it anytime the Red Team deletes the database. As long as the Blue Team restores the database in between scoring engine rounds, the scoring engine gives that Blue Team a perfect service check score. Now, this is slightly cured by the fact that the Red Team reports incidents to the Gold Team and the Gold Team can decide to take away points, but these types of scenarios need to have bigger impact on the 'day to day' operations of the Blue Teams' networks.

Where we plan to go in the future will attempt to combine 3 facets of scoring. The first being financial. The scoring engine will no longer add points, but will score the Blue Teams' companies in a financial sense. The second facet is from an internal employee and systems perspective. Employees must be able to perform their job related duties and interdependent systems must be able to communicate with each other. Finally, the third facet is from the Red Team. This year we tried something new by giving the Red Team specific targets/flags to capture when we gained access to the the Blue Teams' networks. This included the credit card numbers in their customer database, the source code to their latest game, and a few other  things.

Now, I know some people will argue that the CCDCs and even PCDC already take these things into consideration with the scoring engine, but we argue, it is not taken into account enough. The example we like to use is the one where the Blue Team unplugs their network. Now sure, they aren't getting any points from the scoring engine, but in the real world, you can't just go and unplug your entire company from the network. Not only would you be losing sales, but you're paying your employees to do a job that they can't accomplish. And not to mention, the security or IT department has no authority to make that type of decision.

We have thought long and hard about the scoring and we think we have something new and exciting for next year. I don't want to give away too much here until things are more settled. Additionally, we want to find a way to make the audience understand what is going on. PCDC is free and open for the community to come in and view. This year we attempted to show what was taking place by visualizing the the traffic between the Blue Teams and Red Team in real time. I wrote the code for this and am also releasing it on  GitHub. You can see a video demo of it on YouTube.

We have a lot of exciting things planned for next year's PCDC! Stay tuned for more, and if you have any feedback from this year's we'd love to hear it.