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Reversing Challenge - Hack The Box - Impossible Password

In this blog post, we will see the solution of a Hack-The-Box reversing challenge called “Impossible Password”. Its been a long time solving some reversing puzzles :)

Let’s check the binary first.

$ file impossible_password.bin 
impossible_password.bin: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/, for GNU/Linux 2.6.32, BuildID[sha1]=ba116ba1912a8c3779ddeb579404e2fdf34b1568, stripped

This is a 64-bit ELF binary. So, we can use objdump command to see imported functions in this application

$ objdump -T impossible_password.bin 

impossible_password.bin:     file format elf64-x86-64

0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 putchar
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 printf
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 __libc_start_main
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 srand
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 strcmp
0000000000000000  w   D  *UND*  0000000000000000              __gmon_start__
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 time
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 malloc
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.7   __isoc99_scanf
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 exit
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 rand

We can the that the application uses a couple functions from glibc library. We can use this information during our analysis.

$ strings impossible_password.bin 
--- SNIP ---
GCC: (GNU) 4.8.5 20150623 (Red Hat 4.8.5-11)
--- SNIP ---

When I search strings in the application I can see many information, but looks like “SuperSeKretKey” is different than others. Now let’s run the application and see how it works.

$ ./impossible_password.bin 
* asdada

The application shows what I enter as an input. Now I will use ltrace application to see if there is any library functions are used.

$ ltrace ./impossible_password.bin                                                                                                                                                                                                                     1__libc_start_main(0x40085d, 1, 0x7ffdb1afea48, 0x4009e0 <unfinished ...>
printf("* ")                                                                                                                                                 = 2
__isoc99_scanf(0x400a82, 0x7ffdb1afe930, 0, 0* testString
)                                                                                                               = 1
printf("[%s]\n", "testString"[testString]
)                                                                                                                               = 13
strcmp("testString", "SuperSeKretKey")                                                                                                                       = 33
exit(1 <no return ...>
+++ exited (status 1) +++

Basically what happens is, the application compare my input with “SuperSeKretKey”. if my input doesn’t match with the string data, exit the program.

$ ltrace ./impossible_password.bin                                                                                                                                                                                                                   130__libc_start_main(0x40085d, 1, 0x7ffed7da77a8, 0x4009e0 <unfinished ...>
printf("* ")                                                                                                                                                 = 2
__isoc99_scanf(0x400a82, 0x7ffed7da7690, 0, 0* SuperSeKretKey
)                                                                                                               = 1
printf("[%s]\n", "SuperSeKretKey"[SuperSeKretKey]
)                                                                                                                           = 17
strcmp("SuperSeKretKey", "SuperSeKretKey")                                                                                                                   = 0
printf("** ")                                                                                                                                                = 3
__isoc99_scanf(0x400a82, 0x7ffed7da7690, 0, 0** testString
)                                                                                                               = 1
time(0)                                                                                                                                                      = 1626591042
srand(0x9489d839, 10, 0x930c3128, 0)                                                                                                                         = 0
malloc(21)                                                                                                                                                   = 0x22e9ac0
rand(0x22e9ac0, 21, 33, 0x22e9ad0)                                                                                                                           = 0x6c8094
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ac0, 94)                                                                                                          = 0x5436d442
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ac1, 94)                                                                                                          = 0xa242547
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ac2, 94)                                                                                                          = 0x2a045efa
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ac3, 94)                                                                                                          = 0x379c34a0
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ac4, 94)                                                                                                          = 0x75ec02c3
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ac5, 94)                                                                                                          = 0x74091ee1
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ac6, 94)                                                                                                          = 0x22097b52
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ac7, 94)                                                                                                          = 0x156e190a
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ac8, 94)                                                                                                          = 0x624047de
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ac9, 94)                                                                                                          = 0x6893f2f1
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9aca, 94)                                                                                                          = 0x17211d98
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9acb, 94)                                                                                                          = 0x264b215c
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9acc, 94)                                                                                                          = 0x106a3853
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9acd, 94)                                                                                                          = 0xce7992a
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ace, 94)                                                                                                          = 0x3d0562f5
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9acf, 94)                                                                                                          = 0x14484045
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ad0, 94)                                                                                                          = 0x4e8f4c28
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ad1, 94)                                                                                                          = 0x2b6ff460
rand(0x7f3b53b9c740, 0x7ffed7da75f4, 0x22e9ad2, 94)                                                                                                          = 0x33cf64a2
strcmp("testString", "q3\\?C4<=c'~G#$[h8Kck")                                                                                                                = 3
+++ exited (status 3) +++                                                                                                                                                                                                                                  

Once we use the key in the first step we see another step. After I enter the second input the applications runs number of rand function before comparison

We need to analyze the binary. I will use radare2 for binary analysis.

$ r2 -w impossible_password.bin
[0x004006a0]> s main
[0x0040085d]> aaa
[x] Analyze all flags starting with sym. and entry0 (aa)
[x] Analyze function calls (aac)
[x] Analyze len bytes of instructions for references (aar)
[x] Check for vtables
[x] Type matching analysis for all functions (aaft)
[x] Propagate noreturn information
[x] Use -AA or aaaa to perform additional experimental analysis.
[0x0040085d]> s main

Let’s disassembly the application


In the assembly code we can see the SuperSeKretKey in the code. When we review the rest of the code, we see that there are two strcmp function in the code. Those functions used for to compare user input.


As we see above, in the line 0x0040094f, application runs a function. When you see the details this function is to generate random string number. After this line, the generated string is used in the second strcmp operation. That means we will see different strings in every run and we won’t be able to match strings. The code never will come to the address 0x00400971. I assume the flag for this challenge is stored in the function fcn.00400978.

To access the function fcn.00400978, we can add JMP just before 0x00400968 or change the instruction at 0x00400968. Adding NOPs would be a good option i think. I added one NOP but I got “segmentation fault”. So you need to add minimum 2 NOPs in the app


Next step I removed the JNE instruction and added 9090 instead. After the change we can run the code again and get the flag.


Radare2 Tips:
V - for visual view
c - for cursor navigation
i - insert instruction
s <functions> - to select the function
pdf - print function details
aaa - analyze all referenced code