Level14

getflag

When we login as level14 we do not get any file at all. After using the techniques used in the previous exercises, no results ware found.

It's time to get the flag from the getflag binary directly.

When we decompile the binary we get the following code:


undefined4 main(void)

{
  bool bVar1;
  FILE *__stream;
  long lVar2;
  undefined4 uVar3;
  char *__s;
  int iVar4;
  __uid_t _Var5;
  int iVar6;
  int in_GS_OFFSET;
  undefined local_114 [256];
  int local_14;
  
  local_14 = *(int *)(in_GS_OFFSET + 0x14);
  bVar1 = false;
  lVar2 = ptrace(PTRACE_TRACEME,0,1,0);
  if (lVar2 < 0) {
    puts("You should not reverse this");
    uVar3 = 1;
  }
  else {
    __s = getenv("LD_PRELOAD");
    if (__s == (char *)0x0) {
      iVar4 = open("/etc/ld.so.preload",0);
      if (iVar4 < 1) {
        iVar4 = syscall_open("/proc/self/maps",0);
        if (iVar4 == -1) {
          fwrite("/proc/self/maps is unaccessible, probably a LD_PRELOAD attempt exit..\n",1,0x46,
                 stderr);
          uVar3 = 1;
        }
        else {
          do {
            do {
              while( true ) {
                iVar6 = syscall_gets(local_114,0x100,iVar4);
                if (iVar6 == 0) goto LAB_08048ead;
                iVar6 = isLib(local_114,&DAT_08049063);
                if (iVar6 == 0) break;
                bVar1 = true;
              }
            } while (!bVar1);
            iVar6 = isLib(local_114,&DAT_08049068);
            if (iVar6 != 0) {
              fwrite("Check flag.Here is your token : ",1,0x20,stdout);
              _Var5 = getuid();
              __stream = stdout;
              if (_Var5 == 0xbbe) {
                __s = (char *)ft_des("H8B8h_20B4J43><8>\\ED<;j@3");
                fputs(__s,__stream);
              }
              else {
                if (_Var5 < 0xbbf) {
                  if (_Var5 == 0xbba) {
                    __s = (char *)ft_des("<>B16\\AD<C6,G_<1>^7ci>l4B");
                    fputs(__s,__stream);
                  }
                  else {
                    if (_Var5 < 0xbbb) {
                      if (_Var5 == 3000) {
                        __s = (char *)ft_des("I`fA>_88eEd:=`85h0D8HE>,D");
                        fputs(__s,__stream);
                      }
                      else {
                        if (_Var5 < 0xbb9) {
                          if (_Var5 == 0) {
                            fwrite("You are root are you that dumb ?\n",1,0x21,stdout);
                          }
                          else {
LAB_08048e06:
                            fwrite("\nNope there is no token here for you sorry. Try again :)",1,
                                   0x38,stdout);
                          }
                        }
                        else {
                          __s = (char *)ft_des("7`4Ci4=^d=J,?>i;6,7d416,7");
                          fputs(__s,__stream);
                        }
                      }
                    }
                    else {
                      if (_Var5 == 0xbbc) {
                        __s = (char *)ft_des("?4d@:,C>8C60G>8:h:Gb4?l,A");
                        fputs(__s,__stream);
                      }
                      else {
                        if (_Var5 < 0xbbd) {
                          __s = (char *)ft_des("B8b:6,3fj7:,;bh>D@>8i:6@D");
                          fputs(__s,__stream);
                        }
                        else {
                          __s = (char *)ft_des("G8H.6,=4k5J0<cd/D@>>B:>:4");
                          fputs(__s,__stream);
                        }
                      }
                    }
                  }
                }
                else {
                  if (_Var5 == 0xbc2) {
                    __s = (char *)ft_des("74H9D^3ed7k05445J0E4e;Da4");
                    fputs(__s,__stream);
                  }
                  else {
                    if (_Var5 < 0xbc3) {
                      if (_Var5 == 0xbc0) {
                        __s = (char *)ft_des("bci`mC{)jxkn<\"uD~6%g7FK`7");
                        fputs(__s,__stream);
                      }
                      else {
                        if (_Var5 < 0xbc1) {
                          __s = (char *)ft_des("78H:J4<4<9i_I4k0J^5>B1j`9");
                          fputs(__s,__stream);
                        }
                        else {
                          __s = (char *)ft_des("Dc6m~;}f8Cj#xFkel;#&ycfbK");
                          fputs(__s,__stream);
                        }
                      }
                    }
                    else {
                      if (_Var5 == 0xbc4) {
                        __s = (char *)ft_des("8_Dw\"4#?+3i]q&;p6 gtw88EC");
                        fputs(__s,__stream);
                      }
                      else {
                        if (_Var5 < 0xbc4) {
                          __s = (char *)ft_des("70hCi,E44Df[A4B/J@3f<=:`D");
                          fputs(__s,__stream);
                        }
                        else {
                          if (_Var5 == 0xbc5) {
                            __s = (char *)ft_des("boe]!ai0FB@.:|L6l@A?>qJ}I");
                            fputs(__s,__stream);
                          }
                          else {
                            if (_Var5 != 0xbc6) goto LAB_08048e06;
                            __s = (char *)ft_des("g <t61:|4_|!@IF.-62FH&G~DCK/Ekrvvdwz?v|");
                            fputs(__s,__stream);
                          }
                        }
                      }
                    }
                  }
                }
              }
              fputc(10,stdout);
              goto LAB_08048ead;
            }
            iVar6 = afterSubstr(local_114,"00000000 00:00 0");
          } while (iVar6 != 0);
          fwrite("LD_PRELOAD detected through memory maps exit ..\n",1,0x30,stderr);
LAB_08048ead:
          uVar3 = 0;
        }
      }
      else {
        fwrite("Injection Linked lib detected exit..\n",1,0x25,stderr);
        uVar3 = 1;
      }
    }
    else {
      fwrite("Injection Linked lib detected exit..\n",1,0x25,stderr);
      uVar3 = 1;
    }
  }
  if (local_14 == *(int *)(in_GS_OFFSET + 0x14)) {
    return uVar3;
  }
                    /* WARNING: Subroutine does not return */
  __stack_chk_fail();
}

The program getflag is decompiled using Ghidra.

And in the decompiled code we can clearly see that just like the last exercise (Level13) we call a decoding function called ft_des and we call it depending on the UID of the user.

And just like the last exercise (Level13) we can try the same techniques to get the flag. Lets

  • Disassemble the main function

    • Get the addes of where the last call of ft_des is (assuming the calls are in order of exercices)

  • Set a break point to the main function (at the very start)

  • Jump to the desired address.

In the Disassembled code (in the other tab next to getflag.c code) we can see that the last time ft_des is called on address 0x08048df3 (line 259)so we shoulf jump to the address that is a little bit above, to 0x08048de5 (line 256) we jump there se the addresse of the string (that will be decoded) are set properly to the proper registers.

So here is how we will get the flag

Open GDB with the getflag function

level14@SnowCrash:~$ gdb getflag -q

Set the break point to main and we run the program

Breakpoint 1 at 0x804894a
(gdb) run
Starting program: /bin/getflag

Breakpoint 1, 0x0804894a in main ()

We jump to address 0x08048de5

(gdb) jump *0x08048de5
Continuing at 0x8048de5.
7QiHafiNa3HVozsaXkawuYrTstxbpABHD8CPnHJ
*** stack smashing detected ***: /bin/getflag terminated
======= Backtrace: =========
/lib/i386-linux-gnu/libc.so.6(__fortify_fail+0x45)[0xb7f2fd95]
/lib/i386-linux-gnu/libc.so.6(+0x103d4a)[0xb7f2fd4a]
/bin/getflag[0x8048ec7]
/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf3)[0xb7e454d3]
/bin/getflag[0x8048571]
======= Memory map: ========
08048000-0804a000 r-xp 00000000 07:00 12700      /bin/getflag
0804a000-0804b000 r--p 00001000 07:00 12700      /bin/getflag
0804b000-0804c000 rw-p 00002000 07:00 12700      /bin/getflag
0804c000-0806d000 rw-p 00000000 00:00 0          [heap]
b7e07000-b7e23000 r-xp 00000000 07:00 14117      /lib/i386-linux-gnu/libgcc_s.so.1
b7e23000-b7e24000 r--p 0001b000 07:00 14117      /lib/i386-linux-gnu/libgcc_s.so.1
b7e24000-b7e25000 rw-p 0001c000 07:00 14117      /lib/i386-linux-gnu/libgcc_s.so.1
b7e2b000-b7e2c000 rw-p 00000000 00:00 0 
b7e2c000-b7fcf000 r-xp 00000000 07:00 14123      /lib/i386-linux-gnu/libc-2.15.so
b7fcf000-b7fd1000 r--p 001a3000 07:00 14123      /lib/i386-linux-gnu/libc-2.15.so
b7fd1000-b7fd2000 rw-p 001a5000 07:00 14123      /lib/i386-linux-gnu/libc-2.15.so
b7fd2000-b7fd5000 rw-p 00000000 00:00 0 
b7fd9000-b7fdd000 rw-p 00000000 00:00 0 
b7fdd000-b7fde000 r-xp 00000000 00:00 0          [vdso]
b7fde000-b7ffe000 r-xp 00000000 07:00 14081      /lib/i386-linux-gnu/ld-2.15.so
b7ffe000-b7fff000 r--p 0001f000 07:00 14081      /lib/i386-linux-gnu/ld-2.15.so
b7fff000-b8000000 rw-p 00020000 07:00 14081      /lib/i386-linux-gnu/ld-2.15.so
bffdf000-c0000000 rw-p 00000000 00:00 0          [stack]

Program received signal SIGABRT, Aborted.
0xb7fdd428 in __kernel_vsyscall ()

And it shows us the flag.

The flag to login to flag14 account

7QiHafiNa3HVozsaXkawuYrTstxbpABHD8CPnHJ

Command summery

## Follow the gdb exemple...

## Log into the account flag14
level14@SnowCrash:~$ su flag14
Password: 7QiHafiNa3HVozsaXkawuYrTstxbpABHD8CPnHJ
Congratulation. Type getflag to get the key and send it to me the owner of this livecd :)

## Try to get the flag
flag14@SnowCrash:~$ getflag 
Check flag.Here is your token : 7QiHafiNa3HVozsaXkawuYrTstxbpABHD8CPnHJ

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