Adds fcsc 2025 and scale 22x

content/writeups/2025/scale/exploitation/crash_override.md

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+date = '2025-04-28T12:00:00+02:00'
+draft = false 
+title = 'Crash override'
+tags = ['pwn']
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+
+This challenge is funny and I've never seen it before it was really easy once I turned on my brain and realised that the solution isn't supposed to work on my local system without the provided container and that I should try on the remote.  
+
+So we get a file and the usual nc command, as always lets fire up ghidra and see what we're dealing with.
+
+```c
+int main(void)
+{
+  int iVar1;
+  char input [64];
+  
+  signal(0xb,on_segfault);
+  printf("What is the password?\n?: ");
+  fflush(stdout);
+  fgets(input,0x80,stdin);
+  iVar1 = strcmp(input,"password\n");
+  if (iVar1 != 0) {
+    puts("incorrect password");
+    fflush(stdout);
+  }
+  return 0;
+}
+```
+
+This looks like a simple password checking function, however we quickly notice two things.
+1. First we have a nice little buffer overflow on the input, we are able to input 0x80 chars (which is 128 in decimal) inside of a 60 bytes buffer. For a beginner oriented CTF this isn't surprising.
+2. More surprising is the use of `signal` which I guessed allows to do something when receiving a syscall. After a quick look at the manual I was right. This allow us to map a signal to a function resulting in a call to the function being performed when the signal is received by the program (I don't think this is all and I am not sure that it is exactly what happens but it's more than enough to do the challenge). By the name of the function and after a quick look on the internet we can confirm that `0xb` (11 in decimal) is the syscall number for `SIGSEGV` better know as the segmentation fault (invalid memory reference, tried to dereference an invalid pointer).
+
+So putting it all together we have a buffer overflow that allows us to trigger a segfault by overriding the return pointer and a function that executes when a segfault occurs. But what's that function ?
+
+```C
+void on_segfault(int sig_num)
+{
+  int iVar1;
+  FILE *__stream;
+  char acStack_b0 [64];
+  code *pcStack_70;
+  int sig_num_local;
+  char data [64];
+  FILE *fd;
+  
+  sig_num_local = sig_num;
+  __stream = fopen("./flag","r");
+  fgets(data,0x40,__stream);
+  printf("%s",data);
+  fclose(__stream);
+  FUN_00401140(1);
+  signal(0xb,on_segfault);
+  printf("What is the password?\n?: ");
+  fflush(stdout);
+  fgets(acStack_b0,0x80,stdin);
+  iVar1 = strcmp(acStack_b0,"password\n");
+  if (iVar1 != 0) {
+    puts("incorrect password");
+    fflush(stdout);
+  }
+  return;
+}
+```
+
+I removed some bloat but what this function basically does is read the flag and print it.
+So we know what to do, trigger a segfault and you're done. To do that send about 100 characters and you get the flag.