BuckeyeCTF 2022

References https://git.mbund.org/mbund/buckeyectf-2022/src/branch/main/writeups/ronin/ronin.md Decompiled code After decompiling the binary, I was able to take a look at those major funtions that make up this program. main(): int __cdecl main(int argc, const char **argv, const char **envp) { char shellcode_buffer[80]; // [rsp+0h] [rbp-50h] BYREF setvbuf(_bss_start, 0LL, 2, 0LL); scroll(txt); fgets(shellcode_buffer, 80, stdin); if ( !strncmp("Chase after it.", shellcode_buffer, 15uLL) ) { scroll(off_4028); chase(); } scroll(off_4030); return 0; } scroll(): ize_t __fastcall scroll(const char *addr_to_some_buffer) { __useconds_t v1; // eax size_t result; // rax char single_char; // [rsp+1Fh] [rbp-11h] size_t v4; // [rsp+20h] [rbp-10h] size_t i; // [rsp+28h] [rbp-8h] v4 = strlen(addr_to_some_buffer); for ( i = 0LL; ; ++i ) { result = i; if ( i >= v4 ) break; single_char = addr_to_some_buffer[i]; // printing a single char from the string array putchar(single_char); if ( single_char == 10 ) v1 = 1000000; else v1 = 50000; usleep(v1); } return result; } encounter(): ...

BuckeyeCTF 2022 - pwn: samurai What I didn’t understand was the program inserts ./n which can be represented as 0x2ea. This is just a part of the string that it gets printed out to STDOUT. How this program works is it reads in some input from STDIN using fgets. fgets inserts a newline char after reading everything in (EOF or newline). But, if the input that I pass in to overflow the buffer that does not end with a newline char, then it will keep going (or being read) until it overflows the variable I want to overwrite. It can still work but there is this line of code strcpy(&s[strlen(s) - 1], ".\n"); that puts a new line character at strlen(s) - 1. So, when I was naively giving an input that I would think should overwrite the variable, the last bit was always replaced by 2ea which is .\n. AH!!!!!!!!!!!! So, what we would want to do is at least have a newline char in the middle of the string so strcpy does not insert that .\n where the last bit is (this bit is still needed to be overwritten with some value to make the attack happen). ...