some things

DownUnderCTF 2023/beginner/downunderflow/downunderflow.c

@@ -0,0 +1,36 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define USERNAME_LEN 6
+#define NUM_USERS 8
+char logins[NUM_USERS][USERNAME_LEN] = { "user0", "user1", "user2", "user3", "user4", "user5", "user6", "admin" };
+
+void init() {
+    setvbuf(stdout, 0, 2, 0);
+    setvbuf(stdin, 0, 2, 0);
+}
+
+int read_int_lower_than(int bound) {
+    int x;
+    scanf("%d", &x);
+    if(x >= bound) {
+        puts("Invalid input!");
+        exit(1);
+    }
+    return x;
+}
+
+int main() {
+    init();
+
+    printf("Select user to log in as: ");
+    unsigned short idx = read_int_lower_than(NUM_USERS - 1);
+    printf("Logging in as %s\n", logins[idx]);
+    if(strncmp(logins[idx], "admin", 5) == 0) {
+        puts("Welcome admin.");
+        system("/bin/sh");
+    } else {
+        system("/bin/date");
+    }
+}