xpc-crash

xpc-crash is a proof-of-concept exploit for an out-of-bounds read in libxpc that can be used to force any XPC service to crash on iOS versions prior to 11.3 and macOS versions prior to 10.13.4.

The vulnerability

Prior to macOS 10.13.4, the function _xpc_dictionary_apply_wire_f, which is used by _xpc_dictionary_deserialize to deserialize an XPC dictionary from the wire, contained the following code to iteratively deserialize each entry of the dictionary:

void _xpc_dictionary_apply_wire_f
(
        OS_xpc_dictionary *xdict,
        OS_xpc_serializer *xserializer,
        const void *context,
        bool (*applier_fn)(const char *, OS_xpc_serializer *, const void *)
)
{
...
    uint64_t count = (unsigned int)*serialized_dict_count;
    if ( count )
    {
        uint64_t depth = xserializer->depth;
        uint64_t index = 0;
        do
        {
            const char *key = _xpc_serializer_read(xserializer, 0, 0, 0);
            size_t keylen = strlen(key);
            _xpc_serializer_advance(xserializer, keylen + 1);
            if ( !applier_fn(key, xserializer, context) )
                break;
            xserializer->depth = depth;
            ++index;
        }
        while ( index < count );
    }
...
}

The problem is that the use of an unchecked strlen on attacker-controlled data allows the key for the serialized dictionary entry to extend beyond the end of the data buffer. This means the XPC service deserializing the dictionary will crash, either when strlen dereferences out-of-bounds memory or when _xpc_serializer_advance tries to advance the serializer past the end of the supplied data.

Usage

To build, run make. See the top of the Makefile for various build options.

Run the exploit by specifying the name of the XPC service endpoint to crash on the command line:

$ ./xpc-crash com.apple.iokit.powerdxpc

You will notice that the process corresponding to the XPC service, which in this example is powerd, has crashed.

Timeline

I discovered this issue in February of 2018, but tests on macOS High Sierra 10.13.4 Beta revealed that the issue was already in the process of being fixed. I thus declined to report the issue to Apple.

License

The xpc-crash code is released into the public domain. As a courtesy I ask that if you reference or use any of this code you attribute it to me.

---

Brandon Azad