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iMessage - Decoding NSSharedKeyDictionary can read ObjC Object at Attacker Controlled Address



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During processing of incoming iMessages, attacker controlled data is deserialized using the
NSUnarchiver API. One of the classes that is allowed to be decoded from the incoming data is
NSDictionary. However, due to the logic of NSUnarchiver, all subclasses of NSDictionary that also
implement secure coding can then be deserialized as well. NSSharedKeyDictionary is an example of
such a subclass. A NSSharedKeyDictionary is a dictionary for which, for performance reasons, the
keys are predefined using a NSSharedKeySet.

A NSSharedKeyDictionary is essentially a linear array of values and a pointer to its
NSSharedKeySet. An NSSharedKeySet on the other hand looks roughly like this (with some fields
omitted for simplicity and translated to pseudo-C):

struct NSSharedKeySet {
    unsigned int _numKeys;   // The number of keys in the _keys array
    id* _keys;              // A pointer to an array containing the key values
    unsigned int _rankTable;    // A table basically mapping the hashes of
                                // the keys to an index into _keys
    unsigned int _M;        // The size of the _rankTable
    unsigned int _factor;   // Used to compute the index into _rankTable from a hash.
    NSSharedKeySet* _subKeySet; // The next KeySet in the chain

The value lookup on an NSSharedKeyDictionary then works roughly as follows:
* NSSharedKeyDictionary invokes [NSSharedKeySet indexForKey:] on its associated keySet
* indexForKey: computes the hash of the key, basically computes rti = hash % _factor, bounds-checks
  that against _M, and finally uses it to lookup the index in its rankTable: idx = _rankTable[rti]
* It verifies that idx < _numKeys
* It loads _keys[idx] and invokes [key isEqual:candidate] with it as argument
* If the result is true, the index has been found and is returned to the NSSharedKeyDictionary where
  it is used to index into its values array
* If not, indexForKey: recursively processes the subKeySet in the same way until it either finds the
  key or there is no subKeySet left, in which case it returns -1

The NSArchiver format is powerful enough to allow reference cycles between decoded objects.
This now enables the following attack:

SharedKeyDictionary1 --[ keySet ]-> SharedKeySet1 --[ subKeySet ]-> SharedKeySet2 --+
                                          ^                                         |
                                          |                                    [ subKeySet ]
                                          |                                         |

What will happen now is the following:

* The SharedKeyDictionary1 is decoded and its initWithCoder: executed
* [NSSharedKeyDictionary initWithCoder:] decodes its _keySet, which is SharedKeySet1
* The [NSSharedKeySet initWithCoder:] for SharedKeyDictionary1 reads and initializes the following fields:
    * _numKeys, which at this point is unchecked and can be any unsigned integer value.
      Only later will it be checked to be equal to the number of keys in the _keys array.
    * _rankTable, with completely attacker controlled content
    * _M, which must be equal to the size of the _rankTable
    * _factor, which must be a prime but otherwise can be arbitrarily chosen
  At this point, _numKeys = 0xffffffff but _keys is still nullptr (because ObjC objects are
  allocated with calloc)
* Next, *before* initializing _keys, it deserializes the _subKeySet, SharedKeySet2
* [NSSharedKeySet initWithCoder:] of SharedKeySet2 finishes, and at the end verifies that
  it is a valid SharedKeySet. It does that by checking that all its keys correctly map to an index.
  For that it calls [NSSharedKeySet indexForKey:] on itself for every key.
* (At least) one of the keys will, however, not be found on SharedKeySet2. As such,
  indexForKey: will proceed to search for the key in its _subKeySet, which is actually SharedKeySet1
* The lookup proceeds and determines that the index should be (in our case) 2189591170, which is
  less than SharedKeySet1->numKey (which is still 0xffffffff)
* It then loads SharedKeySet1->keys[2189591170], which, as ->_keys is still nullptr, reads an
  objc_object* from 0x414141410 and thus crashes

The attached PoC demonstrates this on the latest macOS 10.14.6

> clang -o tester tester.m -framework Foundation
> ./generator.py
> lldb -- ./tester payload.xml
(lldb) target create "./tester"
Current executable set to './tester' (x86_64).
(lldb) settings set -- target.run-args  "payload.xml"
(lldb) r
2019-07-29 15:40:28.989305+0200 tester[71168:496831] Let's go
Process 71168 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x414141410)
    frame #0: 0x00007fff3390d3e7 CoreFoundation`-[NSSharedKeySet indexForKey:] + 566
CoreFoundation`-[NSSharedKeySet indexForKey:]:
->  0x7fff3390d3e7 <+566>: mov    rdx, qword ptr [rax + 8*r13]

Combined with a heap spray, this bug could likely be remotely exploitable.

Ideally, this issue and similar ones can be prevented by removing the NSSharedKeyDictionary attack
surface completely, as originally suggested by Natalie. Alternatively, I think another solution
might be to stop encoding all the internal fields of the NSSharedKeyDictionary/NSSharedKeySet
(rankTable, numKeys, especially the subKeySet, ...) and only encode the keys and values. The new
[initWithCoder:] implementations could then just call +[NSSharedKeySet keySetWithKeys:] and
+[NSSharedKeyDictionary sharedKeyDictionaryWithKeySet:] to construct new instances with the decoded
keys and values. This should be fine as all the other fields are implementation details anyway.

Proof of Concept:
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