Low Fragmentation Heap ReAllocation for Use After Free Exploitation

Use After Free (UAF) vulnerabilities occur when the application frees an object on the heap, and then tries to erroneously use it again (usually due to a stale pointer reference to the freed object). A common exploitation technique to target a UAF vulnerability is to try to reallocate heap memory between the time when the application frees the memory and when it erroneously dereferences the stale pointer to the freed memory. This reallocation would fill the “hole” left by the application when it initially freed the object. The typical timeline of this type of attack is as follows:

1) Application frees object on the heap

2) Attacker reallocates objects on the heap

3) Application erroneously dereferences freed pointer

Note that in normal circumstances, A UAF would crash the program due to an Access Violation. However, when the application is being exploited, the attacker’s reallocation serves 2 main purposes: 1) Make sure that there is data at the location pointed to by the stale pointer so the application doesn’t crash on erroneous dereference and 2) Craft the data in the reallocation in such a way that the dereference would help the attacker gain control over the Instruction Pointer (EIP on x86).

In Internet Explorer, exploits often use JavaScript to reallocate freed objects on a heap with the Windows Low Fragmentation Heap (LFH) policy activated and groom the backend heap in order to eventually gain control over the Instruction Pointer. The connection between JavaScript and freed objects is as follows:

“When string attributes of HTML objects are assigned in JavaScript, the strings are often stored in the same Front End LFH that the freed objects were stored in.”

The significance of this statement is that attackers can craft maliciously formatted strings in JavaScript and have the strings fill holes left by the freed objects. In order to reallocate the holes left by the freed objects, the attacker must make sure the strings are of the same size as the freed objects, so that the stings will get allocated in the same LFH bucket as the freed object. Once the object of interest has been freed, and the attacker can assign those strings as attributes to an array of HTML nodes, and those strings are likely to be allocated on the same LFH as the freed object. Eventually, if this process is repeated enough, a reallocation of the “hole” left by the freed object would occur. This means that the next time the application dereferences the stale pointer (ie for a virtual function call), it would get the attacker’s string rather than the object it expects to be there. This would be how the attacker initially takes control of the Instruction Pointer. Below is an example of what this process may look like in JavaScript:

//create an array of HTML elements whose string attributes we will assign later
for (var i = 0 ; i < numObjects; i++)
    objectArray[i] = document.createElement('div');

"prime" LFH by allocating a few strings of the same size as the object of interest to enable the LFH bucket for this allocation size
for (var x = 0; x < primeAmount; x++)
    objectArray[x].title = pattern;

//application allocates object here

//application frees object here

//fill the hole left by the freed object assuming primeAmount < numObjects
for (var z = primeAmount; z < numObjects; z++)
    objectArray[z].title = pattern; //attributes should be allocated on LFH

//application erroneously uses object here

The JavaScript “pattern” string above was carefully crafted to meet the following criteria:

1) It must be the same size as the erroneously freed object, so that it will be reallocated in the same LFH Bucket.
2) Its value must be specifically crafted to help gain code execution. For example, if the freed object was a C++ object with a V-Table pointer, one of the first few DWORDs must point to a location in memory which the attacker controls (usually via a Heap Spray of the backend heap) that contains a malicious V-Table. For more information, see prior article about V-Tables.

For more information about the Low Fragmentation Heap, see:


By: Neil Sikka

Posted On : 2013/10/08


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