Magic values in 32-bit processes on 64-bit OS-es and how to exploit them Jun 21 2016 03:01PM
Berend-Jan Wever (berendjanwever gmail com)
(You can read all this information in more detail on

Software components such as memory managers often use magic values to
mark memory as having a certain state. These magic values can be used
during debugging to determine the state of the memory, and have often
(but not always) been chosen to coincide with addresses that fall
outside of the user-land address space on 32-bit versions of the
Operating System. This can help detect vulnerabilities by causing an
access violation when such magic value is used as a pointer as well as
mitigate exploitation of such vulnerabilities by making it impossible to
have this "poisoned" pointer refer allocated memory under the attacker's

For instance, Microsoft's C++ debugging runtime library initializes
stack memory to 0xCCCCCCCC. When an uninitialized object pointer is used
to read the value of a property or call a method of the object, this
reliably causes an access violation on 32-bit versions of Microsoft
Windows and prevents an easy path to exploitation.

The Wikipedia article on magic values has a list containing some of the
values and when they are used. You will notice how all of the values
used on Windows have their high bit set (i.e. >= 0x80000000). As explain
earlier, this is because on 32-bit versions of Windows these addresses
cannot be used to allocate memory in user-land by default. Windows does
have a /3GB switch that allows you to change the upper limit for
user-land memory to 0xC0000000, but AFAIK this is not used very often
and still excludes a large number of magic values.

Magic values on 64-bit OS-es

On 64-bit architectures, there is no need to reserve part of the 32-bit
address space for kernel memory. Consequently, a 32-bit applications
running on 64-bit versions of Windows is able to allocate memory in
almost the entire 32-bit address range. This allows 32-bit applications
to allocate more memory, including at all addresses that these magic
values can reference. Ever since their introduction over 10 years ago,
Javascript heap-sprays in web-browsers in particular offers an attacker
the ability to finely control memory allocations and their content for
use in exploits.


Last year I stumbled upon a two different bugs in two different web
browsers where a magic value was used to mark memory which had not yet
been initialized. Both vulnerabilities allowed me to get the web
browsers to use the memory as a pointer before initializing it to a sane
value. using Javascript, I was able to allocate memory at the magic
value address the web browsers ended up using and store information at
this location that allowed me to exploit both of these two
vulnerabilities. These issues have both been address, so I can discuss
them in more detail.

CVE-2014-1592 Firefox xul.dll!nsHtml5TreeOperation use of poisoned memory

Mozilla 1088635 <https://bugzilla.mozilla.org/show_bug.cgi?id=1088635>
covers a bug in Firefox that could it to use data from a freed and
"poisoned" object through specially crafted HTML, which resulted in
access violations around address |0x5a5a5a5a| on x86 systems. The memory
used to back the object was marked with this magic value after it was
freed. Because this magic value resulted in an address can be allocated
even on 32-bit versions of Windows, I suggested in Mozilla 1182002
<https://bugzilla.mozilla.org/show_bug.cgi?id=1182002> that the value
get updated with something that makes it a little harder to exploit.
This and other reasons for changing the magic values, led to magic
values being changed to |0xe4e4e4e4| for uninitialized memory and
|0xe5e5e5e5| for freed memory.

verifier!AVrfDebugPageHeapAllocate incorrect memory initialization a.k.a
Google Chrome use of uninitialized FLS pointer

In August last year, I found what appear to be a thread-safety
vulnerability in Google Chrome when handling audio data, that could lead
to use of an uninitialized pointer. This issue is only visible when
running Chrome with page heap enabled, as the memory used to store the
pointer appears to be set to 0x00000000 after allocation when page heap
is not enabled. This means this NULL pointer will not be used by the
code to reference memory. However, when running Chrome with page heap
enabled, the pointer will be initialized to 0xD0D0D0D0 and gets used in
code that allows at least freeing of arbitrary memory pointers.

After doing a more thorough analysis, Ricky Zhou explained to me in the
Chromium bug
<https://bugs.chromium.org/p/chromium/issues/detail?id=525288> that the
issue is not in Chromium, but in |verifier.dll|. This DLL is used to
implement page heap on Windows. The problem is that in
|verifier!AVrfDebugPageHeapAllocate|, the |HEAP_ZERO_MEMORY| flag is
sometimes ignored, which in this case caused the memory to get
initialized with the wrong value. I reported this issue to Microsoft at
the end of October last year and after getting the MSRC case number
31596, I never heard back from them again.

Mitigating this type of attack

While working on these issues, I realized that this type of attack is
easy to mitigate by making sure the magic values point to memory that
has been reserved and marked inaccessible. That way there is no risk of
an attacker allocating the memory with data under his/her control for
use in an exploit: whenever the application would attempt to access
memory using a magic value as a pointer, this would reliably cause an
access violation.

Having a memory allocation at the various addresses represented by
common magic values fragments the address space, reducing the largest
possible continuous allocation and the total amount of memory available
to the application. But most 32-bit applications do not depend on being
able to allocate such large chunks of memory for normal operations, as
this is impossible on 32-bit versions of Windows. Regardless, should one
want to prevent this fragmentation, and at the same time organize the
magic values to be more coherent and intuitive to developers, it might
be useful to create an API that can be used to generate the magic
values, and have the generated values be more similar, closer together
and/or located at either edge of the free memory above address

I have suggested adding mitigations such as a special API that allows an
application to request magic values that are guaranteed to point to
reserved memory to MSRC and they responded in November 2015 that they
forwarded it to the Windows team for their consideration.




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