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    Author Archive - Jack Tang (Threats Analyst)

    The recent Duqu 2.0 targeted attack used several zero-day vulnerabilities as part of its attack. One of the vulnerabilities used was CVE-2015-2360, which was fixed by MS15-061 as part of the June Patch Tuesday release. Like CVE-2015-1701, this is also in the Win32k.sys file, which is commonly targeted by attackers to bypass existing vulnerability mitigation techniques.

    The vulnerability lies in how windows are handled by the operating system. Some background information about this is necessary:

    1. If an application wants to show a window, it needs to perform two steps:
      1. Registering a window class. This will lead the OS kernel to create a window class object, which exists in kernel space and an application program cannot access it directly from user mode. The structure is named tagCLS. The window class object specifies the window ‘s style and behavior.
      2. Creating a window with the window class object which was registering in the previous step. The progress will lead the OS kernel to create a window object, which exists in kernel space and application program cannot access it directly from user mode. This structure is named tagWND.
    2. Every window has a window procedure to handle window messages. The window procedure can be run in user mode or kernel mode. It depends on the window class object’s file named CSF_flags. If the CSF_flags field has the flag “Server Side Proc”, the window object’s window procedure can be run in kernel mode. If it does not have the flag, the window object’s window procedure can be run in user mode. If one application program provides its own window procedure which is not the default window procedure, the window procedure only runs in user mode: the window class object’s CSF_flags field doesn’t include “Server Side Proc” flag.

    Figure 1. tagCLS structure

    From Figure 1, we can see the tagCLS structure’s CSF_flags field is a 32-bit number. Every bit represents one Characteristic. The first bit is the flag for the “Server Side Proc” characteristic.

    1. win32.sys has a characteristic that it will switch to user mode to run some user mode callback functions to do some work which is fit for user mode. This is frequently exploited by attackers.

    Let’s take a look the vulnerability. The vulnerability can be summarized in the following figure:

    Figure 2. Vulnerable message handling process

    When a window message is received (for example, from WM_SetIcon), the kernel will handle the message. The process is lengthy. In the above illustration, I only included the parts which are related to the vulnerability. The vulnerability exists in the step 4: it doesn’t check that the tagCLS object is valid after it switches back from user mode and continuously does some operation on the tagCLS object. This poses a serious security risk


    This type of vulnerability is not very easy to exploit, because the attacker needs to be very familiar with the working model of win32k.sys This type of vulnerability can be exploited with several common techniques; I list one possibility below:

    Figure 3. Possible exploit methodology

    After step 7, the attacker can create a window with the modified tagCLS object which has “Server Side Proc” flag. The window’s procedure can be run in kernel mode. This means that the attacker can now run their code with elevated kernel mode privileges, effectively allowing for a complete compromise of the affected system.


    The most effective solution for this vulnerability is to roll out the appropriate official patch. It is also worth noting that this vulnerability is only an escalation-of-privilege vulnerability. Other attack techniques are necessary to get code running on the targeted system in the first place; solutions that focus on preventing and detecting these may also be useful to administrators.


    Posted in Vulnerabilities |

    Our analysis of the win32k.sys vulnerability used in a recent targeted attack reveals that it opens up an easy way to bypass the sandbox, making it a bigger threat than originally thought.

    As mentioned in Microsoft security bulletin MS15-051, CVE-2015-1701 is an elevation of privilege vulnerability that exists when the Win32k.sys kernel-mode driver improperly handles objects in memory. The vulnerability exists in Win32k.sys, which is a weak security point in Windows.

    Elevation of privilege vulnerabilities are technically less dangerous since they can’t be exploited remotely, but since this vulnerability can be used to bypass the sandbox — a security feature designed to keep attackers from being able to execute malicious files in users’ environment — this becomes a viable tool for attackers.

    The vulnerability exists in the Windows OS process of creating windows for applications. To illustrate, we’ll first look at the processes involved when an application wants to create a window:

    1. The application registers a window class to System, which defines the window’s style and behavior. The most important property of window class is window procedure which defines the window ‘s behavior when there’s an incoming message. The window procedure is provided by the application.
    2. The application then calls the API CreateWindow/CreateWindowEx with the window class to create the window. These APIs will switch to kernel mode to call service routine NtUserCreateWindowEx. NtUserCreateWindowEx is complicated function and does the real job of creating the window in Windows system.  During the function execution, it will switch to user mode many times to call many functions that exist in user mode. These function do jobs which is fit to run in the user mode (example: to load an image, etc.).

    Since this is a lengthy process, I’ve simplified the process of creating a window, leaving only the key point related to the vulnerability: Read the rest of this entry »


    One of the vulnerabilities recently patched by Microsoft can be exploited in the same way as Heartbleed, and needs to be addressed immediately.

    Addressed in the April batch of Patch Tuesday fixes (in Microsoft Security Bulletin MS15-034, specifically), the Microsoft Windows HTTP.sys Integer Overflow vulnerability, or CVE-2015-1635, is a remote code execution vulnerability that exists in HTTP.sys, or the HTTP protocol processing module in Microsoft Internet Information Service. Integer overflows have long been known as one kind of notorious and fairly old vulnerability – so why the sudden interest? I found that the vulnerability can be easily used for denial of service (DoS) attacks or data leaks similar to last year’s Heartbleed attack, in which information is copied from memory and sent to the client.

    A researcher already published a checker script for this vulnerability on Pastebin and showed how it can be used to carry out DoS attacks or to leak information. Additionally, another researcher published a POC for information leaks.

    Detailed analysis of DoS attacks and Information leak attacks via CVE-2015-1635

    In this blog post I will explain in detail how the vulnerability can be used for DoS attacks and data leak attacks. Before I dive into the vulnerability, let’s introduce some background information.

    Figure 1. Architecture of IIS

    After installing IIS (Internet Information Service) on the system, there is a service named the World Wide Web Publishing Service, which I call the WWW service. When a client sends an HTTP request to the system, the WWW service starts up or selects a worker process (W3wp.exe) based on load balancing policy to process the HTTP request and send the appropriate HTTP response. The file W3wp.exe uses HTTP.sys to do many tasks related to HTTP protocol handling, which include HTTP protocol parsing, response content assembling, and content caching. HTTP.sys receives the HTTP request and sends the HTTP response via TCPIP.sys.

    How can this be used in DoS attacks?

    Let’s prepare a HTTP request to start our DoS journey. The testing environment I used was Windows 8.1 (32-bit version) plus IIS 8.5.

    First, we take the IIS ‘s default web page iisstart.htm (file size 694 bytes) as an example. We compose a HTTP request as follows:

    GET /iisstart.htm HTTP/1.1\r\nHost: aaaaa\r\nRange: bytes=284-18446744073709551615 \r\n\r\n

    The HTTP range request start position is 284, and the end position is 18446744073709551615. The hexadecimal value of 18446744073709551615 is 0xFFFFFFFFFFFFFFFF. The range length is huge—it must be over the size of the requested web page file, which appears suspicious. I will explain later why I selected the start position as 284.

    After the request is sent to HTTP Server, one w3wp.exe handles the HTTP request processing. TCPIP.sys forwards any HTTP protocol content to HTTP.sys. If the request is a HTTP range request, it will call the function UlpParseRange(). The function parses the request to get the requested range’s start position and end position and calculates the range’s length. We can see that the first integer overflow happens in code which calculates the range’s length. The code is as follows:

    Figure 2. Range Length code

    When the end position=0xFFFFFFFFFFFFFFFF and start position=0, the integer overflow happens and the Range Length equals 0. The code does not check whether there is an overflow and does not take the error handling action.

    In our case, the integer overflow does not happen. The end position=0xFFFFFFFFFFFFFFFF and start position=284, while Range Length equals -284, whose hexadecimal value is 0xFFFFFFFFFFFFFEE4. If this is interpreted as an unsigned integer, the value is very huge.

    After finishing parsing the HTTP request, it will call the function UlAdjustRangesToContentSize(). The function adjusts the Range’s start position and length if the start position and length is “invalid”. The definition of “invalid” contains several scenarios which indicate the Range is over the requested web page content. Examples include:

    • Range Start Position is 0xFFFFFFFFFFFFFFFF
    • Range Length is 0xFFFFFFFFFFFFFFFF
    • Start Position >= the requested web page length
    • End Position >= the requested web page length

    The second integer overflow can be found in the last function. It uses the following code to get the Range End Position:

    Figure 3. Adjustment action

    In this case, the Range Start Position equals 284, Range Length equals 0xFFFFFFFFFFFFFEE4, so an overflow happens, Range End Position equals 0 and the adjusted action is bypass.

    If one same request is received continuously, the request‘s response will be cached and will be taken from the cache and sent to the requester. It will call the function UxpTpDirectTransmit().One job of the function is to calculate the HTTP response packet length. The simplified code is as follows:

    Figure 4. Simplified HTTP code

    Using our case‘s value:

    Range Count = 1;
    Range Boundary and Range Info length = 0; // (This will be explained later.)
    Range Tail Boundary Length = 0; // (This will be explained later.)
    Range Length = 0xFFFFFEE4; // (The value is already cast to 32 bit)
    HTTP Head Length = 283;

    The HTTP Response Length‘s result is 0xFFFFFFFF. The whole length is 4G. Systems will consider that this HTTP response total size is 4G. This means that there is a HTTP response packet whose calculated size is larger than the real size. This is why we used 284 as the Range Start Position.

    HTTP Response Length = HTTP Head Length + (Range End Position – Range Start Position + 1)
    = HTTP Head Length + (0xFFFFFFFF – Range Start Position + 1
    = HTTP Head Length - Range Start Position

    If Range Start Position less than <= HTTP Head Length, it will trigger an overflow. The HTTP Response Length value range is [0, HTTP Head Length).

    Because we want to launch a DoS attack, this is not the result we want. We want the HTTP Content Length value to be large. The preferred range for Range Start Position is [HTTP Head Length+1, target page size). In this range, it does NOT trigger an overflow and the HTTP Content Length value will become large, and will then allow a DoS attack.

    After the HTTP response package is assembled, HTTP.sys will forward the packet information to the protocol stack driver to send it. TCPIP.sys‘s function TcpSegmentTcbSend() will traverse every part content of the packet. There is integer overflow in the function. Here is a list of some simplified code:

    Figure 5. Simplified code

    In the above code, line 15 will trigger the overflow. In our case, the HTTP Response Length is 0xFFFFFFFF. The “virtual address’” initial value must be a kernel address ( >=0x80000000). Because HTTP Response Length is very large, the loop will run many times. When the loop has run many times, the “virtual address” will become a very small value after triggering the integer overflow. In the loop, it will use the “virtual address” to build a partial memory descriptor list (MDL). Because the range is not a sub range in the source range, it leads to a Blue Screen Of Death (or BSOD). The DoS attack is complete.

    How can this be used in an information theft attack?

    We prepare a request as follows:

    GET /iisstart.htm HTTP/1.1\r\nHost: aaaaa\r\nRange: bytes=3-18446744073709551615, 1-600"+ "\r\n\r\n"

    The reason for using multiple ranges will be explained later. We take 3-18446744073709551615 as Range1 and take 1-600 as Range2.

    In function UlpParseRange(),the code can be seen in Figure 3. For this case, we have 2 range lengths:

    Range1 Length = 0xFFFFFFFFFFFFFFFF - 0x3 + 1 = 0xFFFFFFFFFFFFFFFD
    Range2 Length = 600 -1 + 1 = 600

    After parsing the HTTP request, it will call the function UlAdjustRangesToContentSize(). The code is in Figure 3. Range1 will trigger integer overflow (3 + 0xFFFFFFFFFFFFFFFD => 0), so bypass adjust action, even if the Range1 Length is invalid. Range2 is valid, so it should not be adjusted.

    If the same request is received continuously, the request‘s response will be cached and will be taken from the cache and sent to the requester. It will call the function UxpTpDirectTransmit() . The code in Figure 4 is executed, using the following values:

    • Range Count = 2
    • Range1 Length = 0xFFFFFFFFFFFFFFFD
    • Range2 Length = 600
    • Http Head Length= 0x127 //HTTP head content, see Figure 6
    • Range1 Boundary and Range1 Info length = 0x7a
    • Range2 Boundary and Range2 Info length = 0x69
    • Range Tail Boundary Length = 0x32; //see Figure 8

    By using a multi-range request, there is a Range bound tag and Range Information (Content-Type, Content-Range) before each range content. (See Figure 7 for this.)

    Figure 6. HTTP Head

    Figure 7. Range boundary tag and Range Information

    Figure 8. Range Tail

    From Figure 4, we can see that HTTP Response Length = HTTP Head Length + Range Boundary and Range Info length + Range1 Length + Range Boundary and Range Info length + Range2 Length = 0x127+7a+0xFFFFFFFD+0x69+0x258+0x32 => 0x491. At line 5 of Figure 4, an integer overflow occurs when 0xFFFFFFFD is added. At this time, the system takes 0x491 as the HTTP response length.

    The memory status can be described as following:

    Figure 9. Memory status

    After assembling the HTTP response package, HTTP.sys will forward the packet information under the protocol stack driver to send it. TCPIP.sys ‘s function TcpSegmentTcbSend() will traverse every part content of the packet (see Figure)

    See the code in Figure 4. In this case, the HTTP response length is 0x491. When handling the part which length is 0xFFFFFFFD, line 7’s condition is met, Part Length‘s value will be set by Remain Length. At that point, Remain Length is 0x2f0 (0x491 – 0x172- 0x7a). It will use [0x3, 0x3+0x2f0] as a buffer range to read and send to client. Our target web page iistart.html has a length 0x2b6. The status is the following:

    Figure 10. Buffer status

    In Figure 10, the red part is the leaked information buffer. Compared with the code in Figure 5, the condition at line 24 is satisfied and breaks the loop. That is to say, the parts after the part which has 0xFFFFFFFD length will not be sent to client. At this point, we can see why we used a multi-range request.

    Using multi-range requests can let the HTTP response length (calculated in step 3) become larger to leak more information. If only a single range is used, the HTTP response length becomes either too big or too small. If the HTTP response length is too big, it will lead to BSOD, as it was described in the previous section. If the HTTP response length is small, it doesn’t make a range to cover the buffer after the target web page content to do information leak. Attackers use many range to leak more information. For example, I created a multi-range request as follows:

    GET /iisstart.htm HTTP/1.1\r\nHost: aaaaa\r\nRange: bytes=3- 18446744073709551615,1-32,32-64, 64-96,96-128,128-256, 129-130,130-140,160-170,180-190, 190-200"+ "\r\n\r\n"

    In the request, I use 11 ranges. The leaked information can be seen in the following picture:

    Figure 11. Information leakage


    Given these attack scenarios, I strongly advise users and network administrators to apply the patch, if they haven’t. Aside from the update provided by Microsoft, Deep Security also protects users from this through the following Deep Security rule:

    • 1006620 – Microsoft Windows HTTP.sys Remote Code Execution Vulnerability (CVE-2015-1635)

    This blog post is a follow-up to our previous blog entry on CVE-2015-1635:
    IIS At Risk: The HTTP Protocol Stack Vulnerability

    Posted in Exploits, Vulnerabilities |

    As operating system developers are always keen on improving exploit mitigation technology, Microsoft has enabled a new mechanism in Windows 10 and in Windows 8.1 Update 3 (released last November) by fault. This technology is called Control Flow Guard (CFG).

    Previous mitigation techniques like address space layout randomization (ASLR) and Data Execution Prevention (DEP) have been successful in making exploitation of vulnerabilities more difficult, even if these techniques are not perfect. ASLR causes the development of heap sprays, and DEP results in return-oriented-programming (ROP) techniques showing up in the exploit code.

    To explore this particular technology, I used the Windows 10 Technical Preview (build 6.4.9841),  with test applications built using the Visual Studio 2015 Preview. Because the CFG implementation of the latest Windows 10 technical preview build (10.0.9926) has a slight change at that same point, I will point out the difference.

    To fully implement CFG, both the compiler and the operating system must support it properly. As an exploit mitigation mechanism in the system level, the CFG implementation requires cooperation from the compiler, the operating system user mode library, and the kernel mode module. A blog post on MSDN outlined the steps that developers need to do to support CFG.

    Microsoft’s implementation of CFG is focused on indirect call protection. Consider the following code in the test program I created:

    Figure 1. Code of test application

    Let’s take a look at what the encircled code compiles into if CFG is not enabled.

    Figure 2. Assembly code of test program

    In the above figure, there is one type of indirect call. Its target address is not decided at compilation and is instead decided at runtime. An exploit can abuse this as follows:

    Figure 3. How to abuse the indirect call

    Microsoft’s implementation of CFG focuses on mitigating problems if the indirect call is exploited and an invalid target is called (in an exploit, this would be to first stage shellcode).

    The invalid target has a distinguishing characteristic: in most cases it is not a valid function starting address. Microsoft’s CFG implementation is based on the idea that an indirect call’s target must be the start of a valid function. What is the resulting assembly code if CFG is enabled?

    Figure 4. Assembly code, with CFG enabled

    Before the indirect call, the target address is passed to the _guard_check_icall function, which is where CFG is actually implemented. In versions of Windows without CFG support, this function does nothing. In Windows 10, which does have CFG support, it points to ntdll!LdrpValidateUserCallTarget. This function takes a target address as argument and does the following:

    1. Access a bitmap (called CFGBitmap) which represents the starting location of all the functions in the process space. The status of every 8 bytes in the process space corresponds to a bit in CFGBitmap. If there is a function starting address in each group of 8 bytes, the corresponding bit in CFGBitmap is set to 1; otherwise it is set to 0. The figure below is an example of a portion of CFGBitmap:

      Figure 5. Representation of CFGBitmap

    2. Convert the target address to one bit in CFGBitmap. Let’s take 00b01030 as example:

      Figure 6. Target address

      The highest 3 bytes (the 24 bits encircled in blue) is the offset for CFGBitmap (unit is 4 bytes/32 bits). In this example, the highest three bytes are equal to to 0xb010. Therefore, the pointer to a four byte unit in CFGBitmap is the base address of CFGBitmap plus 0xb010.

      Meanwhile, the fourth bit to the eighth bit is (the five bits encircled in red) have the value X. If target address is aligned with 0x10 (target address & 0xf == 0), then X is the bit offset value within the unit. If the target address is not aligned with 0x10 (target address & 0xf != 0), the X | 0x1 is the bit offset value.

      In this example, the target address is 0x00b01030. X has the value of six. The formula 0x00b01030 & 0xf has a result of zero; this means the bit offset is also six.

    3. We look at the bit identified in step #2. If the bit is equal to 1, it means the indirect call target is valid because it is a function’s starting address. If the bit is 0, it means the indirect call target is invalid because it is not one function’s starting address. If the indirect call target is valid, the function will do nothing and let it go. If the indirect call target is invalid, an exception will be raised which should prevent further exploit code from running.

      Figure 7. Value in CFGBitmap

    The implementation of CFG should be useful in preventing some classes of exploits. This will help reduce the threat from software vulnerabilities in the long run.

    You may read a detailed analysis about Control Flow Guard in our technical report, Exploring Control Flow Guard in Windows 10.

    Posted in Vulnerabilities | Comments Off on Exploring Control Flow Guard in Windows 10

    Applications that have been frequently targeted by exploits frequently add sandboxes to their features in order to harden their defenses against these attacks. To carry out a successful exploit, an attacker will have to breach these sandboxes to run malicious code. As a result, researchers will pay particular attention to exploits that are able to escape sandboxes.

    In both October and November Patch Tuesday cycles, Microsoft addressed several vulnerabilities that were used by attackers to escape the Internet Explorer sandbox. One of these was CVE-2014-6349, which was addressed by Microsoft as part of MS14-065, November’s cumulative Internet Explorer patch. We chose this particular vulnerability for two reasons: exploiting it is relatively easy, and its methodology – using shared memory to escape the Internet Explorer sandbox – has not been seen before. A separate vulnerability that also allowed for sandbox escapes – CVE-2014-6350 – was also fixed in the same patch, and Google released details about this second vulnerability earlier this week.

    Internet Explorer 11 exposes a shared memory section object to all tab process (which are sandboxed). This is used to store various Internet Explorer settings. Normally, the tab processes only read this to see these settings. However, in Enhanced Protected Mode (EPM, which is IE’s sandbox mode), the shared memory section‘s DACL (Discretionary Access Control List) is not configured correctly. The tab processes have “write” permission to modify the shared memory section content. This can be used by an attacker to break the IE sandbox. How can this be done? We will explain this in the rest of this post.

    To understand the concepts covered in this post, background knowledge about Protected Mode (PM) and EPM is necessary. These MSDN documents and HITB presentations provide background information on these topics. I carried out my tests on a system running Windows 8.1 with Internet Explorer 11.0.9600.17107.

    After enable IE 11’s EPM mode,  we run IE. The broker process and tab process are seen below:

    Figure 1. Internet Explorer broker and tab processes

    The parent iexplore.exe broker process’s integrity is Medium. The iexplore.exe tab process’s integrity is AppContainer. This means the web page rendering in the sandboxed tab process is in the sandbox and its privilege is controlled. Both process share a memory section: \Sessions\1\BaseNamedObjects\ie_ias_<frame process id>-0000-0000-0000-000000000000.

    Read the rest of this entry »

    Posted in Exploits, Vulnerabilities | Comments Off on Escaping the Internet Explorer Sandbox: Analyzing CVE-2014-6349


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