Windows ASN.1 bitstring heap corruption exploit


This is an exploit for a heap corruption vulnerability in the Microsoft ASN.1 library. This vulnerability is different from the one described in the eEye advisory, but it was also patched in MS04-007. The ASN.1 exploits in the CANVAS and Metasploit frameworks are based on this code.



$ ./ 
. kill-bill : Microsoft ASN.1 remote exploit for CAN-2003-0818 (MS04-007)
  by Solar Eclipse <>

Usage: kill-bill -p <port> -s <service> host

   iis           IIS HTTP server (port 80)
   iis-ssl       IIS HTTP server with SSL (port 443)
   exchange      Microsoft Exchange SMTP server (port 25)
   smb-nbt       SMB over NetBIOS (port 139)
   smb           SMB (port 445)

If a service is running on its default port you don't have to
specify both the service and the port.

Examples: kill-bill -s iis
          kill-bill -p 80
          kill-bill -p 1234 -s smb

$ ./ -s smb
. kill-bill : Microsoft ASN.1 remote exploit for CAN-2003-0818 (MS04-007)
  by Solar Eclipse <>

. Loading shellcode
. Generating SPNEGO token
  SPNEGO token is 4222 bytes long.
. Exploiting SMB server at
  Sending Negotiate Protocol Request
  Sending Session Setup AndX request (4287) bytes
. Attempting to connect to shell on port 8721

Microsoft Windows 2000 [Version 5.00.2195]
(C) Copyright 1985-2000 Microsoft Corp.


Bit string decoding in MSASN1.DLL

The bit string heap corruption vulnerability in the Microsoft ASN.1 library was disclosed by eEye on February 10, 2004. This vulnerability occurs during the processing of a bit string of length 1 with 7 unused bits. For more details refer to the eEye advisory AD20040210-2.

Bit strings are decoded by BERDecBitString() in MSASN1.DLL. This function allocates a new buffer, copies the bit string into it and returns a pointer to the data and its length in bits. Constructed bit strings (tag 0x23) are handled by calling BERDecBitString() recursively on each element and concatenating the results. The pseudocode of BERDecBitString() is given bellow:

struct BITBUF
    int length_in_bits;
    char* ptr;

void BERDecBitString(struct DECD *decd, struct BITBUF *bitbuf) {

    /* If the element is empty, return a NULL pointer in bitbuf->ptr
       and set bitbuf->length_in_bits to 0 */
    if (ELEMENT_LENGTH = 0)
        bitbuf->ptr = NULL;
        bitbuf->length_in_bits = 0;
        return 1;

        /* If the element is not constructed (tag 0x03), allocate a new buffer,
           copy the bit string into it and return it in bitbuf->ptr */

        bitbuf->ptr = alloc(ELEMENT_LENGTH);
        bitbuf->length_in_bits = ELEMENT_LENGTH * 8;
        memcpy(bitbuf->ptr, decd->data, ELEMENT_LENGTH);
    else {

        /* If the element is constructed (tag 0x23), call BERDecConstructed to
           allocate 76 bytes for a new DECD object and initialize it with a
           pointer to the beginning of the constructed bit string. */

        decd2 = BERDecConstructed(decd->data);
        bitbuf->length_in_bits = 0;

        /* Notice that bitbuf->ptr is not initialized to NULL. This is very
           important. */
        while (MORE_DATA(decd2)) {
            BITBUF bitbuf2;

            /* Decode each of the bit strings in the constructed element */

            BERDecBitString(decd2, &bitbuf2);
            if (bitbuf2.length == 0)

            /* Reallocate the buffer and append each bit string to it */

            bitbuf->ptr = realloc(bitbuf->ptr, bitbuf->length + bitbuf2.length);
            memcpy(bitbuf->ptr+bitbuf->length, bitbuf2.ptr, bitbuf2.length);


Let's look at an example bit string:

"\x23\x08"                      # constructed bit string, length=8
    "\x03\x03\x00"              # bit string, length=3, unused bits=0
    "\x03\x02\x00"              # bit string, length=2, unused bits=0

For clarity we will use a representation similar to the Perl code used to build bit strings in our exploit:


When this constructed bit string BERDecBitString() will be called twice to decode the "AA" and "B" bit strings. Each time the size of the B buffer will be increased by calling realloc and the new bit string will be appended to it.

The realloc() vulnerability

There is a problem in the MSASN1.DLL which is much easier to exploit than the eEye vulnerability. The BERDecBitString function does not properly handle constructed bit strings which contain constructed bit strings.

The following example illustrates the problem:


When BERDecBitString is called on this bit string, the bitbuf->ptr pointer is initially NULL. Since this is a constructed element, BERDecBitString is called again to decode the "AAAAAAAA" bit string. It allocates a new buffer of size 8 and copies "AAAAAAAA" into it. The new buffer is returned in bitbuf2->ptr. We call realloc(bitbuf->ptr, 8) to allocate space for the 8 bytes of data. Since bitbuf->ptr is NULL, this call is equivalent to alloc(8). We copy the data from bitbuf2->ptr to bitbuf->ptr and free bitbuf2->ptr.

Now bitbuf->ptr is a pointer to a 8 byte buffer containing "AAAAAAAA". The original buffer has been freed and bitbuf2->ptr points to the freed memory.

We go back to the beginning of the while loop and call BERDecBitString to decode the next element, which is the second constructed bit string. Before it starts decoding it, the function initializes bitbuf->length_in_bits to 0, but bitbuf->ptr still contains the value passed by the caller, a pointer to freed memory. If bitbuf->ptr were initialized to NULL, as it is in the version of MSASN1.DLL included in the MS04-007 patch, the vulnerability would not exist.

The first element of the constructed bit string is the "BBBBBBBB" bit string, which is decoded by calling BERDecBitString. It allocates 8 bytes, copies "BBBBBBBB" into the 8 byte buffer and returns a pointer to it as bitbuf2->ptr.

Then we calculate the new size of the bitbuf->ptr buffer and call realloc(bitbuf->ptr, 8).

At this point bitbuf->ptr still points to the freed memory chunk which used to contain "AAAAAAAA". What happens when we pass a pointer to a free chunk to the NtReallocateHeap() function? It should return NULL to indicate an error, but due to a design flaw it returns its argument, unmodified. Now the program believes that it has allocated a new buffer of size 8, but in fact it is about to overwrite the first 8 bytes of a freed memory chunk with the "BBBBBBBB" bit string.

NT heap overview

The Windows NT heap allocator is similar to the Linux malloc implementation. All available memory is split into chunks which can be either free or in use. Each chunk has a 8 byte header, containing the size of the current chunk, the size of the previous chunk and flags. The user data starts at chunk+8.

When a chunk is freed, it is coalesced with the previous and next chunks if they are also free. Then the chunk is inserted into a doubly linked list of free chunks. There are 127 such lists, called dedicated lists, for chunks of size 8, 16, 24, 32, up to 1016. Chunks larger than 1024 bytes are stored in a non-dedicated list, sorted in ascending order by their size.

When an application tries to allocate some amount of memory, the dedicated list for the exact size is searched first. If it is empty, the list for the next chunk size is searched and so on, until we find a free chunk or we go through all dedicated lists without finding one. In this case we have to search the non-dedicated list. The heap allocator starts at the head of the list and follows the forward pointer until it finds a big enough chunk. Since the non-dedicated list is sorted by chunk size, we are guaranteed to find the smallest chunk satisfying our request size. If there are no free chunks available, we have to request more memory from the kernel.

The free chunk we have found is taken off the free list and marked "in use". A pointer to the user data in the chunk is returned to the application.

It is important to note than the forward and backward pointers that link the chunks in a free list are stored in the first 8 bytes of the user data of a chunk. This means that if we can write 8 bytes to a buffer that has already been freed, we will overwrite the linked list pointers. Once this is done, the operations of adding a chunk to the free list and removing one from the list can be used to write to an arbitrary location in memory.

Constructing the exploit

There are many attack vectors for this vulnerability, but to develop this exploit we will use the Negotiate protocol for HTTP authentication, described in a MSDN article.

We will build a SPNEGO token containing a specially crafted constructed bit string, encode it in Base64 and send it as a part of a HTTP GET request. The default IIS configuration allows Windows authentication and our SPNEGO token will be passed to MSASN1.DLL for decoding.

$bitstring =
                bits($fw, $bk)

$spnego =
    "\x60" . asn1(                      # Application Constructed Object
        "\x06\x06\x2b\x06\x01\x05\x05\x02" .    # SPNEGO OID
        "\xa0" . asn1(                  # NegTokenInit (0xa0)
            "\x30" . asn1(              # Constructed Sequence
                "\xA1" . asn1(          # ContextFlags (0xa1)

$request =
    "GET / HTTP/1.1\r\n" .
    "Authorization: Negotiate " . encode_base64($spnego, "") . "\r\n" .

The bit string used in the exploit is complicated due to the fact that we need the ASN.1 decoder function to perform a specific sequence of memory allocations in order to corrupt the heap but not crash the application.

This exploit is different from most Windows heap exploit because we can control the memory allocation pattern of the decoder by supplying it with bit strings of different length. This allows us to do multiple arbitrary 4 byte overwrites and exploit the vulnerability with a high degree of reliability.

The desired sequence of memory operations is given bellow:

memcpy(B, "AAAABBBB", 8);
alloc(size of B)
memcpy(B, "CCCCDDDD", 8)
alloc(size of B)

After we overwrite the FW and BK pointers of a freed block, we allocate a block of the same size. The NtHeapAllocate() function walks the linked list of free blocks and reaches the block with the overwritten pointers. Since it satisfies the request size it is removed from the linked list, marked as in-use and returned to the application. If the overwritten FW and BK pointers point to writable memory the unlink operation will succeed, but the block will still be on the free list.

When the block is freed, the NtFreeHeap() function walks the linked list of free blocks until it finds a block of equal or greater size. Then the freed block is inserted into the linked list in front of that block. In our case the block we're freeing will still be on the free list, so it will be the block we find.

When NtFreeHeap() inserts a block into the free list, it manipulates three blocks, labeled A, B and C. Block B is to be inserted between A and C. The code to do this is given bellow:

B->fw = C
B->bk = C->bk
C->bk->fw = B
C->bk = B

In our case blocks B and C are the same, so the code is equivalent to

B->fw = B
B->bk = B->bk
B->bk->fw = B
B->bk = B

Assuming the address of B is in ebx, the executed instructions will have the same effect as

mov [ebx], ebx
mov eax, [ebx+4]
mov [eax], ebx
mov [ebx+4], ebx

This code writes the address of the block we are freeing into the location specified by the value at block+4. The ability to write the address of our block to an arbitrary address in memory makes it very easy to write a reliable exploit.

In our exploit we overwrite the FastPebLockRoutine pointer at PEB+0x20. If we allow the program to continue its execution, our shellcode buffer may be freed or overwritten before the program calls the FastPebLockRoutine. That's why we make the final allocation request for a block of the same size as our shellcode block. During the search for a suitable free block, NtAllocateHeap() finds the shellcode block (which is still on the free list) and attempts to unlink it. If we make sure that the first two dwords in the shellcode point to invalid memory space (such as 0x90909090), an exception will occur.

The exception is handled by an exception handler in LSASRV.DLL, which logs the following message in the event log:

"The security package Negotiate generated an exception. The package is now disabled."

One of the functions involved in writing to the event log locks the PEB by calling the FastPebLockRoutine, which now points to our shellcode.

After the Negotiate protocol is disabled the system keep running, but interactive logins fail.


This vulnerability presents specific requirements for our shellcode which make it impossible to use a generic payload. During the execution of the exception handler the LSASS.EXE process is blocked and unable to serve any authentication requests. If our shellcode executes a process or uses a system function which sends a request to Local Security Authority, a deadlock will occur. If LSASS.EXE is killed the system will reboot. It is very important that the exception handler in LSASS is allowed to continue its execution.

To accomplish this our shellcode is split in two stages. Stage 0 restores the FastPebLockRoutine pointer and repairs the heap. The stage 1 shellcode is then executed in a new thread. In the parent thread the stage 0 shellcode jumps to RtlEnterCriticalSection and the execution of the exception handler continues without any trace of being exploited.

There are almost no restrictions on what the stage 1 shellcode can do, as long as it does not crash the process it is running in. After the shellcode completion it should terminate its thread by calling ExitThread or simply execute a return instruction.


The exploit has been tested and is known to work on Windows 2000 SP2, SP3, SP4 and Windows XP SP0 and SP1.

On Windows 2000 SP0 and SP1 the FastPebLockRoutine pointer is overwritten, but it is never called by LSASS.EXE. An alternative exploitation method, such as overwriting the unhandled exception handler has to be used.