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From: mmaiffret at (Marc Maiffret)
Subject: EEYE: Microsoft ASN.1 Library Bit String Heap Corruption

Microsoft ASN.1 Library Bit String Heap Corruption

Release Date:
February 10, 2004

Date Reported:
September 25, 2003

High (Remote Code Execution)

Systems Affected:
Microsoft Windows NT 4.0
Microsoft Windows 2000
Microsoft Windows XP
Microsoft Windows Server 2003

eEye Digital Security has discovered a second critical vulnerability in
Microsoft's ASN.1 library (MSASN1.DLL) that allows an attacker to
overwrite heap memory with data he or she controls and cause the
execution of arbitrary code.  ASN.1 is an industry standard used in a
variety of binary protocols, and as a result, this flaw in Microsoft's
implementation can be reached through a number of Windows applications
and services.  Ironically, the security-related functionality in Windows
is especially adept at rendering a machine vulnerable to this attack,
including Kerberos (UDP/88) and NTLMv2 authentication (TCP/135, 139,

Technical Description:
Thanks to another pair of integer overflows, software that uses MSASN1
directly or indirectly is again vulnerable to a complete overwrite of a
large portion of its heap memory.  This time, the attack is specific to
bit string values (tags 03h and 23h), but the outcome is the same as
with the heap corruption involving large data lengths.

To recap, ASN.1 BER encoding is a representation for binary data that
encapsulates pieces of that data into a hierarchy of typed values,
analogous to "binary XML."  If a value consists of other values, then it
is considered constructed (or compound); if it contains only raw data,
then the value is described as simple.  The format of a BER-encoded
value is a tag number that gives the type and attributes of the value,
and then the length of the value data, followed by the data itself.  If
bit 5 (20h) of the tag byte is set, this indicates that the value is
constructed, and MSASN1 will decode the following data as its own
BER-encoded block.

In the case of a bit string, the first byte of data is the number of
bits (from 0 to 7) to exclude from the end of the bit string value data,
since the data is naturally given in bytes.  The remaining bytes, then,
contain the (8 * (value_length - 1) - number_of_unused_bits) bits that
compose the bit string.

As the reader might guess, there's an interesting integer overflow here
when a bit string is given a length of one byte (only the "number of
unused bits" field, with no data bits following), and a non-zero number
of unused bits.  (We consider this an integer overflow, rather than a
signedness issue, because the number of bits is always treated as a
strictly unsigned value.)  ASN1BERDecBitString() and
ASN1BERDecBitString2() will both report that the length in bits of such
a bit string is (0 - number_of_unused_bits), a number that can fall in
the range 0xFFFFFFF9 (-7) to 0xFFFFFFFF (-1), although neither will
attempt to copy an amount of data based on this count.  The former
function will attempt to copy the length of the original data minus one
byte -- in this case, zero -- and doesn't hurt anything.  The latter
just returns a pointer into the original BER-encoded block and the
length in bits of the data, and is also harmless.

While it's possible that some client application somewhere might misuse
this number of bits and create an exploitable condition, it doesn't
really matter because there's another integer overflow in MSASN1 that
definitely will.  ASN1BERDecBitString() has a special way of handling
constructed bit strings (tag 23h), in that it concatenates each of the
simple bit strings that the compound one comprises.  By supplying a
valid constructed bit string that contains a single, simple bit string
with length 1 and 7 unused bits, a second integer overflow occurs while
adding the number of bits in the bit string to the cumulative total.
The following code from BERDecBitString() performs the vulnerable

76195338  mov     eax, [ebp-18h]        ; = length of simple bit string
7619533B  cmp     eax, ebx              ; (EBX = 0)
7619533D  jz      short 7619539A        ; skip this bit string if empty
7619533F  cmp     [ebp+14h], ebx        ; = no-copy flag
76195342  jnz     short 761953AF        ; don't concatenate if no-copy
76195344  mov     ecx, [esi]            ; = count of accumulated bits
76195346  lea     eax, [ecx+eax+7]      ; *** INTEGER OVERFLOW ***
7619534A  shr     eax, 3                ; div by 8 to get size in bytes
7619534D  push    eax
7619534E  push    dword ptr [esi+4]
76195351  push    dword ptr [ebp-4]
76195354  call    DecMemReAlloc         ; allocates a zero-byte block

If the first simple bit string encountered has a length of 0xFFFFFFF9
(-7) bits, then the arithmetic at 0x76195346 will add the total number
of accumulated bits (0), the length of the bit string being concatenated
(-7), and then an additional 7 for the purpose of rounding up, to arrive
at a total length of zero.  This sum is passed to DecMemReAlloc() to
allocate a zero-length heap block, but then the bit strings' original
lengths in [ESI] and [EBP-18h] are passed on to a function named
ASN1bitcpy() (not shown here), which in this case performs a typical
memcpy() and overwrites a whole bunch of heap memory as a result.

To demonstrate this vulnerability, all that's necessary is a constructed
bit string with length 3, then a simple bit string with length 1 and an
unused bits field set to 7, all of which BER-encodes to the following

23h/03h         ; constructed bit string (tag bit 5 = 1), length = 3
03h/01h/07h     ; simple bit string, length = 1, 7 unused bits, no data

Normal Kerberos packets already have bit strings available, but to get
LSASS to accept a bit string within SPNEGO, it takes just a bit of
crafting.  If we provide a NegTokenInit token (tag A0h) containing a
ContextFlags value (tag A1h), then we can pass a bit string that does
get decoded using the vulnerable function.  (See RFC 2478 Section 3.2.1
for more details.)  This leaves us with the byte sequence below:

A0h/09h         ; NegotiationToken: negTokenInit, length = 9
30h/07h         ; sequence, length = 7
A1h/05h         ; reqFlags (ContextFlags), length = 5
23h/03h         ; constructed bit string, length = 3
03h/01h/07h     ; simple bit string, length = 1, 7 unused bits, no data

Note: Due to the technical nature of the vulnerability described above,
this advisory may contain disassembly and/or hexadecimal byte codes.
This information is in no way related to "exploit code", "payloads", or
"shell code".

Retina Network Security Scanner has been updated to identify this

Vendor Status:
Microsoft has released a patch for this vulnerability. The patch is
available at:

Discovery: Derek Soeder
More Additional Research: Yuji Ukai (this guy rocks!)

Dah and Murr; 14540253; fuzen; recurring thoughts, flashback humor,
deja-vu, and all the other sensations that go along with releasing
Windows advisories; people who read long advisories

Copyright (c) 1998-2004 eEye Digital Security
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