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From: pdt at (Paul Tinsley)
Subject: EEYE: Microsoft ASN.1 Library Bit String Heap

"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"."


Sure am glad you put that notice in there, here I was getting all hot 
and bothered that you were giving people a road map to the exploit.  
Here I was wondering why a security vendor would be increasing the risk 
model by releasing details which will save the "bad guys" weeks of 
research on the day of the patch release, giving the "good guys" even 
less time to regression test this patch in their environment and 
mitigate any harmful side effects.  Glad you could clear all that up for 
us so that the "good guys" will feel all warm and fuzzy when they are 
scrambling to get this patch deployed and working long hours when a worm 
comes out before 100% patching could occur?

Seriously, I think as a firm in the security industry that touts 
themselves as an enterprise network protector you owe the community an 
explanation as to what value the information in these bulletins have.  
How many of your customers have been directly affected by worms which 
have spawned from information you have provided?  What incentive is 
there for corporations to fund a company which is causing them grief?  
What harm is there in waiting weeks/months after a patch release to give 
such detail about a flaw?  Nothing in this bulletin helps me mitigate 
this vulnerability, unless I am writing my own IDS rules to look for the 
inevitable instance of verbatim hex traffic ending up in a worm down the 

I am all for full-disclosure, but that doesn't have to mean immediate 
disclosure, understanding the potential harm in what you are doing and 
adjusting your ego boosting email release cycle to match it would do us 
all some good.  Do I want you to stop releasing bulletins about 
vulnerabilities?  No.  Do I want you to wait to release academically 
valuable research info which might help others either avoid creating 
such flaws in their code or find such flaws that already exist?  Yes. 

Concerned Security Professional,
Paul Tinsley

    Information and opinions contained within this message reflect my 
personal views and should not be
  construed as the opinions of companies I have affiliations with.

Marc Maiffret wrote:

>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
>Permission is hereby granted for the redistribution of this alert
>electronically. It is not to be edited in any way without express
>consent of eEye. If you wish to reprint the whole or any part of this
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> for permission.
>The information within this paper may change without notice. Use of this
>information constitutes acceptance for use in an AS IS condition. There
>are NO warranties with regard to this information. In no event shall the
>author be liable for any damages whatsoever arising out of or in
>connection with the use or spread of this information. Any use of this
>information is at the user's own risk.
>Please send suggestions, updates, and comments to:
>eEye Digital Security
>Full-Disclosure - We believe in it.

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