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Microsoft ASN.1 Library Bit String Heap Corruption Vulnerability
11 February 2004
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From: "Marc Maiffret" <mmaiffret@eeye.com>

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, 445).

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 arithmetic:

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 bytes:

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 vulnerability:

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

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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.

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