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Windows Oday release



dear all

SChannel Off-By-One Heap Corruption
===================================

Discovery Date:
28th August 2006

Date reported to Microsoft:
19th March 2007

Summary:
The Secure Channel (SChannel) library on WinXP-SP1/SP2 is vulnerable to
a off-by-one heap buffer overwrite. The SChannel library implements
PCT/TLS/SSL protocols exported via the Security Service Provider Interface
(SSPI). It is one of several Security Service Providers loaded-in and supported
by the privileged Local Security Authority (Lsass.exe) process.

In SChannel's implementation of the client-side SSLv3 handshake protocol,
specifically in the processing of the server-key-exchange SSL handshake
record, there is insufficient checks for malformed server-sent digital signature, with its length-field set to 0. This results in a allocation of a 0-length heap buffer (with a valid heap address). A reverse memory copy is then performed to copy-in the digital signature, by decrementing the 0-length by 1. This results in an integer-underflow, causing the heap-buffer pointer to decrement before its start address, ultimately leading to an overwrite of exactly one-byte of user-controlled value, into the heap control-block. Depending on the robustness of the application in question, this may lead to an unrecoverable heap corruption condition, causing the application to terminate. In the case of Lsass.exe on WinXP-SP2, we can crash it locally after several iterations, from a less-privileged user, causing a system reboot. Vulnerable code although also exists in WinXP-SP1 but it does not cause an
unrecoverable heap corruption in Lsass.exe.

Vendor Affected:
Microsoft


Systems Affected:
========
WinXP-SP2 (DOS/Reboot)
WinXP-SP1 (minimal impact)


Exploitation:
=============
1) For local machine reboot via normal user account, on WinXP-SP2
  OR
  For remote machine reboot by enticing user to visit HTTPS site via IE,
  on WinXP-SP2 (but over 2000 iterations required)
POC (crash-test/reboot):
========================
1) Run sctest.exe from a normal user account, on client machine
  running WinXP-SP2.
2) sctest.exe will attempt to use SChannel's SSL implementation to
  parse pre-generated malformed SSL handshake records, over
  several iterations, causing multiple off-by-one overwrites with
  0xFF byte, within the Lsass.exe process.

3) Attach Debugger to Lsass.exe to see crash. The system will notify
  the user and perform a 60sec. reboot count-down, after detecting
  the Lsass.exe crash.
** Lsass.exe crash-test can also be done by forcing/enticing Internet Explorer
  to access a HTTPS site, serving out the same malformed SSL handshake
  records (as shown in source code below). However, over 2000 iterations
are needed (IE needs to access HTTPS site over 2000 times), before Lsass.exe
  heap corruption occurs.

Vuln Analysis:
==============
(Based on schannel.dll/v5.1.2600.2180/WinXP-SP2)
The vulnerability exists in schannel.dll component, that implements the SSPI-compliant PCT/TLS/SSL protocol handling implementation. For more information on SSPI and how it relates to LSA, refer to 1) http://msdn.microsoft.com/library/default.asp?url=/library/en-us/secauthn/security/authentication_packages.asp 2) http://msdn.microsoft.com/library/default.asp?url=/library/en-us/secauthn/security/sspi.asp

Essentially, in the case of SSPI authentication libraries like schannel, kerberos, msv1_0 (ntlm), data is exchanged between less-privileged user applications requring authentication, and Lsass.exe. With LSA providing the authentication back-end support. Both LSA and the less-privileged application communicate indirectly via the SSPI interface. Specifically, in SSL authentication, untrusted SSL record packets are passed from the less-privileged application to the privileged LSA. While extensive efforts are made in LSA to validate the SSL records, on WinXP's version of schannel, an off-by-one vulnerability exists in the parsing of the less-common and less-used SSL server-key-exchange record. The vulnerability can hence be triggered via less-privileged client applications utilizing the schannel's client-side SSL protocol implementation. This includes Internet Explorer, whenever the user
uses IE to browse a HTTPS site.

The vulnerable code exists in the _ReverseMemCopy() function and is reachable from Ssl3ParseServerKeyExchange():
(via SPProcessHandshake()->PkcsGenerateClientExchangeValue())

; On WinXP-SP1, the code below is located at 0x767FF976 (no symbols available)
Ssl3ParseServerKeyExchange()
... .text:767FFFC8 movzx ebx, byte ptr [esi] ; MSB-byte of malformed signature length field .text:767FFFCB movzx eax, byte ptr [esi+1] ; LSB-byte of malformed signature length field
.text:767FFFCF                 shl     ebx, 8
.text:767FFFD2                 add     ebx, eax
.text:767FFFD4                 push    ebx                      ; size=0
.text:767FFFD5 call _SPExternalAlloc@4 ; HeapAlloc will return a valid 0-length heap buffer address
.text:767FFFDA                 test    eax, eax
.text:767FFFDC                 mov     [ebp+pbSignature], eax
.text:767FFFDF                 jz      loc_768000B9
.text:767FFFE5                 push    ebx                      ; size=0
.text:767FFFE6 lea ecx, [esi+2] ; address of the signature data in our malformed record ; containing 0xFF,0x41,0x41...
.text:767FFFE9                 push    ecx
.text:767FFFEA push eax ; 0-length heap buffer
.text:767FFFEB                 call    _ReverseMemCopy@12
       _ReverseMemCopy()
       .text:767FF46F                 mov     edi, edi
       .text:767FF471                 push    ebp
       .text:767FF472                 mov     ebp, esp
       .text:767FF474                 mov     eax, [ebp+arg_8]
       .text:767FF477                 mov     ecx, [ebp+arg_4]
       .text:767FF47A                 push    esi
       .text:767FF47B                 mov     esi, [ebp+arg_0]
.text:767FF47E lea eax, [esi+eax-1] ; EAX=0, ESI which points to 0-length heap buffer ; is decremented to, before start of heap buffer
       .text:767FF482                 mov     dl, [ecx]
.text:767FF484 mov [eax], dl ; Off-by-one overwrite with 0xFF from our signature data
       .text:767FF486                 dec     eax
       .text:767FF487                 inc     ecx
       .text:767FF488                 cmp     eax, esi
.text:767FF48A jnb short loc_767FF482 ; Just one-byte overwrite! .text:767FF48C pop esi
       .text:767FF48D                 pop     ebp
       .text:767FF48E                 retn    0Ch


Discovered by:
Steven
Security Researcher
Vulnerability Research Lab
COSEINC

--
Thank you
Thomas Lim
COSEINC Private Limited

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