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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