Home / vulnerabilities schannel-offbyone.txt
Posted on 13 June 2007
Source : packetstormsecurity.org Link
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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