On 2007-06-13 02:58+0800, Thomas Lim wrote:
>dear all
Dear all, this is not a 0day, it is a public release of a responsibly
disclosed vulnerability.
Thank you for sharing your research,
Gadi.
>
>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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--
--
"beepbeep it, i leave work, stop reading sec lists and im still hearing
gadi"
- HD Moore to Gadi Evron on IM, on Gadi's interview on npr, March 2007.
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