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 -----BEGIN PGP PUBLIC KEY BLOCK----- Version: PGP 8.1 - not licensed for commercial use: www.pgp.com mQGiBEQM9cARBADvlIe8Ck5/u2EtX3ikd/eKjI7uZKyIFHNLxEYBB1AaHmEvYCPi VpvNr7ArKjbqlEpdsl6c9gQUY8vir5Lfk/p6siCD1aIYfCdPa64gKJQ66UVIUy7a hIlE8sJ86mcbvVGzA4f1LjwPUPwymeKEQeDJyRLlRnPkxWzaoiZqHuEa/QCg/2/t IAlQdVT7Q+ss51/NcL87RoEEANcf+ChnlH6vhXLSwnH1iXUMBbGA6t2F0/q29ROR lsMoUQW5hvjuOw+4yDzGzmBDQUYbN0GI7pNOBs7UwerGOInTGCFD6nan0JpONT51 bp5sfF93PNH12I1qVFf+h/qdX4me2mhyKfSNvc0qQMydwfsCJ3vBbEWTF7CqWZFO VadVA/9uJTKjJ7ZnN1enBBGUhLl6bA9estqH6lyP69B6Y1tGahDSqVyDe9Q9zs0T XDcM6aS+PRnybzX9gfgPfSYtDzX3AU6C7N2XgSK5DnjVZVr2Tdd/2ttM7ApvzaeV +ifO/nLGIQ38ik7mKlul5vlXsISShzHpUIdswuQtMp0R2sa+6bQfVGhvbWFzIExp bSA8dGhvbWFzQGNvc2VpbmMuY29tPokAXQQQEQIAHQUCRAz1wAcLCQgHAwIKAhkB BRsDAAAABR4BAAAAAAoJELxffA89J0fkz+cAn3cklzVq/VYiD9wgH0J2ULsuTbMl AJ9NMdYJHBlunYjbPJIcRgGwhAkY4LkCDQREDPXAEAgA9kJXtwh/CBdyorrWqULz Bej5UxE5T7bxbrlLOCDaAadWoxTpj0BV89AHxstDqZSt90xkhkn4DIO9ZekX1KHT UPj1WV/cdlJPPT2N286Z4VeSWc39uK50T8X8dryDxUcwYc58yWb/Ffm7/ZFexwGq 01uejaClcjrUGvC/RgBYK+X0iP1YTknbzSC0neSRBzZrM2w4DUUdD3yIsxx8Wy2O 9vPJI8BD8KVbGI2Ou1WMuF040zT9fBdXQ6MdGGzeMyEstSr/POGxKUAYEY18hKcK ctaGxAMZyAcpesqVDNmWn6vQClCbAkbTCD1mpF1Bn5x8vYlLIhkmuquiXsNV6TIL OwACAggAwTip4JFx4LCDazFSyTG7qzIlZonEf3QTHNH4jP15CLvVFxjaHE8g2EgL pt2+E6XDg7IGuZ2iXS9gwHkyLKzGR4bwpanAHyMZZbcQOglPHUkxuJZW+AjfcfOD 5jB+cUOtxk97ca/z9Fz+2qS8Q3sz2QSkHcZgBBxTS07cvd2P60ecVECBKG+dgxGw X13e5hgw2tzFRMqnty66lKXYEIUj4ZSl70UPAmy5xUaU4EahLURN29f3zM+EPy72 374v28ud28yO59iyRqoUGiHr3c87wumrDtjwm8qKIkWHsi+7AiES29nCqtm4zN45 3yXkalvQ/O97ZJSinzZb851ToowyFIkATAQYEQIADAUCRAz1wAUbDAAAAAAKCRC8 X3wPPSdH5MLbAKCIYtkvUPIoxm15I4UlvCIZjT8hdACdEWiQKWdlwZCJTePk4CF9 swkS3cQ= =Q3SR -----END PGP PUBLIC KEY BLOCK-----