Synopsis
The flaw exists in BwPAlarm.dll due to improper validation of user-supplied data when processing an IOCTL 70022 RPC message. The attacker can control the size of a heap buffer to be allocated as well as the data to be copied to the buffer. Note that Advantech was already aware of this issue when we reported it; however, we discovered it independently.
Proof of Concept
Below is the disassembly of the IOCTL 70022 handler. Comments have been added for clarity.
.text:0700399E ioctl_70022: ; CODE XREF: _BwRPCPAlarmService+75↑j .text:0700399E ; DATA XREF: .text:jpt_7003855↓o .text:0700399E mov ebx, [ebp+arg_pInbuf] ; jumptable 07003855 case 10022 .text:070039A1 xor edi, edi .text:070039A3 mov [ebp+arg_pOutbuf], edi .text:070039A6 mov eax, [ebx] .text:070039A8 mov edx, [ebx+4] .text:070039AB mov [ebp+dw0], eax .text:070039AE mov eax, [ebx+8] ; attacker-controlled .text:070039B1 push eax ; heap buf size controlled by attacker .text:070039B2 push 3Dh ; '=' .text:070039B4 mov [ebp+dw4], edx .text:070039B7 call _calloc .text:070039BC add ebx, 0Ch .text:070039BF push 3Bh ; ';' ; Val .text:070039C1 push ebx ; Str .text:070039C2 mov [ebp+arg_InbufLen_pHeapBuf], eax ... .text:07003A3C push 16h .text:07003A3E lea eax, [edi-16h] .text:07003A41 push edx .text:07003A42 push eax ; copy to heap buf .text:07003A43 mov [edi-18h], cx .text:07003A47 call strlcpy(char *,char const *,uint) .text:07003A4C lea ecx, [ebp+var_168] .text:07003A52 push 25h ; '%' .text:07003A54 push ecx .text:07003A55 push edi ; copy to heap buf .text:07003A56 call strlcpy(char *,char const *,uint) .text:07003A5B mov eax, [ebp+arg_OutbufLen] .text:07003A5E lea ebx, [esi+1] .text:07003A61 inc eax .text:07003A62 push 3Bh ; ';' ; look for next string token separated by ';' .text:07003A64 push ebx .text:07003A65 mov [ebp+arg_OutbufLen], eax .text:07003A68 add edi, 3Dh ; '=' .text:07003A6B call _strchr .text:07003A70 mov esi, eax .text:07003A72 add esp, 2Ch .text:07003A75 test esi, esi .text:07003A77 jnz process_next_str_token_in_input_buffer ...
The following shows the crash of webvrpcs.exe and the access violation is exploitable:
0:010> g (1428.678): Access violation - code c0000005 (first chance) First chance exceptions are reported before any exception handling. This exception may be expected and handled. *** WARNING: Unable to verify checksum for C:\WebAccess\Node\BwPAlarm.dll eax=00000001 ebx=02e4e495 ecx=00000015 edx=034af6ff esi=02400000 edi=034af64d eip=070027db esp=034aead0 ebp=034af734 iopl=0 nv up ei ng nz na pe nc cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010286 BwPAlarm+0x27db: 070027db 8816 mov byte ptr [esi],dl ds:002b:02400000=10 0:009> kb ChildEBP RetAddr Args to Child 034aead4 07003a4c 023fffff 034af64c 00000016 BwPAlarm+0x27db *** WARNING: Unable to verify checksum for C:\WebAccess\Node\webvrpcs.exe *** ERROR: Module load completed but symbols could not be loaded for C:\WebAccess\Node\webvrpcs.exe 034af734 004049de 00002726 023f0bc0 02e40050 BwPAlarm+0x3a4c WARNING: Stack unwind information not available. Following frames may be wrong. 034af9d0 00402cc1 002828e0 006de0d0 00011186 webvrpcs+0x49de 034afa1c 00401370 002828e0 006de0d0 00011186 webvrpcs+0x2cc1 *** ERROR: Symbol file could not be found. Defaulted to export symbols for C:\Windows\syswow64\RPCRT4.dll - 034afb60 7db85feb 002829a4 1e092dea 002872d0 webvrpcs+0x1370 034afb98 7db8648b 00401260 002829a4 034afc3c RPCRT4!NdrOutInit+0x11d 034afbf0 7db86365 00000001 00000000 00000000 RPCRT4!RpcBindingInqObject+0x2aa 034afc14 7dbc7e25 002829a4 00000000 00000000 RPCRT4!RpcBindingInqObject+0x184 034afca0 7dbc8174 00000b74 002828e0 0028295c RPCRT4!I_RpcFilterDCOMActivation+0x3891 034afcb4 7dbc83b9 00000000 002734a0 00272ab0 RPCRT4!I_RpcTransServerNewConnection+0x118 034afce0 7dbc8b6a 002734a0 02000b8c 00000000 RPCRT4!I_RpcTransServerNewConnection+0x35d 034afd40 7dbd74e7 00000000 002734a0 00000b8c RPCRT4!I_RpcTransServerNewConnection+0xb0e 034afd54 7dbebfff 00280210 0000000c 00000000 RPCRT4!RpcMgmtSetAuthorizationFn+0x6528 034afd78 7dbec1f8 00280210 0000000c 00000000 RPCRT4!I_RpcInitHttpImports+0x78e *** ERROR: Symbol file could not be found. Defaulted to export symbols for C:\Windows\syswow64\KERNELBASE.dll - 034afdb0 7d888369 00000b8c 00000000 00272c30 RPCRT4!I_RpcInitHttpImports+0x987 034afdd4 7dee4cd6 034afe74 002632e0 00272c30 KERNELBASE!CreateThreadpoolWait+0x62 034afe30 7debfb4b 034afe74 00251018 00272c30 ntdll!WinSqmStartSession+0x399 034aff88 7dd7343d 00266a60 034affd4 7dea9812 ntdll!TpSetTimer+0x3bf 034aff94 7dea9812 00266a60 e91c4c39 00000000 kernel32!BaseThreadInitThunk+0x12 034affd4 7dea97e5 7dec04ac 00266a60 ffffffff ntdll!RtlInitializeExceptionChain+0x63 034affec 00000000 7dec04ac 00266a60 00000000 ntdll!RtlInitializeExceptionChain+0x36 0:009> .load msec.dll 0:009> !exploitable !exploitable 1.6.0.0 Exploitability Classification: EXPLOITABLE Recommended Bug Title: Exploitable - User Mode Write AV starting at BwPAlarm+0x00000000000027db (Hash=0x46a88e71.0x5200d855) User mode write access violations that are not near NULL are exploitable.
Solution
Disclosure Timeline
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