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BAE Systems Applied Intelligence: Snake Rootkit Report 2014 17
DEAD BEEF ON A COOL BASE
As the driver intercepts all connections (e.g. on TDI_RECEIVE TDI event or ClientEventReceive() event notification triggered
through its TDI Filter Driver), it parses all incoming HTTP and SMTP traffic to see if it can be authenticated as Snake traffic.
The authentication is implemented by decrypting the data and making sure it starts with the markers 0xDEADBEAF and
0xC001BA5E (which appear to derive from
DEAD BEEF
and
COOL BASE
Here are specific steps:
The data it accepts should start from a 10 byte signature with the following rules:
the first 8 bytes must all be ASCII characters, the parser calculates their total sum (sum):
01 for (int i = 0; i < 8; i++)
02 {
03
if (*(BYTE *)ptrBuffer <= 32 ||
04
*(BYTE *)ptrBuffer >= 128)
05
06
return 0; // if not ASCII, quit
07
08
sum += *(BYTE *)ptrBuffer; // add to sum
09
++ptrBuffer; // advance buffer pointer
10 }
9th byte must be equal to sum / 26 + 65
10th byte must be equal to 122 - sum % 26
01 if ((*(BYTE *)ptrBuffer != sum / 26 + 65) ||
02 (*(BYTE *)(ptrBuffer + 1) != 122 - sum % 26))
03 {
04 result = 0;
05 }
Starting from the 11th byte, the data must be base64-encoded; the parser decodes that data
01 base_64_decode(abyBuffer + 10,
02 &ptrDecoded,
03 iMaxLength - 10);
Once decoded, the decrypted data should contain the aforementioned markers:
.text:F6751426 lea eax, [ebp+dwMarker] ; return marker here
.text:F6751429 push eax
.text:F675142A mov ecx, [ebp+buf_len] ; traffic
s buffer length
.text:F675142D push ecx
.text:F675142E mov edx, [ebp+abyBuffer] ; traffic
s buffer pointer
.text:F6751431 push edx
.text:F6751432 call decrypt_traffic ; decrypt traffic first
.text:F6751437 test eax, eax
.text:F6751439 jz short exit ; if failed, exit
.text:F675143B mov eax, [ebp+dwMarker] ; check the returned marker
.text:F675143E cmp eax, _DEADBEAF ; _DEADBEAF dd 0DEADBEAFh
.text:F6751444 jnz short exit ; if not 0xDEADBEAF, exit
.text:F6751446 cmp [ebp+dwNextDword], 0C001BA5Eh ; check next DWORD
.text:F675144D jnz short next ; if not 0xC001BA5E, exit
When the traffic is authenticated, its contents is then parsed by using
POST
http://
HTTP/
Content-Length
Connection
close
tags, in order to retrieve HTTP requests
SMTP traffic is also parsed, only by using
MAIL
RCPT
tags in order to retrieve SMTP characteristics
BAE Systems Applied Intelligence: Snake Rootkit Report 2014 18
By observing such behaviour, one might wonder why the driver is expecting HTTP or SMTP clients? Why does it act like HTTP/SMTP
server processing client traffic, and serving back normal responses as per the protocol?
For example, in HTTP the driver will respond with messages like
HTTP/1.1 200 OK
or
HTTP/1.1 500 Server Error
For SMTP traffic, it communicates back normal SMTP server responses, such as
250 Sender OK
503 Bad sequence of
commands
, etc.
The reason behind such behaviour is that the driver is acting in this mode like a proxy, routing requests from other infected hosts to
a remote C&C server.
Another opportunity this mode unlocks is a peer-to-peer network mode with no centralised C&C. The infected hosts are capable
of transferring the following peer-2-peer commands defining fragment size, reliability parameters, new peer information, peer
impersonation flags, etc.:
frag_size
frag_no_scrambling
peer_frag_size
read_peer_nfo
write_peer_nfo