CobaltStrike_Resources_Reverse_Bin_v2_5_through_v4_x

rule CobaltStrike_Resources_Reverse_Bin_v2_5_through_v4_x
{
	meta:
		description = "Cobalt Strike's resources/reverse.bin signature for versions 2.5 to 4.x"
		hash =  "887f666d6473058e1641c3ce1dd96e47189a59c3b0b85c8b8fccdd41b84000c7"
		author = "gssincla@google.com"
		reference = "https://cloud.google.com/blog/products/identity-security/making-cobalt-strike-harder-for-threat-actors-to-abuse"
		date = "2022-11-18"
		
		id = "182dbcd0-1180-5516-abe3-cf2eebbd0e39"
	strings:
	/*
		31 ??     xor     eax, eax
		AC        lodsb
		C1 ?? 0D  ror     edi, 0Dh
		01 ??     add     edi, eax
		38 ??     cmp     al, ah
		75 ??     jnz     short loc_10000054
		03 [2]    add     edi, [ebp-8]
		3B [2]    cmp     edi, [ebp+24h]
		75 ??     jnz     short loc_1000004A
		5?        pop     eax
		8B ?? 24  mov     ebx, [eax+24h]
		01 ??     add     ebx, edx
		66 8B [2] mov     cx, [ebx+ecx*2]
		8B ?? 1C  mov     ebx, [eax+1Ch]
		01 ??     add     ebx, edx
		8B ?? 8B  mov     eax, [ebx+ecx*4]
		01 ??     add     eax, edx
		89 [3]    mov     [esp+28h+var_4], eax
		5?        pop     ebx
		5?        pop     ebx
	*/

	$apiLocator = {
			31 ?? 
			AC
			C1 ?? 0D 
			01 ?? 
			38 ?? 
			75 ?? 
			03 [2]
			3B [2]
			75 ?? 
			5? 
			8B ?? 24 
			01 ?? 
			66 8B [2]
			8B ?? 1C 
			01 ?? 
			8B ?? 8B 
			01 ?? 
			89 [3]
			5? 
			5? 
		}

    // the signature for the stagers overlap significantly. Looking for reverse.bin specific bytes helps delineate sample types
	/*
		5D             pop     ebp
		68 33 32 00 00 push    '23'
		68 77 73 32 5F push    '_2sw'
	*/

	$ws2_32 = {
			5D
			68 33 32 00 00
			68 77 73 32 5F
		}


  // reverse.bin makes outbound connection (using connect) while bind.bin listens for incoming connections (using listen)
  // so the presence of the connect API hash is a solid method for distinguishing between the two.
	/*
		6A 10          push    10h
		[0]5?          push    esi
		5?             push    edi
		68 99 A5 74 61 push    connect
	*/
	$connect = {
			6A 10
			5? 
			5? 
			68 99 A5 74 61
		}
	
	condition:
		$apiLocator and $ws2_32 and $connect
}