Merge pull request #409 from fireeye/fix/extract-bytes

improve bytes feature extraction

capa/features/extractors/ida/helpers.py

@@ -166,6 +166,10 @@ def basic_block_size(bb):
 
 def read_bytes_at(ea, count):
     """ """
+    # check if byte has a value, see get_wide_byte doc
+    if not idc.is_loaded(ea):
+        return b""
+
     segm_end = idc.get_segm_end(ea)
     if ea + count > segm_end:
         return idc.get_bytes(ea, segm_end - ea)

capa/features/extractors/ida/insn.py

@@ -148,6 +148,9 @@ def extract_insn_bytes_features(f, bb, insn):
     example:
         push    offset iid_004118d4_IShellLinkA ; riid
     """
+    if idaapi.is_call_insn(insn):
+        return
+
     ref = capa.features.extractors.ida.helpers.find_data_reference_from_insn(insn)
     if ref != insn.ea:
         extracted_bytes = capa.features.extractors.ida.helpers.read_bytes_at(ref, MAX_BYTES_FEATURE_SIZE)

capa/features/extractors/viv/insn.py

@@ -258,10 +258,10 @@ def extract_insn_bytes_features(f, bb, insn):
     example:
         #     push    offset iid_004118d4_IShellLinkA ; riid
     """
-    for oper in insn.opers:
-        if insn.mnem == "call":
-            continue
+    if insn.mnem == "call":
+        return
 
+    for oper in insn.opers:
         if isinstance(oper, envi.archs.i386.disasm.i386ImmOper):
             v = oper.getOperValue(oper)
         elif isinstance(oper, envi.archs.i386.disasm.i386RegMemOper):

tests/fixtures.py

@@ -10,6 +10,7 @@
 import os
 import sys
 import os.path
+import binascii
 import contextlib
 import collections
 
@@ -444,6 +445,8 @@ FEATURE_PRESENCE_TESTS = [
     ("mimikatz", "function=0x40105D", capa.features.Bytes("SCardTransmit".encode("utf-16le")), True),
     ("mimikatz", "function=0x40105D", capa.features.Bytes("ACR  > ".encode("utf-16le")), True),
     ("mimikatz", "function=0x40105D", capa.features.Bytes("nope".encode("ascii")), False),
+    # IDA features included byte sequences read from invalid memory, fixed in #409
+    ("mimikatz", "function=0x44570F", capa.features.Bytes(binascii.unhexlify("FF" * 256)), False),
     # insn/bytes, pointer to bytes
     ("mimikatz", "function=0x44EDEF", capa.features.Bytes("INPUTEVENT".encode("utf-16le")), True),
     # insn/characteristic(nzxor)