ScanJpeg improvements, fixes #270

src/python/strelka/scanners/scan_jpeg.py

@@ -1,30 +1,86 @@
+import struct
+
 from strelka import strelka
 
 
 class ScanJpeg(strelka.Scanner):
-    """Extracts data embedded in JPEG files.
+    """Extracts data appended to JPEG files.
 
-    This scanner extracts data that is inserted past the JFIF trailer.
+    This scanner extracts data that is inserted past the JFIF EOI marker.
     """
+
     def scan(self, data, file, options, expire_at):
-        if not data.endswith(b'\xff\xd9'):
-            trailer_index = data.rfind(b'\xff\xd9')
-            if trailer_index == -1:
-                self.flags.append('no_trailer')
+
+        offset = 0
+
+        # Skip check for length with these markers
+        markers_zero_length = [
+            b"\xff\xd0",
+            b"\xff\xd1",
+            b"\xff\xd2",
+            b"\xff\xd3",
+            b"\xff\xd4",
+            b"\xff\xd5",
+            b"\xff\xd6",
+            b"\xff\xd7",
+            b"\xff\xd8",
+            b"\xff\x01",
+        ]
+
+        # Image must start with SOI
+        if not data[offset:].startswith(b"\xff\xd8"):
+            self.flags.append("corrupt_jpeg_data_no_soi")
+            return
+
+        # Skip SOI
+        offset += 2
+        while True:
+
+            marker = data[offset : offset + 2]
+
+            # Marker must start with 0xff
+            if marker[0] != 0xFF:
+                self.flags.append("corrupt_jpeg_data_misaligned_marker")
+                break
+
+            if marker in markers_zero_length:
+                offset += 2
+                continue
+            # Start scan data (SOS)
+            elif marker == b"\xff\xda":
+                offset += 2
+                while True:
+                    # Fast forward until we find a marker that's not FF00
+                    if data[offset] == 0xFF and data[offset + 1] != 0x00:
+                        break
+                    offset += 1
+                continue
+            # EOI marker
+            elif marker == b"\xff\xd9":
+                offset += 2
+                break
             else:
-                trailer_data = data[trailer_index + 2:]
-                if trailer_data:
-                    self.event['trailer_index'] = trailer_index
-
-                    extract_file = strelka.File(
-                        source=self.name,
-                    )
-
-                    for c in strelka.chunk_string(trailer_data):
-                        self.upload_to_coordinator(
-                            extract_file.pointer,
-                            c,
-                            expire_at,
-                        )
-
-                    self.files.append(extract_file)
+                marker_length = struct.unpack(">H", data[offset + 2 : offset + 4])[0]
+                offset += 2
+                offset += marker_length
+
+        # If the end of the image is reached with no more data, return
+        if offset >= len(data):
+            self.flags.append("no_trailer")
+            return
+
+        if trailer_data := data[offset:]:
+            self.event["trailer_index"] = offset
+
+            extract_file = strelka.File(
+                source=self.name,
+            )
+
+            for c in strelka.chunk_string(trailer_data):
+                self.upload_to_coordinator(
+                    extract_file.pointer,
+                    c,
+                    expire_at,
+                )
+
+            self.files.append(extract_file)

src/python/strelka/tests/test_scan_jpeg.py

@@ -11,7 +11,7 @@ def test_scan_jpeg(mocker):
     Failure: Unable to load file or sample event fails to match.
     """
 
-    test_scan_event = {"elapsed": mock.ANY, "flags": []}
+    test_scan_event = {"elapsed": mock.ANY, "flags": ["no_trailer"]}
 
     scanner_event = run_test_scan(
         mocker=mocker,
@@ -29,7 +29,7 @@ def test_scan_jpeg_pe_overlay(mocker):
     Failure: Unable to load file or sample event fails to match.
     """
 
-    test_scan_event = {"elapsed": mock.ANY, "flags": [], "trailer_index": 308564}
+    test_scan_event = {"elapsed": mock.ANY, "flags": [], "trailer_index": 308566}
 
     scanner_event = run_test_scan(
         mocker=mocker,