Add ransomware extension hijacking detection signature

modules/signatures/windows/ransomware_extensionhijack.py

@@ -0,0 +1,51 @@
+# Copyright (C) 2026 Kevin Ross
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+from lib.cuckoo.common.abstracts import Signature
+
+class RansomwareSetsIcon(Signature):
+    name = "ransomware_sets_icon"
+    description = "Modifies registry keys for file-extension hijacking, possible ransomware behavior"
+    severity = 3
+    categories = ["ransomware"]
+    authors = ["Kevin Ross"]
+    minimum = "1.3"
+    evented = True
+    ttps = ["T1486", "T1564"] 
+    mbcs = ["OB0008", "E1486"]
+
+    filter_apinames = {"RegSetValueExA", "RegSetValueExW"}
+
+    def __init__(self, *args, **kwargs):
+        Signature.__init__(self, *args, **kwargs)
+        self.ret = False
+
+    def on_call(self, call, process):
+        filepath = self.get_argument(call, "Buffer")         
+        regkey = self.get_argument(call, "FullName")
+
+        if isinstance(filepath, str) and isinstance(regkey, str):
+            filepath_lower = filepath.lower()
+            regkey_lower = regkey.lower()
+
+            is_icon_hijack = filepath_lower.endswith(".ico") and (r"\defaulticon" in regkey_lower or r"\applications" in regkey_lower)
+            is_userchoice_hijack = r"\userchoice" in regkey_lower
+
+            if is_icon_hijack or is_userchoice_hijack:
+                self.mark_call()
+                self.ret = True
+
+    def on_complete(self):
+        return self.ret