diff --git a/app/api/v2/handlers/payload_api.py b/app/api/v2/handlers/payload_api.py
index 1b034a332..9d16cc513 100644
--- a/app/api/v2/handlers/payload_api.py
+++ b/app/api/v2/handlers/payload_api.py
@@ -12,6 +12,20 @@
 from app.api.v2.schemas.payload_schemas import PayloadQuerySchema, PayloadSchema, PayloadCreateRequestSchema, \
     PayloadDeleteRequestSchema
 
+# Extensions that could be executed server-side and must never be accepted as
+# uploaded payloads (CWE-94 / Remote Code Execution mitigation).
+_BLOCKED_EXTENSIONS = frozenset({'.py', '.pyc', '.pyo', '.so', '.dll'})
+
+
+def _validate_payload_extension(filename: str) -> None:
+    """Raise HTTPBadRequest if the file extension is on the server-side executable blocklist."""
+    # Normalize the filename so the extension check reflects how it will be stored
+    # on filesystems like Windows that ignore trailing dots and spaces.
+    normalized = filename.rstrip(". ")
+    ext = os.path.splitext(normalized)[1].lower()
+    if ext in _BLOCKED_EXTENSIONS:
+        raise web.HTTPBadRequest(reason=f"File type {ext!r} is not allowed as a payload")
+
 
 class PayloadApi(BaseApi):
     def __init__(self, services):
@@ -73,6 +87,9 @@ async def post_payloads(self, request: web.Request):
         # Sanitize the file name to prevent directory traversal
         sanitized_filename = self.sanitize_filename(file_field.filename)
 
+        # Block server-side executable file types (CWE-94 RCE mitigation)
+        _validate_payload_extension(sanitized_filename)
+
         # Generate the file name and path
         file_name, file_path = await self.__generate_file_name_and_path(sanitized_filename)
 
diff --git a/tests/security/test_payload_extension_blocking.py b/tests/security/test_payload_extension_blocking.py
new file mode 100644
index 000000000..b09850059
--- /dev/null
+++ b/tests/security/test_payload_extension_blocking.py
@@ -0,0 +1,53 @@
+"""
+Security regression tests for payload upload extension blocking (CWE-94 fix).
+
+Verifies that _validate_payload_extension() correctly rejects server-side
+executable file types (.py, .pyc, .pyo, .so, .dll).
+"""
+import pytest
+from aiohttp import web
+
+from app.api.v2.handlers.payload_api import _validate_payload_extension
+
+
+class TestPayloadExtensionBlocking:
+
+    @pytest.mark.parametrize(
+        "filename",
+        [
+            "malicious.py",
+            "malicious.pyc",
+            "malicious.pyo",
+            "exploit.so",
+            "evil.dll",
+            "malicious.PY",
+            "malicious.Py",
+            # Files with a safe-looking extension followed by a blocked one
+            "legit.txt.py",
+        ],
+    )
+    def test_blocked_filenames_rejected(self, filename):
+        with pytest.raises(web.HTTPBadRequest):
+            _validate_payload_extension(filename)
+
+    def test_exe_allowed(self):
+        """Agent binaries (.exe) must still be uploadable."""
+        _validate_payload_extension('sandcat.exe')  # should not raise
+
+    def test_elf_allowed(self):
+        _validate_payload_extension('sandcat-linux')  # no extension — should not raise
+
+    def test_zip_allowed(self):
+        _validate_payload_extension('payloads.zip')
+
+    def test_go_allowed(self):
+        _validate_payload_extension('manx.go')
+
+    def test_blocked_extensions_set(self):
+        """Verify the constant is a frozenset and contains at least the known dangerous types."""
+        from app.api.v2.handlers.payload_api import _BLOCKED_EXTENSIONS as _BLK
+        assert isinstance(_BLK, frozenset), "_BLOCKED_EXTENSIONS must be a frozenset"
+        expected_minimum = frozenset({'.py', '.pyc', '.pyo', '.so', '.dll'})
+        assert _BLK.issuperset(expected_minimum), (
+            f"_BLOCKED_EXTENSIONS missing required extensions: {expected_minimum - _BLK}"
+        )