Adding boundary generation and tests

mitmproxy/http.py

@@ -1,3 +1,5 @@
+import binascii
+import os
 import re
 import time
 import urllib.parse
@@ -943,10 +945,17 @@ class Request(Message):
         return ()
 
     def _set_multipart_form(self, value):
+        is_valid_content_type = self.headers.get("content-type", "").lower().startswith("multipart/form-data")
+        if not is_valid_content_type:
+            """
+            Generate a random boundary here.
+
+            See <https://datatracker.ietf.org/doc/html/rfc2046#section-5.1.1> for specifications
+            on generating the boundary.
+            """
+            boundary = "-" * 20 + binascii.hexlify(os.urandom(16)).decode()
+            self.headers["content-type"] = f"multipart/form-data; boundary={boundary}"
         self.content = multipart.encode(self.headers, value)
-        if "content-type" not in self.headers:
-            # Don't overwrite header if it already exists or it will destroy the boundary value
-            self.headers["content-type"] = "multipart/form-data"
 
     @property
     def multipart_form(self) -> multidict.MultiDictView[bytes, bytes]:

test/mitmproxy/test_http.py

@@ -429,15 +429,9 @@ class TestRequestUtils:
 
     def test_set_multipart_form(self):
         request = treq()
-        request.multipart_form = [("file", "shell.jpg"), ("file_size", "1000")]
-        assert request.headers["Content-Type"] == 'multipart/form-data'
-        assert request.content is None
-
-        request = treq()
-        request.headers["Content-Type"] = 'multipart/form-data; boundary=foo'
         request.multipart_form = [(b"file", b"shell.jpg"), (b"file_size", b"1000")]
-        assert request.headers["Content-Type"] == 'multipart/form-data; boundary=foo'
-
+        assert request.headers["Content-Type"].startswith('multipart/form-data')
+        assert list(request.multipart_form.items()) == [(b"file", b"shell.jpg"), (b"file_size", b"1000")]
 
 class TestResponse: