Fix multipart forms losing `boundary` values on edit (#4625)

* Fix for issue #4613

* Adding tests

* Updated CHANGELOG.md

* Restoring contentviews

* Reverting contentview tests

* Adding boundary generation and tests

* Extra newline for flake8

* Janky byte fix

* Revert "Extra newline for flake8"

This reverts commit 683ba167de.

* Reverting a commit that was supposed to go to dev branch

* Update CHANGELOG.md

* Update test_http.py

Co-authored-by: Maximilian Hils <github@maximilianhils.com>

CHANGELOG.md

@@ -67,6 +67,7 @@ If you depend on these features, please raise your voice in
 * Customize markers with emoji, and filters: The `flow.mark` command may be used to mark a flow with either the default
   "red ball" marker, a single character, or an emoji like `:grapes:`. Use the `~marker` filter to filter on marker characters. (@rbdixon)
 * New `flow.comment` command to add a comment to the flow. Add `~comment <regex>` filter syntax to search flow comments. (@rbdixon)
+* Fix multipart forms losing `boundary` values on edit (@roytu)
 * --- TODO: add new PRs above this line ---
 * ... and various other fixes, documentation improvements, dependency version bumps, etc.
 

mitmproxy/http.py

@@ -1,3 +1,5 @@
+import binascii
+import os
 import re
 import time
 import urllib.parse
@@ -943,8 +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)
-        self.headers["content-type"] = "multipart/form-data"
 
     @property
     def multipart_form(self) -> multidict.MultiDictView[bytes, bytes]:

test/mitmproxy/test_http.py

@@ -429,9 +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.multipart_form = [(b"file", b"shell.jpg"), (b"file_size", b"1000")]
+        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: