From c599730b9ecc4bf91194fd5acfef5766d92409c3 Mon Sep 17 00:00:00 2001
From: Zo Bot <github-automation@zo.computer>
Date: Fri, 3 Jul 2026 23:53:15 +0000
Subject: [PATCH] parse_mimetype: skip whitespace-only segments after a
 semicolon

Trailing ';' followed by whitespace (e.g. "text/html; ") inserted a
spurious empty-key parameter in the parsed parameters dict, while a
bare trailing ';' was already correctly skipped. The check is now
'if not item.strip()' so whitespace-only segments are also skipped.
---
 CHANGES/13009.bugfix.rst |  1 +
 aiohttp/helpers.py       |  2 +-
 tests/test_helpers.py    | 27 +++++++++++++++++++++++++++
 3 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 CHANGES/13009.bugfix.rst

diff --git a/CHANGES/13009.bugfix.rst b/CHANGES/13009.bugfix.rst
new file mode 100644
index 00000000000..f7faff672f0
--- /dev/null
+++ b/CHANGES/13009.bugfix.rst
@@ -0,0 +1 @@
+Fixed :py:func:`~aiohttp.helpers.parse_mimetype` inserting a spurious empty-key
diff --git a/aiohttp/helpers.py b/aiohttp/helpers.py
index 2fa8a19b6a7..65bc269e1da 100644
--- a/aiohttp/helpers.py
+++ b/aiohttp/helpers.py
@@ -332,7 +332,7 @@ def parse_mimetype(mimetype: str) -> MimeType:
     parts = mimetype.split(";")
     params: MultiDict[str] = MultiDict()
     for item in parts[1:]:
-        if not item:
+        if not item.strip():
             continue
         key, _, value = item.partition("=")
         params.add(key.lower().strip(), value.strip(' "'))
diff --git a/tests/test_helpers.py b/tests/test_helpers.py
index a499746607a..7f05c144385 100644
--- a/tests/test_helpers.py
+++ b/tests/test_helpers.py
@@ -83,6 +83,33 @@ def test_parse_mimetype(mimetype: str, expected: helpers.MimeType) -> None:
     assert result == expected
 
 
+@pytest.mark.parametrize(
+    "mimetype",
+    [
+        "text/html; ",
+        "text/html;  ",
+        "text/html;\t",
+        "text/html; \t",
+        "text/html; charset=utf-8; ",
+        "text/html; charset=utf-8;\t\t",
+    ],
+)
+def test_parse_mimetype_skips_whitespace_only_segments(
+    mimetype: str,
+) -> None:
+    # https://github.com/aio-libs/aiohttp/issues/13009
+    # A trailing ';' followed by whitespace (e.g. 'text/html; ') was being
+    # treated as a parameter with an empty key, producing {'': ''} in the
+    # parsed parameters. RFC 2045 says whitespace-only segments after a ';'
+    # are not valid parameters; a bare trailing ';' was already skipped
+    # because it becomes an empty string, but 'text/html; ' was truthy
+    # and slipped through.
+    result = helpers.parse_mimetype(mimetype)
+    assert "" not in result.parameters
+    if "charset=utf-8" in mimetype:
+        assert result.parameters.get("charset") == "utf-8"
+
+
 # ------------------- parse_content_type ------------------------------