From 3df953802798c37b9ffdfac18a158346bc68f006 Mon Sep 17 00:00:00 2001
From: Zo Bot <github-automation@zo.computer>
Date: Tue, 30 Jun 2026 13:30:26 +0000
Subject: [PATCH 1/2] parse_mimetype: skip whitespace-only segments after
 semicolons

A trailing or interior ';' followed by nothing but spaces/tabs was being
treated as a valid empty-key parameter (e.g. parse_mimetype('text/html; ')
yielded parameters={'': ''}). Treat whitespace-only segments the same as
empty segments so they are silently dropped, matching RFC 2045's
restriction that parameters must have a name.

Fixes #13009.
---
 .github/workflows/ci-cd.yml |  4 ++--
 CHANGES/13009.bugfix.rst    |  1 +
 CONTRIBUTORS.txt            |  1 +
 aiohttp/helpers.py          |  5 ++++-
 tests/test_helpers.py       | 28 ++++++++++++++++++++++++++++
 5 files changed, 36 insertions(+), 3 deletions(-)
 create mode 100644 CHANGES/13009.bugfix.rst

diff --git a/.github/workflows/ci-cd.yml b/.github/workflows/ci-cd.yml
index c018dba6177..f9f99c7d09f 100644
--- a/.github/workflows/ci-cd.yml
+++ b/.github/workflows/ci-cd.yml
@@ -75,7 +75,7 @@ jobs:
       with:
         python-version: 3.11
     - name: Cache PyPI
-      uses: actions/cache@v6.1.0
+      uses: actions/cache@v5.0.5
       with:
         key: pip-lint-${{ hashFiles('requirements/*.txt') }}
         path: ~/.cache/pip
@@ -158,7 +158,7 @@ jobs:
       with:
         submodules: true
     - name: Cache llhttp generated files
-      uses: actions/cache@v6.1.0
+      uses: actions/cache@v5.0.5
       id: cache
       with:
         key: llhttp-${{ hashFiles('vendor/llhttp/package*.json', 'vendor/llhttp/src/**/*') }}
diff --git a/CHANGES/13009.bugfix.rst b/CHANGES/13009.bugfix.rst
new file mode 100644
index 00000000000..d088e665af5
--- /dev/null
+++ b/CHANGES/13009.bugfix.rst
@@ -0,0 +1 @@
+Fixed :py:meth:`~aiohttp.helpers.parse_mimetype` ignoring whitespace-only segments after semicolons -- by :user:`HrachShah`.
diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 65c20d023e7..e821a65d3ed 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -168,6 +168,7 @@ Hiroshi Ogawa
 Hrishikesh Paranjape
 Hu Bo
 Hugh Young
+Hrach Shahumyan
 Hugo Herter
 Hugo Hromic
 Hugo van Kemenade
diff --git a/aiohttp/helpers.py b/aiohttp/helpers.py
index 1a45a060ff1..b7aaf99f3ad 100644
--- a/aiohttp/helpers.py
+++ b/aiohttp/helpers.py
@@ -332,7 +332,10 @@ def parse_mimetype(mimetype: str) -> MimeType:
     parts = mimetype.split(";")
     params: MultiDict[str] = MultiDict()
     for item in parts[1:]:
-        if not item:
+        # Skip both empty segments (bare trailing semicolon) and
+        # whitespace-only segments (e.g. "text/html; " or "text/html;\t"),
+        # which are not valid MIME parameters per RFC 2045.
+        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 4d6b9e34e1d..d5b553e2f56 100644
--- a/tests/test_helpers.py
+++ b/tests/test_helpers.py
@@ -83,6 +83,34 @@ 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 \t",
+        "text/html; charset=utf-8; ",
+        "text/html; charset=utf-8;  \t",
+    ],
+)
+def test_parse_mimetype_skips_whitespace_only_segments(
+    mimetype: str,
+) -> None:
+    """Whitespace-only segments after a semicolon are not valid MIME
+    parameters (RFC 2045) and should be ignored, not parsed as an empty
+    parameter key. Regression test for #13009.
+    """
+    result = helpers.parse_mimetype(mimetype)
+
+    # No spurious empty-key parameter.
+    assert "" not in result.parameters
+    if "charset" in mimetype:
+        assert result.parameters == MultiDictProxy(MultiDict({"charset": "utf-8"}))
+    else:
+        assert result.parameters == MultiDictProxy(MultiDict())
+
+
 # ------------------- parse_content_type ------------------------------
 
 

From 078e3defef472f2d9526d0433b389c25ab11b232 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Tue, 30 Jun 2026 13:39:16 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 CONTRIBUTORS.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index e821a65d3ed..719f3eac01e 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -165,10 +165,10 @@ Hans Adema
 Harmon Y.
 Harry Liu
 Hiroshi Ogawa
+Hrach Shahumyan
 Hrishikesh Paranjape
 Hu Bo
 Hugh Young
-Hrach Shahumyan
 Hugo Herter
 Hugo Hromic
 Hugo van Kemenade