From 25561d975a7a9e10eec3b381bc769737b57b3c9b Mon Sep 17 00:00:00 2001
From: Shadow_Lu <xshadowlu13@gmail.com>
Date: Mon, 27 Jul 2026 02:20:53 +0530
Subject: [PATCH] Strip trailing OWS from header values in the C parser
 (#13246)

---
 CHANGES/13246.bugfix.rst  |  3 +++
 CONTRIBUTORS.txt          |  1 +
 aiohttp/_http_parser.pyx  | 10 +++++++---
 tests/test_http_parser.py |  9 +++++++++
 4 files changed, 20 insertions(+), 3 deletions(-)
 create mode 100644 CHANGES/13246.bugfix.rst

diff --git a/CHANGES/13246.bugfix.rst b/CHANGES/13246.bugfix.rst
new file mode 100644
index 00000000000..8389dad2ad2
--- /dev/null
+++ b/CHANGES/13246.bugfix.rst
@@ -0,0 +1,3 @@
+Stripped the trailing whitespace from header values in the C HTTP parser,
+so that it matches the pure-Python parser and :rfc:`9110#section-5.5`
+-- by :user:`LuShadowX`.
diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 624b9400fbd..d273f6b88d2 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -253,6 +253,7 @@ Lubomir Gelo
 Ludovic Gasc
 Luis Pedrosa
 Lukasz Marcin Dobrzanski
+Lukshya Supyal
 Lénárd Szolnoki
 Makc Belousow
 Manuel Miranda
diff --git a/aiohttp/_http_parser.pyx b/aiohttp/_http_parser.pyx
index 73e73cc6773..3808417b0cf 100644
--- a/aiohttp/_http_parser.pyx
+++ b/aiohttp/_http_parser.pyx
@@ -430,16 +430,20 @@ cdef class HttpParser:
 
     cdef _process_header(self):
         cdef str value
+        cdef bytes raw_value
         if self._raw_name != b"":
             name = find_header(self._raw_name)
-            value = self._raw_value.decode('utf-8', 'surrogateescape')
+            # llhttp strips the OWS before the field value but not after.
+            # https://www.rfc-editor.org/info/rfc9110/#section-5.5-3
+            raw_value = self._raw_value.rstrip(b" \t")
+            value = raw_value.decode('utf-8', 'surrogateescape')
 
             # reject null bytes in header values - matches the Python parser
             # check at http_parser.py. llhttp in lenient mode doesn't reject
             # these itself, so we need to catch them here.
             # ref: RFC 9110 section 5.5 (CTL chars forbidden in field values)
             if "\x00" in value:
-                raise InvalidHeader(self._raw_value)
+                raise InvalidHeader(raw_value)
 
             if not self._lax and name in SINGLETON_HEADERS:
                 if name in self._seen_singletons:
@@ -454,7 +458,7 @@ cdef class HttpParser:
 
             self._has_value = False
             self._header_name_size = 0
-            self._raw_headers.append((self._raw_name, self._raw_value))
+            self._raw_headers.append((self._raw_name, raw_value))
             self._raw_name = b""
             self._raw_value = b""
 
diff --git a/tests/test_http_parser.py b/tests/test_http_parser.py
index 71e9c783ea0..f080cd208ff 100644
--- a/tests/test_http_parser.py
+++ b/tests/test_http_parser.py
@@ -645,6 +645,15 @@ def test_whitespace_before_header(parser: HttpRequestParser) -> None:
         parser.feed_data(text)
 
 
+def test_whitespace_around_header_value(parser: HttpRequestParser) -> None:
+    text = b"GET / HTTP/1.1\r\nHost: a\r\ntest: \t value \t \r\n\r\n"
+    messages, upgrade, tail = parser.feed_data(text)
+    msg = messages[0][0]
+
+    assert msg.headers["test"] == "value"
+    assert msg.raw_headers == ((b"Host", b"a"), (b"test", b"value"))
+
+
 @pytest.fixture
 def xfail_c_parser_status(request: pytest.FixtureRequest) -> None:
     if isinstance(request.getfixturevalue("parser"), HttpRequestParserPy):