diff --git a/CHANGES/9851.bugfix.rst b/CHANGES/9851.bugfix.rst
new file mode 100644
index 00000000000..02541a92dd4
--- /dev/null
+++ b/CHANGES/9851.bugfix.rst
@@ -0,0 +1 @@
+Fixed incorrect parsing of chunk extensions with the pure Python parser -- by :user:`bdraco`.
diff --git a/aiohttp/http_parser.py b/aiohttp/http_parser.py
index c20806841e7..148a30b2ca1 100644
--- a/aiohttp/http_parser.py
+++ b/aiohttp/http_parser.py
@@ -845,6 +845,13 @@ def feed_data(
                         i = chunk.find(CHUNK_EXT, 0, pos)
                         if i >= 0:
                             size_b = chunk[:i]  # strip chunk-extensions
+                            # Verify no LF in the chunk-extension
+                            if b"\n" in (ext := chunk[i:pos]):
+                                exc = BadHttpMessage(
+                                    f"Unexpected LF in chunk-extension: {ext!r}"
+                                )
+                                set_exception(self.payload, exc)
+                                raise exc
                         else:
                             size_b = chunk[:pos]
 
diff --git a/tests/test_http_parser.py b/tests/test_http_parser.py
index 2524bf34b3a..a8305da84f7 100644
--- a/tests/test_http_parser.py
+++ b/tests/test_http_parser.py
@@ -1478,7 +1478,55 @@ async def test_parse_chunked_payload_split_chunks(response: Any) -> None:
     assert await reader.read() == b"firstsecond"
 
 
-def test_partial_url(parser: Any) -> None:
+@pytest.mark.skipif(NO_EXTENSIONS, reason="Only tests C parser.")
+async def test_parse_chunked_payload_with_lf_in_extensions_c_parser(
+    loop: asyncio.AbstractEventLoop, protocol: BaseProtocol
+) -> None:
+    """Test the C-parser with a chunked payload that has a LF in the chunk extensions."""
+    # The C parser will raise a BadHttpMessage from feed_data
+    parser = HttpRequestParserC(
+        protocol,
+        loop,
+        2**16,
+        max_line_size=8190,
+        max_field_size=8190,
+    )
+    payload = (
+        b"GET / HTTP/1.1\r\nHost: localhost:5001\r\n"
+        b"Transfer-Encoding: chunked\r\n\r\n2;\nxx\r\n4c\r\n0\r\n\r\n"
+        b"GET /admin HTTP/1.1\r\nHost: localhost:5001\r\n"
+        b"Transfer-Encoding: chunked\r\n\r\n0\r\n\r\n"
+    )
+    with pytest.raises(http_exceptions.BadHttpMessage, match="\\\\nxx"):
+        parser.feed_data(payload)
+
+
+async def test_parse_chunked_payload_with_lf_in_extensions_py_parser(
+    loop: asyncio.AbstractEventLoop, protocol: BaseProtocol
+) -> None:
+    """Test the py-parser with a chunked payload that has a LF in the chunk extensions."""
+    # The py parser will not raise the BadHttpMessage directly, but instead
+    # it will set the exception on the StreamReader.
+    parser = HttpRequestParserPy(
+        protocol,
+        loop,
+        2**16,
+        max_line_size=8190,
+        max_field_size=8190,
+    )
+    payload = (
+        b"GET / HTTP/1.1\r\nHost: localhost:5001\r\n"
+        b"Transfer-Encoding: chunked\r\n\r\n2;\nxx\r\n4c\r\n0\r\n\r\n"
+        b"GET /admin HTTP/1.1\r\nHost: localhost:5001\r\n"
+        b"Transfer-Encoding: chunked\r\n\r\n0\r\n\r\n"
+    )
+    messages, _, _ = parser.feed_data(payload)
+    reader = messages[0][1]
+    assert isinstance(reader.exception(), http_exceptions.BadHttpMessage)
+    assert "\\nxx" in str(reader.exception())
+
+
+def test_partial_url(parser: HttpRequestParser) -> None:
     messages, upgrade, tail = parser.feed_data(b"GET /te")
     assert len(messages) == 0
     messages, upgrade, tail = parser.feed_data(b"st HTTP/1.1\r\n\r\n")