Index: aiohttp-3.8.5/CHANGES/9851.bugfix.rst
===================================================================
--- /dev/null
+++ aiohttp-3.8.5/CHANGES/9851.bugfix.rst
@@ -0,0 +1 @@
+Fixed incorrect parsing of chunk extensions with the pure Python parser -- by :user:`bdraco`.
Index: aiohttp-3.8.5/aiohttp/http_parser.py
===================================================================
--- aiohttp-3.8.5.orig/aiohttp/http_parser.py
+++ aiohttp-3.8.5/aiohttp/http_parser.py
@@ -26,7 +26,7 @@ from yarl import URL
 
 from . import hdrs
 from .base_protocol import BaseProtocol
-from .helpers import NO_EXTENSIONS, BaseTimerContext
+from .helpers import NO_EXTENSIONS, BaseTimerContext, set_exception
 from .http_exceptions import (
     BadHttpMessage,
     BadStatusLine,
@@ -770,6 +770,14 @@ class HttpPayloadParser:
                         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 chunk[i:pos]:
+                                ext = repr(chunk[i:pos])
+                                exc = BadHttpMessage(
+                                    "Unexpected LF in chunk-extension: %s" % ext
+                                )
+                                set_exception(self.payload, exc)
+                                raise exc
                         else:
                             size_b = chunk[:pos]
 
Index: aiohttp-3.8.5/tests/test_http_parser.py
===================================================================
--- aiohttp-3.8.5.orig/tests/test_http_parser.py
+++ aiohttp-3.8.5/tests/test_http_parser.py
@@ -12,6 +12,7 @@ from yarl import URL
 
 import aiohttp
 from aiohttp import http_exceptions, streams
+from aiohttp.base_protocol import BaseProtocol
 from aiohttp.http_parser import (
     NO_EXTENSIONS,
     DeflateBuffer,
@@ -1202,3 +1203,27 @@ class TestDeflateBuffer:
         dbuf.feed_eof()
 
         assert buf.at_eof()
+
+
+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,
+        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())
Index: aiohttp-3.8.5/aiohttp/helpers.py
===================================================================
--- aiohttp-3.8.5.orig/aiohttp/helpers.py
+++ aiohttp-3.8.5/aiohttp/helpers.py
@@ -796,8 +796,10 @@ def set_result(fut: "asyncio.Future[_T]"
 
 
 def set_exception(fut: "asyncio.Future[_T]", exc: BaseException) -> None:
-    if not fut.done():
-        fut.set_exception(exc)
+    if asyncio.isfuture(fut) and fut.done():
+        return
+
+    fut.set_exception(exc)
 
 
 class ChainMapProxy(Mapping[str, Any]):