diff --git a/CHANGES/4818.feature b/CHANGES/4818.feature
new file mode 100644
index 00000000000..158e4ebae84
--- /dev/null
+++ b/CHANGES/4818.feature
@@ -0,0 +1 @@
+Add validation of HTTP header keys and values to prevent header injection.
diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index b9361827b66..7fb06298395 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -115,6 +115,7 @@ Felix Yan
 Fernanda Guimarães
 FichteFoll
 Florian Scheffler
+Franek Magiera
 Frederik Gladhorn
 Frederik Peter Aalund
 Gabriel Tremblay
diff --git a/aiohttp/_http_writer.pyx b/aiohttp/_http_writer.pyx
index 84b42fa1c35..eff85219586 100644
--- a/aiohttp/_http_writer.pyx
+++ b/aiohttp/_http_writer.pyx
@@ -111,6 +111,14 @@ cdef str to_str(object s):
         return str(s)
 
 
+cdef void _safe_header(str string) except *:
+    if "\r" in string or "\n" in string:
+        raise ValueError(
+            "Newline or carriage return character detected in HTTP status message or "
+            "header. This is a potential security issue."
+        )
+
+
 def _serialize_headers(str status_line, headers):
     cdef Writer writer
     cdef object key
@@ -119,6 +127,10 @@ def _serialize_headers(str status_line, headers):
 
     _init_writer(&writer)
 
+    for key, val in headers.items():
+        _safe_header(to_str(key))
+        _safe_header(to_str(val))
+
     try:
         if _write_str(&writer, status_line) < 0:
             raise
diff --git a/aiohttp/http_writer.py b/aiohttp/http_writer.py
index f859790efdd..428a7929b1a 100644
--- a/aiohttp/http_writer.py
+++ b/aiohttp/http_writer.py
@@ -171,13 +171,19 @@ async def drain(self) -> None:
             await self._protocol._drain_helper()
 
 
+def _safe_header(string: str) -> str:
+    if "\r" in string or "\n" in string:
+        raise ValueError(
+            "Newline or carriage return detected in headers. "
+            "Potential header injection attack."
+        )
+    return string
+
+
 def _py_serialize_headers(status_line: str, headers: "CIMultiDict[str]") -> bytes:
-    line = (
-        status_line
-        + "\r\n"
-        + "".join([k + ": " + v + "\r\n" for k, v in headers.items()])
-    )
-    return line.encode("utf-8") + b"\r\n"
+    headers_gen = (_safe_header(k) + ": " + _safe_header(v) for k, v in headers.items())
+    line = status_line + "\r\n" + "\r\n".join(headers_gen) + "\r\n\r\n"
+    return line.encode("utf-8")
 
 
 _serialize_headers = _py_serialize_headers
diff --git a/tests/test_http_writer.py b/tests/test_http_writer.py
index ab62ffc31b5..3fb5531ca1d 100644
--- a/tests/test_http_writer.py
+++ b/tests/test_http_writer.py
@@ -5,6 +5,7 @@
 from unittest import mock
 
 import pytest
+from multidict import CIMultiDict
 
 from aiohttp import http
 from aiohttp.test_utils import make_mocked_coro
@@ -272,3 +273,16 @@ async def test_drain_no_transport(protocol: Any, transport: Any, loop: Any) -> N
     msg._protocol.transport = None
     await msg.drain()
     assert not protocol._drain_helper.called
+
+
+async def test_write_headers_prevents_injection(
+    protocol: Any, transport: Any, loop: Any
+) -> None:
+    msg = http.StreamWriter(protocol, loop)
+    status_line = "HTTP/1.1 200 OK"
+    wrong_headers = CIMultiDict({"Set-Cookie: abc=123\r\nContent-Length": "256"})
+    with pytest.raises(ValueError):
+        await msg.write_headers(status_line, wrong_headers)
+    wrong_headers = CIMultiDict({"Content-Length": "256\r\nSet-Cookie: abc=123"})
+    with pytest.raises(ValueError):
+        await msg.write_headers(status_line, wrong_headers)
diff --git a/tests/test_web_response.py b/tests/test_web_response.py
index d37c8e8cc27..5ff0aabbe6e 100644
--- a/tests/test_web_response.py
+++ b/tests/test_web_response.py
@@ -15,6 +15,7 @@
 
 from aiohttp import HttpVersion, HttpVersion10, HttpVersion11, hdrs
 from aiohttp.helpers import ETag
+from aiohttp.http_writer import _serialize_headers
 from aiohttp.payload import BytesPayload
 from aiohttp.test_utils import make_mocked_coro, make_mocked_request
 from aiohttp.web import ContentCoding, Response, StreamResponse, json_response
@@ -59,12 +60,7 @@ def write(chunk):
         buf.extend(chunk)
 
     async def write_headers(status_line, headers):
-        headers = (
-            status_line
-            + "\r\n"
-            + "".join([k + ": " + v + "\r\n" for k, v in headers.items()])
-        )
-        headers = headers.encode("utf-8") + b"\r\n"
+        headers = _serialize_headers(status_line, headers)
         buf.extend(headers)
 
     async def write_eof(chunk=b""):