Be more strict in parsing Content-Length

Validate that we are only parsing digits and nothing else. RFC7230 is
explicit in that the Content-Length can only exist of 1*DIGIT and may
not include any additional sign information.

The Python int() function parses `+10` as `10` which means we were more
lenient than the standard intended.

src/waitress/parser.py

@@ -23,6 +23,7 @@
 
 from waitress.buffers import OverflowableBuffer
 from waitress.receiver import ChunkedReceiver, FixedStreamReceiver
+from waitress.rfc7230 import HEADER_FIELD_RE, ONLY_DIGIT_RE
 from waitress.utilities import (
     BadRequest,
     RequestEntityTooLarge,
@@ -31,8 +32,6 @@
     find_double_newline,
 )
 
-from .rfc7230 import HEADER_FIELD
-
 
 def unquote_bytes_to_wsgi(bytestring):
     return unquote_to_bytes(bytestring).decode("latin-1")
@@ -221,7 +220,7 @@ def parse_header(self, header_plus):
         headers = self.headers
 
         for line in lines:
-            header = HEADER_FIELD.match(line)
+            header = HEADER_FIELD_RE.match(line)
 
             if not header:
                 raise ParsingError("Invalid header")
@@ -317,11 +316,12 @@ def parse_header(self, header_plus):
                 self.connection_close = True
 
         if not self.chunked:
-            try:
-                cl = int(headers.get("CONTENT_LENGTH", 0))
-            except ValueError:
+            cl = headers.get("CONTENT_LENGTH", "0")
+
+            if not ONLY_DIGIT_RE.match(cl.encode("latin-1")):
                 raise ParsingError("Content-Length is invalid")
 
+            cl = int(cl)
             self.content_length = cl
 
             if cl > 0:

tests/test_parser.py

@@ -193,6 +193,26 @@ def test_parse_header_bad_content_length(self):
         else:  # pragma: nocover
             self.assertTrue(False)
 
+    def test_parse_header_bad_content_length_plus(self):
+        data = b"GET /foobar HTTP/8.4\r\ncontent-length: +10\r\n"
+
+        try:
+            self.parser.parse_header(data)
+        except ParsingError as e:
+            self.assertIn("Content-Length is invalid", e.args[0])
+        else:  # pragma: nocover
+            self.assertTrue(False)
+
+    def test_parse_header_bad_content_length_minus(self):
+        data = b"GET /foobar HTTP/8.4\r\ncontent-length: -10\r\n"
+
+        try:
+            self.parser.parse_header(data)
+        except ParsingError as e:
+            self.assertIn("Content-Length is invalid", e.args[0])
+        else:  # pragma: nocover
+            self.assertTrue(False)
+
     def test_parse_header_multiple_content_length(self):
         data = b"GET /foobar HTTP/8.4\r\ncontent-length: 10\r\ncontent-length: 20\r\n"