Fix qvalue parsing

CHANGES.rst

@@ -6,6 +6,7 @@ Version 2.3.7
 Unreleased
 
 -   Use ``flit_core`` instead of ``setuptools`` as build backend.
+-   Fix ``parse_options_header`` rejects q values "0" and "1". :issue:`2751`
 
 
 Version 2.3.6

src/werkzeug/_internal.py

@@ -314,6 +314,7 @@ def _decode_idna(domain: str) -> str:
 
 _plain_int_re = re.compile(r"-?\d+", re.ASCII)
 _plain_float_re = re.compile(r"-?\d+\.\d+", re.ASCII)
+_plain_qvalue_re = re.compile(r"(1(\.0{0,3})?)|(0(\.\d{0,3})?)", re.ASCII)
 
 
 def _plain_int(value: str) -> int:
@@ -339,3 +340,11 @@ def _plain_float(value: str) -> float:
         raise ValueError
 
     return float(value)
+
+
+def _qvalue(value: str) -> float:
+    """Parse a RFC9110 quality value"""
+    if _plain_qvalue_re.fullmatch(value) is None:
+        raise ValueError
+
+    return float(value)

src/werkzeug/http.py

@@ -18,8 +18,8 @@
 from urllib.request import parse_http_list as _parse_list_header
 
 from ._internal import _dt_as_utc
-from ._internal import _plain_float
 from ._internal import _plain_int
+from ._internal import _qvalue
 
 if t.TYPE_CHECKING:
     from _typeshed.wsgi import WSGIEnvironment
@@ -658,7 +658,7 @@ def parse_accept_header(
         if "q" in options:
             try:
                 # pop q, remaining options are reconstructed
-                q = _plain_float(options.pop("q"))
+                q = _qvalue(options.pop("q"))
             except ValueError:
                 # ignore an invalid q
                 continue

tests/test_http.py

@@ -776,6 +776,12 @@ def test_accept_invalid_float(value):
     assert list(a.values()) == ["en"]
 
 
+def test_accept_valid_int_one_zero():
+    assert http.parse_accept_header("en;q=1") == http.parse_accept_header("en;q=1.0")
+    assert http.parse_accept_header("en;q=0") == http.parse_accept_header("en;q=0.0")
+    assert http.parse_accept_header("en;q=5") == http.parse_accept_header("en;q=5.0")
+
+
 @pytest.mark.parametrize("value", ["🯱🯲🯳", "+1-", "1-1_23"])
 def test_range_invalid_int(value):
     assert http.parse_range_header(value) is None