fix handling of trailing commas in SecretStringSliceCSV.Set()

CHANGELOG.md

@@ -62,6 +62,7 @@
 * [BUGFIX] Distributor: Release the push worker pool goroutines on shutdown by stopping the async executor during the stopping phase when `-distributor.num-push-workers` is set. #7602
 * [BUGFIX] Querier: Fix flake in integration tests TestQuerierWithStoreGatewayDataBytesLimits and TestQuerierWithBlocksStorageLimits by waiting for the querier to see the store-gateway ACTIVE in the ring before querying. #7614
 * [BUGFIX] Ruler: Register xfunctions (xincrease, xrate, xdelta) in the global parser before loading rule files. #7621
+* [BUGFIX] Security: Reject empty entries in `-distributor.sign-write-requests-keys` caused by stray or trailing commas (e.g. `newkey,`). Previously these were silently accepted and produced an empty signing key, which downgraded HMAC stream-push authentication to a forgeable signature. Misconfigured flags now fail at process startup; audit your configs before upgrading. #7587
 
 ## 1.21.0 2026-04-24
 

pkg/util/flagext/secretstringslicecsv.go

@@ -1,6 +1,9 @@
 package flagext
 
-import "strings"
+import (
+	"errors"
+	"strings"
+)
 
 // SecretStringSliceCSV is a slice of strings that is parsed from a comma-separated string.
 // It implements flag.Value and yaml Marshalers, but masks the value when marshaled to YAML
@@ -15,12 +18,25 @@ func (v SecretStringSliceCSV) String() string {
 }
 
 // Set implements flag.Value
+// Each comma-separated entry is trimmed of surrounding whitespace.
+// Empty entries (after trimming) are rejected with an error.
 func (v *SecretStringSliceCSV) Set(s string) error {
 	if s == "" {
 		v.values = nil
 		return nil
 	}
-	v.values = strings.Split(s, ",")
+	parts := strings.Split(s, ",")
+	values := make([]string, 0, len(parts))
+	for _, p := range parts {
+		p = strings.TrimSpace(p)
+		if p == "" {
+			// Do not include the original input value in the error message
+			// to avoid accidentally exposing secret values.
+			return errors.New("invalid value: empty entry after trimming")
+		}
+		values = append(values, p)
+	}
+	v.values = values
 	return nil
 }
 

pkg/util/flagext/secretstringslicecsv_test.go

@@ -44,4 +44,32 @@ func TestSecretStringSliceCSV(t *testing.T) {
 		require.NoError(t, s.Keys.Set(""))
 		assert.Equal(t, []string(nil), s.Keys.Value())
 	})
+
+	t.Run("trailing comma is rejected", func(t *testing.T) {
+		var s TestStruct
+		err := s.Keys.Set("newkey,")
+		require.Error(t, err, "trailing comma must produce an error")
+		assert.Nil(t, s.Keys.Value(), "values must not be updated on error")
+	})
+
+	t.Run("leading comma is rejected", func(t *testing.T) {
+		var s TestStruct
+		require.Error(t, s.Keys.Set(",newkey"))
+	})
+
+	t.Run("double comma is rejected", func(t *testing.T) {
+		var s TestStruct
+		require.Error(t, s.Keys.Set("newkey,,oldkey"))
+	})
+
+	t.Run("whitespace-only entry is rejected", func(t *testing.T) {
+		var s TestStruct
+		require.Error(t, s.Keys.Set("newkey, ,oldkey"))
+	})
+
+	t.Run("surrounding whitespace is trimmed from valid entries", func(t *testing.T) {
+		var s TestStruct
+		require.NoError(t, s.Keys.Set(" key1 , key2 "))
+		assert.Equal(t, []string{"key1", "key2"}, s.Keys.Value())
+	})
 }