oauthex: accept empty list elements in WWW-Authenticate

oauthex/resource_meta.go

@@ -198,7 +198,7 @@ func splitChallenges(header string) ([]string, error) {
 			// This is a potential challenge separator.
 			// A new challenge does not start with `key=value`.
 			// We check if the part after the comma looks like a parameter.
-			lookahead := strings.TrimSpace(header[i+1:])
+			lookahead := trimEmptyListElements(header[i+1:])
 			eqPos := strings.Index(lookahead, "=")
 
 			isParam := false
@@ -239,6 +239,7 @@ func parseSingleChallenge(s string) (Challenge, error) {
 	}
 
 	params := make(map[string]string)
+	paramsStr = trimEmptyListElements(paramsStr)
 
 	// Parse the key-value parameters.
 	for paramsStr != "" {
@@ -301,7 +302,7 @@ func parseSingleChallenge(s string) (Challenge, error) {
 
 		// If there is a comma, consume it and continue to the next parameter.
 		if strings.HasPrefix(paramsStr, ",") {
-			paramsStr = strings.TrimSpace(paramsStr[1:])
+			paramsStr = trimEmptyListElements(paramsStr[1:])
 		} else if paramsStr != "" {
 			// If there's content but it's not a new parameter, the format is wrong.
 			return Challenge{}, fmt.Errorf("malformed auth parameter: expected comma after value, but got %q", paramsStr)
@@ -311,3 +312,12 @@ func parseSingleChallenge(s string) (Challenge, error) {
 	// Per RFC 9110, the scheme is case-insensitive.
 	return Challenge{Scheme: strings.ToLower(scheme), Params: params}, nil
 }
+
+// trimEmptyListElements removes leading whitespace and any empty list
+// elements, such as the extra comma in "a=1,, b=2". RFC 9110, section 5.6.1.2,
+// requires recipients to accept them.
+func trimEmptyListElements(s string) string {
+	return strings.TrimLeftFunc(s, func(r rune) bool {
+		return r == ',' || unicode.IsSpace(r)
+	})
+}

oauthex/resource_meta_test.go

@@ -53,6 +53,16 @@ func TestSplitChallenges(t *testing.T) {
 			input: `Basic realm="C:\\", Bearer error="insufficient_scope"`,
 			want:  []string{`Basic realm="C:\\"`, ` Bearer error="insufficient_scope"`},
 		},
+		{
+			name:  "empty list element between params",
+			input: `Bearer realm="example",, error="invalid_token"`,
+			want:  []string{`Bearer realm="example",, error="invalid_token"`},
+		},
+		{
+			name:  "empty list element between challenges",
+			input: `Basic, , Bearer realm="example"`,
+			want:  []string{`Basic`, ` `, ` Bearer realm="example"`},
+		},
 		{
 			name:  "empty input",
 			input: "",
@@ -117,6 +127,28 @@ func TestParseWWWAuthenticateEmptyQuotedValue(t *testing.T) {
 	}
 }
 
+// RFC 9110, section 5.6.1.2, requires recipients to accept empty list
+// elements. An extra comma must not move the parameters after it into a
+// separate challenge, or the auth handlers never see resource_metadata.
+func TestParseWWWAuthenticateEmptyListElements(t *testing.T) {
+	got, err := ParseWWWAuthenticate([]string{
+		`Bearer , realm="example",, resource_metadata="https://example.com/.well-known/oauth-protected-resource" ,, Basic realm="other",`,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []Challenge{
+		{Scheme: "bearer", Params: map[string]string{
+			"realm":             "example",
+			"resource_metadata": "https://example.com/.well-known/oauth-protected-resource",
+		}},
+		{Scheme: "basic", Params: map[string]string{"realm": "other"}},
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("ParseWWWAuthenticate() = %+v, want %+v", got, want)
+	}
+}
+
 func TestSplitChallengesError(t *testing.T) {
 	if _, err := splitChallenges(`"Bearer"`); err == nil {
 		t.Fatal("got nil, want error")
@@ -209,6 +241,15 @@ func TestParseSingleChallenge(t *testing.T) {
 			},
 			wantErr: false,
 		},
+		{
+			name:  "empty list elements around params",
+			input: `Bearer , realm="example",, error=invalid_token, ,`,
+			want: Challenge{
+				Scheme: "bearer",
+				Params: map[string]string{"realm": "example", "error": "invalid_token"},
+			},
+			wantErr: false,
+		},
 		{
 			name:    "malformed param - no value",
 			input:   "Bearer realm=",