fix: reject invalid json pointer escapes

internal/binding/json_pointer.go

@@ -33,8 +33,10 @@ func ReadJSONPointer(data interface{}, pointer string) (interface{}, error) {
 
 	for i, raw := range segments {
 		// RFC 6901 unescaping: ~1 → /, ~0 → ~ (order matters).
-		key := strings.ReplaceAll(raw, "~1", "/")
-		key = strings.ReplaceAll(key, "~0", "~")
+		key, err := decodeJSONPointerSegment(raw)
+		if err != nil {
+			return nil, fmt.Errorf("json pointer %q: segment %q: %w", pointer, raw, err)
+		}
 
 		m, ok := current.(map[string]interface{})
 		if !ok {
@@ -53,3 +55,26 @@ func ReadJSONPointer(data interface{}, pointer string) (interface{}, error) {
 
 	return current, nil
 }
+
+func decodeJSONPointerSegment(raw string) (string, error) {
+	var out strings.Builder
+	for i := 0; i < len(raw); i++ {
+		if raw[i] != '~' {
+			out.WriteByte(raw[i])
+			continue
+		}
+		if i+1 >= len(raw) {
+			return "", fmt.Errorf("invalid escape: ~ must be followed by 0 or 1")
+		}
+		switch raw[i+1] {
+		case '0':
+			out.WriteByte('~')
+		case '1':
+			out.WriteByte('/')
+		default:
+			return "", fmt.Errorf("invalid escape: ~%c must be ~0 or ~1", raw[i+1])
+		}
+		i++
+	}
+	return out.String(), nil
+}

internal/binding/json_pointer_test.go

@@ -98,6 +98,41 @@ func TestReadJSONPointer_RFC6901_Escaping(t *testing.T) {
 	}
 }
 
+func TestReadJSONPointer_InvalidEscape(t *testing.T) {
+	data := map[string]interface{}{
+		"a~2b": "literal",
+		"a~":   "literal",
+	}
+	tests := []struct {
+		name    string
+		pointer string
+		want    string
+	}{
+		{
+			name:    "unsupported escape code",
+			pointer: "/a~2b",
+			want:    `json pointer "/a~2b": segment "a~2b": invalid escape: ~2 must be ~0 or ~1`,
+		},
+		{
+			name:    "dangling tilde",
+			pointer: "/a~",
+			want:    `json pointer "/a~": segment "a~": invalid escape: ~ must be followed by 0 or 1`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := ReadJSONPointer(data, tt.pointer)
+			if err == nil {
+				t.Fatal("expected error for invalid escape, got nil")
+			}
+			if err.Error() != tt.want {
+				t.Errorf("error = %q, want %q", err.Error(), tt.want)
+			}
+		})
+	}
+}
+
 func TestReadJSONPointer_InvalidFormat(t *testing.T) {
 	data := map[string]interface{}{"key": "val"}
 	_, err := ReadJSONPointer(data, "no-leading-slash")