Defending Against Malicious Pageable Input: PageRequestDto Design

Problem Discovery

While implementing pagination for the Gift API at Kakao Tech Campus, a question came up:

GET /api/wishlist?page=0&size=3&sort=product.name,desc

What if a user sends malicious input?

• What if they put garbage in sort?
• What if they spam tons of sort criteria?
• What if they send size=100000?

Spring’s default Pageable just accepts all of this as-is.

---

Asked My Mentor

I asked:

“PropertyReferenceException is too broad to catch, and checking Pageable.getSort() one by one feels tedious. And wouldn’t there be issues if someone maliciously sends many sort criteria?”

Mentor’s response:

“Great question! Creating a custom PageRequestDto would be the easiest approach:

• Manage sortProperty as an enum; throw an error or fall back to default if invalid
• Set a maxSize (e.g., 100) for pageSize; cap it there
• This way you get clean handling for validation, defaults, and error messages”

---

Solution: PageRequestDto Design

SortField Interface

A common interface for managing sortable fields as enums.

public interface SortField {
    String getFieldName();
}

Domain-specific Sort Field Enum

public enum WishlistSortField implements SortField {
    CREATED_AT("createdAt"),
    PRODUCT_NAME("product.name"),
    PRODUCT_PRICE("product.price");

    private final String fieldName;

    WishlistSortField(String fieldName) {
        this.fieldName = fieldName;
    }

    @Override
    public String getFieldName() {
        return fieldName;
    }
}

PageRequestDto

public record PageRequestDto(
        @Min(value = 1, message = "Page number must be at least 1")
        Integer page,

        @Range(min = 1, max = 100, message = "Page size must be between 1 and 100")
        Integer size,

        String sortBy,

        Boolean ascending
) {
    public <T extends Enum<T> & SortField> PageRequest toSafePageable(
            Class<T> enumClass, T defaultSortField) {
        int page = this.page != null ? this.page : 1;
        int size = this.size != null ? this.size : 10;
        T sortField = defaultSortField;
        boolean ascending = this.ascending != null ? this.ascending : true;

        if (sortBy != null) {
            for (T enumValue : enumClass.getEnumConstants()) {
                if (enumValue.getFieldName().equals(sortBy)) {
                    sortField = enumValue;
                }
            }
        }

        return PageRequest.of(page - 1, size,
                Sort.by(
                        ascending ? Sort.Direction.ASC : Sort.Direction.DESC,
                        sortField.getFieldName()
                )
        );
    }
}

Usage Example

@GetMapping
public Page<WishlistItemDto> getWishlistItems(
        @Valid PageRequestDto pageRequest) {
    Pageable pageable = pageRequest.toSafePageable(
            WishlistSortField.class,
            WishlistSortField.CREATED_AT
    );
    return wishlistService.getItems(pageable);
}

---

Defense Summary

Attack	Defense
size=100000	@Range(max = 100) caps the value
sort=malicious_field	Ignored if not in enum; falls back to default
Multiple sort params	Only single sortBy accepted
page=-1	@Min(1) enforces minimum

---

Lessons Learned

• Spring’s default Pageable doesn’t validate input
• You need a custom DTO to block malicious input
• Security should be baked in from the design phase

---

From Kakao Tech Campus 3rd cohort Gift API clone coding, summarizing mentor feedback.

• spring-gift-wishlist