Spoof-proof file type validation in a Spring Boot app

I've created a custom validator for use in a Spring Boot app that checks the type of an uploaded file. The @ValidFileType annotation is defined as

@Documented
@Constraint(validatedBy = MultiPartFileValidator.class)
@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
public @interface ValidFileType {

    String[] allowed() default {
        "image/webp",
        "image/svg+xml",
        "application/zip",
    };

    String message() default "{com.example.invalidFileType}";

    Class<?>[] groups() default {};

    Class<? extends Payload>[] payload() default {};
}

The validator itself is

class MultiPartFileValidator implements ConstraintValidator<ValidFileType, MultipartFile> {

    private Set<String> allowed;

    @Override
    public void initialize(ValidFileType constraintAnnotation) {
        allowed = Set.of(constraintAnnotation.allowed());
    }

    @Override
    public boolean isValid(MultipartFile file, ConstraintValidatorContext context) {
        return file == null || allowed.contains(file.getContentType());
    }
}

This is typically used in a controller method like so:

@PostMapping
public void uploadFile(@Valid @ValidFileType @RequestPart MultipartFile uploadedFile) {
    // if we get this far, the file that has passed validation
}

However, someone has pointed out that this validator relies solely on the content-type header which can be spoofed. If I can't rely on this header (or the extension in the file name), is there anything else I can use to verify the file type that is more resilient?

You can test out Tika for this. Requires this dependency: org.apache.tika:tika-core:3.0.0

Example usage in a similar validator

import jakarta.validation.ConstraintValidator;
import jakarta.validation.ConstraintValidatorContext;
import org.apache.tika.Tika;
import org.springframework.http.MediaType;
import org.springframework.web.multipart.MultipartFile;

import java.io.IOException;
import java.util.Set;

public class MultiPartFileValidator implements ConstraintValidator<ValidFileType, MultipartFile> {

    private static final Set<String> ALLOWED_FILETYPES =
            Set.of(MediaType.IMAGE_PNG_VALUE, MediaType.IMAGE_JPEG_VALUE);

    private static final Tika TIKA = new Tika();

    private long maxFileSize;
    private String invalidFileTypeMessage;
    private String invalidSizeMessage;

    @Override
    public void initialize(ValidFileType constraintAnnotation) {
        this.maxFileSize = constraintAnnotation.maxFileSize();
        this.invalidFileTypeMessage = constraintAnnotation.invalidFileTypeMessage();
        this.invalidSizeMessage = constraintAnnotation.invalidSizeMessage();
    }

    @Override
    public boolean isValid(MultipartFile file, ConstraintValidatorContext context) {
        // Null or empty check upfront
        if (file == null) {
            return true;
        }

        context.disableDefaultConstraintViolation();

        // Size check: Ensure the file is within the allowed size limit
        if (file.getSize() > maxFileSize) {
            return setValidationError(invalidSizeMessage, context);
        }

        // Validate file type
        try {
            // other overloads of detect exist
            String mimeType = TIKA.detect(file.getBytes());
            if (!ALLOWED_FILETYPES.contains(mimeType)) {
                return setValidationError(invalidFileTypeMessage, context);
            }
        } catch (IOException e) {
            // Handle error if file type detection fails
            return setValidationError("Unable to detect file type", context);
        }

        // If both size and type are valid
        return true;
    }

    private static boolean setValidationError(String message, ConstraintValidatorContext context) {
        context.buildConstraintViolationWithTemplate(message).addConstraintViolation();
        return false;
    }
}