Data Exposure in Fiber with Dynamodb

Data exposure occurs when an API returns sensitive information to unauthorized users or in insecure contexts. In a Fiber-based application that uses DynamoDB as its primary data store, this risk arises from a mismatch between access patterns and authorization checks. If route-level handlers query DynamoDB without validating that the authenticated subject is allowed to access the specific item, records can be returned to any caller who knows or guesses an identifier.

DynamoDB does not enforce ownership or tenant boundaries by itself. It enforces permissions at the account and table level via IAM policies, but not at the item level for individual API requests. When a Fiber handler constructs a GetItem or Query request using a user-supplied key (such as an ID from a URL parameter) and passes it directly to DynamoDB, the service will return the item if the caller’s IAM role has dynamodb:GetItem permission. If the caller’s permissions are broader than intended (for example, an application-level role rather than per-tenant scoped credentials), or if the handler fails to scope the request with the correct partition key and ownership attribute, sensitive records can be exposed.

A common pattern in Fiber applications is to use a middleware that attaches a user ID to the context and then build a DynamoDB key from route parameters. Consider a handler that retrieves a user profile by ID:

func GetProfile(c *fiber.Ctx) error {
    userID := c.Params("id")
    svc := dynamodb.NewFromConfig(cfg)
    out, err := svc.GetItem(context.TODO(), &dynamodb.GetItemInput{
        TableName: aws.String("Users"),
        Key: map[string]types.AttributeValue{
            "PK": &types.AttributeValueMemberS{Value: "USER#" + userID},
        },
    })
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": err.Error()})
    }
    if out.Item == nil {
        return c.Status(fiber.StatusNotFound).SendString("not found")
    }
    return c.JSON(out.Item)
}

This code is vulnerable because it does not verify that the authenticated user is requesting their own profile (or that they have permission for that specific user). An attacker who iterates over plausible IDs can enumerate profiles or sensitive fields returned in out.Item. Data exposure in this context includes personally identifiable information, role assignments, or internal references that should not be accessible to other users.

Additionally, if the response is serialized back to the client without filtering, fields such as email, ssn, or internal metadata may be included. The DynamoDB response contains raw attribute values; if the API mirrors these directly into JSON, sensitive data can be exfiltrated. The combination of Fiber’s concise routing style and DynamoDB’s flexible schema can inadvertently expose more data than intended when authorization is applied at the route level but not enforced at the data access level.

To remediate data exposure when using DynamoDB with Fiber, enforce tenant or ownership checks before constructing queries, and scope requests to the minimal required key attributes. Ensure that the authenticated subject is validated against the item’s ownership attribute rather than relying solely on route parameters.

One approach is to include the user’s subject identifier in the DynamoDB key design and enforce it at query time. For example, use a composite primary key where the partition key encodes ownership:

func GetProfileSecure(c *fiber.Ctx) error {
    userID := c.Params("id")
    subjectID := c.Locals("user_id").(string) // from auth middleware

    // Ensure the requested ID matches the authenticated subject
    if userID != subjectID {
        return c.Status(fiber.StatusForbidden).SendString("forbidden")
    }

    svc := dynamodb.NewFromConfig(cfg)
    out, err := svc.GetItem(context.TODO(), &dynamodb.GetItemInput{
        TableName: aws.String("Users"),
        Key: map[string]types.AttributeValue{
            "PK": &types.AttributeValueMemberS{Value: "USER#" + userID},
            "SK": &types.AttributeValueMemberS{Value: "PROFILE"},
        },
        // Explicitly limit returned attributes to reduce exposure
        ProjectionExpression: aws.String("profileData,settings"),
    })
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": err.Error()})
    }
    if out.Item == nil {
        return c.Status(fiber.StatusNotFound).SendString("not found")
    }
    return c.JSON(out.Item)
}

In this corrected version, the handler compares the route parameter with the authenticated subject from the context. Even if an attacker guesses or iterates over IDs, the authorization check prevents access to other users’ data. The ProjectionExpression further limits data exposure by returning only intended fields.

For queries that list items, apply a filter that includes the subject in the key condition. For example, if using a Global Secondary Index (GSI) to support queries by status:

func ListUserItems(c *fiber.Ctx) error {
    subjectID := c.Locals("user_id").(string)
    svc := dynamodb.NewFromConfig(cfg)

    out, err := svc.Query(context.TODO(), &dynamodb.QueryInput{
        IndexName: aws.String("UserIdStatusIndex"),
        KeyConditionExpression: aws.String("UserId = :uid AND status = :status"),
        ExpressionAttributeValues: map[string]types.AttributeValue{
            ":uid": &types.AttributeValueMemberS{Value: "USER#" + subjectID},
            ":status": &types.AttributeValueMemberS{Value: "active"},
        },
        Limit: aws.Int64(50),
    })
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": err.Error()})
    }
    return c.JSON(out.Items)
}

By scoping the query to the authenticated user’s partition key and applying a reasonable limit, you reduce the risk of mass data enumeration. Always validate inputs, enforce ownership on the server side, and restrict returned attributes to what the endpoint actually needs.