Format String in Fiber with Dynamodb

A format string vulnerability occurs when user-controlled input is passed directly into a formatted output function without proper sanitization. In a Fiber application using the AWS SDK for DynamoDB, this typically arises when constructing command parameters or log messages using unchecked request data. For example, if a handler uses fmt.Sprintf to build a string that includes user input before passing it to a DynamoDB operation, an attacker can supply format verbs such as %s, %x, or %n. These verbs can cause the function to read from or write to memory, potentially leaking sensitive information or causing unexpected behavior in downstream processing.

Consider a route that accepts a userID query parameter and uses it to build a GetItemInput for DynamoDB. If the code incorrectly builds the table name or a log entry using fmt.Sprintf, the user input is interpreted as a format string. During a black-box scan, middleBrick tests such input vectors and can detect indicators of unsafe string formatting in runtime behavior, such as unusual response patterns or data leakage. Even though the DynamoDB SDK does not directly interpret format verbs, the vulnerability surface appears when application-level code mishandles input before constructing SDK inputs, for example when generating dynamic table names or constructing debug logs that include request identifiers.

Because middleBrick scans the unauthenticated attack surface and runs checks in parallel, it can identify input validation weaknesses and unsafe consumption patterns in the context of DynamoDB integrations. The scanner does not rely on internal implementation details; instead, it observes how the API reacts to specially crafted payloads designed to probe format string behavior. If the API returns inconsistent data, verbose errors, or signs of memory disclosure, middleBrick flags the endpoint with a finding that references insecure string handling and maps it to relevant standards such as OWASP API Top 10 and CWE-134.

To remediate format string risks in a Fiber application that interacts with DynamoDB, ensure that all user input is treated as data, not executable format directives. Avoid passing request parameters directly to formatting functions when constructing values for DynamoDB commands. Instead, use explicit parameterization and strict validation.

Below is a secure example using the AWS SDK for Go with Fiber, where user input is used to retrieve an item from a predefined table without exposing format string risks:

package main

import (
    "context"
    "fmt"
    "github.com/gofiber/fiber/v2"
    "github.com/aws/aws-sdk-go-v2/aws"
    "github.com/aws/aws-sdk-go-v2/config"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb/types"
)

type GetItemRequest struct {
    TableName string `json:"table_name"`
    Key       map[string]types.AttributeValue `json:"key"`
}

func main() {
    app := fiber.New()

    app.Post("/item", func(c *fiber.Ctx) error {
        var req GetItemRequest
        if err := c.BodyParser(&req); err != nil {
            return c.Status(fiber.StatusBadRequest).SendString(`{"error": "invalid request"}`)
        }

        // Validate table name against an allowlist to prevent injection via table name
        allowedTables := map[string]bool{"users": true, "products": true}
        if !allowedTables[req.TableName] {
            return c.Status(fiber.StatusBadRequest).SendString(`{"error": "invalid table"}`)
        }

        cfg, err := config.LoadDefaultConfig(context.TODO())
        if err != nil {
            return c.Status(fiber.StatusInternalServerError).SendString(`{"error": "config error"}`)
        }

        client := dynamodb.NewFromConfig(cfg)
        out, err := client.GetItem(context.TODO(), &dynamodb.GetItemInput{
            TableName: aws.String(req.TableName),
            Key:       req.Key,
        })
        if err != nil {
            return c.Status(fiber.StatusInternalServerError).SendString(`{"error": "dynamodb error"}`)
        }

        if out.Item == nil {
            return c.Status(fiber.StatusNotFound).SendString(`{"error": "not found"}`)
        }

        return c.JSON(out.Item)
    })

    app.Listen(":3000")
}

Key practices include validating table names against an allowlist, avoiding string interpolation for command construction, and ensuring that all DynamoDB attribute values are provided as typed types.AttributeValue instances. middleBrick can verify that these mitigations are effective by testing the API with format string probes and confirming that no unintended memory reads or writes occur.