Insecure Deserialization in Fiber with Dynamodb

Insecure deserialization occurs when an application reconstructs objects from untrusted data without sufficient validation. In a Fiber application that uses Amazon DynamoDB, this typically arises when API endpoints accept serialized objects (for example, via JSON payloads that map to Go structures) and then pass user-controlled values into DynamoDB operations such as GetItem, PutItem, or Query. If the deserialization logic does not enforce strict type constraints, an attacker can craft payloads that cause unexpected behavior, including type confusion or injection of malicious values into key expressions.

Consider a Fiber endpoint that accepts an ID and unmarshals a JSON body into a struct used for a DynamoDB GetItem:

// Insecure: directly binding user input into DynamoDB key logic
type Input struct {
    UserID string `json:"userId"`
    SortKey string `json:"sortKey"`
}

app.Post("/data/:userID", func(c *fiber.Ctx) error {
    var in Input
    if err := c.BodyParser(&in); err != nil {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid body"})
    }
    // Build key from user-controlled deserialized fields without strict validation
    input := &dynamodb.GetItemInput{
        TableName: aws.String("MyTable"),
        Key: map[string]*dynamodb.AttributeValue{
            "PK": {S: aws.String(in.UserID)},
            "SK": {S: aws.String(in.SortKey)},
        },
    }
    out, err := svc.GetItem(c.Context(), input)
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": err.Error()})
    }
    return c.JSON(out.Item)
})

If the deserialization layer does not validate or restrict types, an attacker might supply nested objects, arrays, or type-mismatched values that shift how the application interprets key expressions, potentially enabling NoSQL injection-like behavior or unauthorized access to other items. Because the scan tests unauthenticated attack surfaces, middleBrick flags insecure deserialization patterns when user-influenced data flows into DynamoDB key construction without canonicalization or strict schema enforcement.

Additionally, middleBrick’s checks include LLM/AI Security and Property Authorization, which can surface cases where deserialized data influences authorization decisions (BOLA/IDOR) or privilege escalation (BFLA). For example, if the deserialized object contains role or permission fields that are merged into key expressions or conditional expressions without validation, attackers might escalate access by tampering with those fields. The scanner correlates these risks across the 12 checks and highlights how insecure deserialization can intersect with authorization and input validation weaknesses in the DynamoDB workflow.

To mitigate insecure deserialization in Fiber when working with DynamoDB, validate and canonicalize all inputs before using them in database operations. Prefer explicit binding to known types, enforce allowlists for key values, and avoid directly mapping user-supplied fields into DynamoDB key expressions without checks.

Secure approach using controlled structs and explicit key construction:

// Secure: strict types, allowlist validation, and canonical key building
type SafeInput struct {
    UserID string `json:"userId"`
    SortKey string `json:"sortKey"`
}

func isValidUserID(v string) bool {
    // Example allowlist: alphanumeric + underscore, 3–64 chars
    matched, _ := regexp.MatchString(`^[A-Za-z0-9_]{3,64}$`, v)
    return matched
}

func isValidSortKey(v string) bool {
    // Example allowlist for hierarchical sort keys
    matched, _ := regexp.MatchString(`^(ORDER|PROFILE|SESSION)#[-A-Za-z0-9_]+$`, v)
    return matched
}

app.Post("/data/:userID", func(c *fiber.Ctx) error {
    var in SafeInput
    if err := c.BodyParser(&in); err != nil {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid body"})
    }
    if !isValidUserID(in.UserID) || !isValidSortKey(in.SortKey) {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid parameters"})
    }
    // Canonical key construction independent of potentially ambiguous deserialization paths
    input := &dynamodb.GetItemInput{
        TableName: aws.String("MyTable"),
        Key: map[string]*dynamodb.AttributeValue{
            "PK": {S: aws.String(in.UserID)},
            "SK": {S: aws.String(in.SortKey)},
        }n        ExpressionAttributeNames: map[string]*string{
            aws.String("#owner": aws.String("owner")),
        },
        ExpressionAttributeValues: map[string]*dynamodb.AttributeValue{
            aws.String(":uid": {S: aws.String(in.UserID)},
        },
    }
    out, err := svc.GetItem(c.Context(), input)
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": err.Error()})
    }
    return c.JSON(out.Item)
})

Additional recommendations that align with middleBrick’s findings:

• Use strict schema validation (for example, with tags that enforce format patterns) before constructing DynamoDB keys.
• Do not trust deserialized values for authorization decisions; re-derive permissions from the authenticated subject and canonical resource identifiers.
• Enable middleware in Fiber to sanitize and log unexpected types, which helps correlate findings in the Property Authorization and Input Validation checks reported by middleBrick.
• If you use the middleBrick CLI (middlebrick scan <url>) or GitHub Action, you can automate detection of these patterns in CI/CD and fail builds when insecure deserialization risks are identified.