Use After Free in Fiber with Dynamodb

Use After Free (UAF) is a memory safety class vulnerability where code continues to use a pointer after the associated memory has been freed. In a Fiber-based Go API service that also interacts with DynamoDB, UAF can arise when request-scoped objects (such as a struct used to unmarshal a DynamoDB item) are released while an in-flight handler or middleware still holds references to its fields. Because Fiber is a high-performance HTTP framework with low-latency request handling, object reuse patterns (e.g., sync.Pool) and aggressive escape-to-stack optimizations can increase the chance that a freed backing array or map entry is accessed later.

Consider a DynamoDB Scan or Query where the raw attribute values are stored in a struct that is reused across requests. If the handler unmarshals into a temporary object, stores a pointer to one of its fields (for example, a pointer to a string or byte slice), and then returns that pointer in a JSON response after the next request reuses the same memory, the response may expose data belonging to another request (information disclosure), or even lead to arbitrary code execution if the overwritten content is controlled by an attacker.

DynamoDB-specific aspects can exacerbate the issue. The AWS SDK for Go v2 deserializes DynamoDB attribute values into interface{}-based structures; if you bind these directly to request-scoped objects that are pooled or reused, you risk keeping references into transient buffers. Additionally, conditional expressions and attribute value manipulations may allocate temporary objects on the stack; if a closure captures a pointer into such temporary storage and that storage is freed and reused, the closure will read invalid memory when invoked later in the request lifecycle.

An example pattern that can lead to UAF in Fiber with DynamoDB interaction:

• A request handler calls Query with a key condition, unmarshals results into a request-local struct, and stores a pointer to a field in a context value or a global cache.
• Because the struct is allocated on the stack or in a sync.Pool, the next request reuses that memory before the pointer is cleared.
• The response writer later serializes the stale pointer into JSON, leaking data, or the pointer is used in downstream logic, causing unpredictable behavior.

To detect this class of issue with middleBrick, you can scan the unauthenticated endpoint using the CLI (e.g., middlebrick scan <url>) or the GitHub Action to add API security checks to your CI/CD pipeline. While middleBrick does not fix the vulnerability, its findings include prioritized guidance to help developers address memory safety issues and validate request isolation.

Remediation focuses on ensuring that any data derived from DynamoDB responses is not stored in pointers that outlive the request scope, and that request-local objects are not reused in a way that introduces use-after-free conditions.

1. Avoid capturing pointers into temporary objects

Do not store pointers to fields of a struct that may be reused or go out of scope. Instead, copy values into new variables or structures that are owned exclusively by the handler.

// Avoid: storing a pointer into a potentially reused struct
type Item struct {
    ID   string
    Data []byte
}

// Handler with risky pointer capture
func handler(c *fiber.Ctx) error {
    var item Item
    if err := db.Query(c.Context(), &item); err != nil {
        return err
    }
    // Risk: &item.Data may be reused by the next request if item is pooled or stack-reused
    c.Locals("ref", &item.Data)
    return c.SendStatus(fiber.StatusOK)
}

// Safer: copy the data
func handler(c *fiber.Ctx) error {
    var item Item
    if err := db.Query(c.Context(), &item); err != nil {
        return err
    }
    dataCopy := make([]byte, len(item.Data))
    copy(dataCopy, item.Data)
    c.Locals("copy", dataCopy)
    return c.SendStatus(fiber.StatusOK)
}

2. Do not reuse request-bound structs across goroutines or requests

When using sync.Pool or global caches, ensure that any DynamoDB-unmarshaled objects are fully detached (deep-copied) before being stored or shared. Never place a pointer into a transient request object into a pool that may be handed to another request.

var itemPool = sync.Pool{
    New: func() interface{} {
        return new(DynamoItem)
    },
}

type DynamoItem struct {
    PK string
    Attrs map[string]interface{}
}

func processItem(ctx context.Context) error {
    item := itemPool.Get().(*DynamoItem)
    defer itemPool.Put(item)

    // Reset fields to avoid leaking data between uses
    item.PK = ""
    item.Attrs = nil

    // Fetch from DynamoDB into a fresh map
    input := &dynamodb.ScanInput{
        TableName: aws.String("MyTable"),
    }
    out, err := client.Scan(ctx, input)
    if err != nil {
        return err
    }

    // Copy data out of the DynamoDB response instead of storing pointers into pooled objects
    item.PK = aws.ToString(out.Items[0]["PK"])
    item.Attrs = make(map[string]interface{})
    for k, v := range out.Items[0] {
        item.Attrs[k] = v
    }

    // Safe to use item within this scope; no cross-request references
    _ = item
    return nil
}

3. Prefer value semantics for small DynamoDB responses

For small payloads, prefer unmarshaling into concrete structs and passing by value rather than holding references to map-based representations that may be mutated or reused by the SDK.

type UserProfile struct {
    UserID string `json:"user_id"`
    Email  string `json:"email"`
}

func getUser(c *fiber.Ctx) error {
    var profile UserProfile
    // Assume a helper that calls DynamoDB GetItem and unmarshals into profile
    if err := fetchUserProfile(c.Context(), "user123", &profile); err != nil {
        return err
    }
    // profile is a copy; no lingering references to SDK-managed buffers
    return c.JSON(profile)
}

By ensuring that data extracted from DynamoDB is either copied or stored in request-scoped values that do not escape the handler, you mitigate the risk of use-after-free conditions that could lead to information disclosure or unstable behavior in a Fiber service.