Use After Free in Echo Go with Dynamodb

A Use After Free (UAF) occurs when memory is freed but pointers to that memory remain in use, leading to unpredictable behavior or potential code execution. In the context of an Echo Go service that interacts with Amazon DynamoDB, UAF can arise through unsafe handling of request-scoped objects, response unmarshaling, or connection/session management across concurrent requests.

When an Echo Go handler deserializes a DynamoDB GetItem or Query response into a struct, the SDK typically binds attribute values into Go fields. If the application retains references to buffers or temporary objects beyond the request lifecycle—such as caching raw byte slices or reusing buffers across goroutines—and the underlying memory is freed or reused, a UAF condition can be triggered during subsequent DynamoDB operations. This is especially risky when using lower-level SDK patterns that expose byte slices or when integrating with custom serializers that do not isolate lifecycle ownership.

DynamoDB itself does not directly cause UAF, but the way Go code interfaces with the service can introduce the flaw. For example, capturing request context or response payload references in global caches, reusing buffers across multiple dynamodbattribute.UnmarshalMap calls, or mishandling pointers returned from query scans can keep references alive after the backing memory is released. When those references are later dereferenced—such as during attribute validation, retry logic, or response serialization—the runtime may read or write to freed memory. An attacker can exploit this by inducing race conditions through concurrent requests or manipulating timing to increase the likelihood of memory reuse, potentially leading to information disclosure or code execution within the Echo Go process.

The risk is amplified in Echo Go when developers inadvertently share objects between requests or fail to scope objects to the request lifecycle. Because DynamoDB operations often involve large or complex data structures, developers may attempt optimizations such as object pooling or buffer reuse, which can inadvertently create UAF windows. These patterns may pass static analysis but still be unsafe when concurrency and garbage collection interactions are misunderstood.

To prevent Use After Free in Echo Go with DynamoDB, ensure that all data bound from DynamoDB responses is owned by the request scope and that no references to transient buffers are retained beyond their lifecycle. Use deep copies when storing data beyond the immediate handler, avoid reusing buffers across concurrent operations, and prefer value semantics over pointer reuse where possible.

Safe DynamoDB GetItem with isolated structs

Define distinct structs for request and response handling, and avoid capturing references to the SDK’s internal buffers.

// Request-scoped handler with isolated unmarshaling
type GetItemInput struct {
    TableName *string
    Key       map[string]types.AttributeValue
}

type UserProfile struct {
    UserID   string
    Email    string
    Metadata map[string]string
}

func getUserProfile(svc *dynamodb.Client, key map[string]types.AttributeValue) (*UserProfile, error) {
    out, err := svc.GetItem(context.TODO(), &dynamodb.GetItemInput{
        TableName: aws.String("users"),
        Key:       key,
    })
    if err != nil {
        return nil, err
    }
    if out.Item == nil {
        return nil, fmt.Errorf("item not found")
    }

    // Unmarshal into a new struct, ensuring no shared memory with SDK internals
    var profile UserProfile
    av, ok := out.Item["profile"]
    if !ok {
        return nil, fmt.Errorf("missing profile attribute")
    }
    // Use a deep copy if the value is a nested structure
    profileBytes, err := json.Marshal(av)
    if err != nil {
        return nil, err
    }
    if err := json.Unmarshal(profileBytes, &profile); err != nil {
        return nil, err
    }
    return &profile, nil
}

Safe DynamoDB Scan with scoped buffers

Avoid reusing buffers across scans; allocate fresh memory for each iteration to prevent stale pointers from being accessed after free.

func scanUsers(svc *dynamodb.Client) ([]UserProfile, error) {
    var results []UserProfile
    input := &dynamodb.ScanInput{
        TableName: aws.String("users"),
    }

    for {
        out, err := svc.Scan(context.TODO(), input)
        if err != nil {
            return nil, err
        }

        for _, item := range out.Items {
            // Allocate new memory for each item to avoid any reuse risks
            itemCopy := make(map[string]types.AttributeValue)
            for k, v := range item {
                // Deep copy attribute values
                copied, err := types.MarshalAttributeValue(v)
                if err != nil {
                    return nil, err
                }
                itemCopy[k] = copied
            }

            var profile UserProfile
            av, ok := itemCopy["profile"]
            if !ok {
                continue
            }
            profileBytes, err := json.Marshal(av)
            if err != nil {
                return nil, err
            }
            if err := json.Unmarshal(profileBytes, &profile); err != nil {
                return nil, err
            }
            results = append(results, profile)
        }

        if out.LastEvaluatedKey == nil {
            break
        }
        input.ExclusiveStartKey = out.LastEvaluatedKey
    }
    return results, nil
}

Concurrency safety and avoiding pointer reuse

Do not share pointers or slices across goroutines without synchronization, and prefer copying data rather than holding references to SDK-managed objects.

func handleUser(c echo.Context) error {
    userID := c.Param("id")
    key := map[string]types.AttributeValue{
        "user_id": &types.AttributeValueMemberS{Value: userID},
    }

    profile, err := getUserProfile(userDynamoClient, key)
    if err != nil {
        return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
    }

    // Return a copy; do not return references to cached or shared state
    resp := struct {
        UserID string `json:"user_id"`
        Email  string `json:"email"`
    }{
        UserID: profile.UserID,
        Email:  profile.Email,
    }
    return c.JSON(http.StatusOK, resp)
}

Validation and defensive copying

When working with DynamoDB attribute values, always validate and copy data before storing or passing it to other subsystems. This prevents accidental use of freed memory if the SDK recycles buffers internally.

func safeAttributeCopy(attr types.AttributeValue) ([]byte, error) {
    // Marshal to JSON to ensure an independent copy of the data
    return json.Marshal(attr)
}