Double Free in Echo Go with Dynamodb

A Double Free occurs when a program attempts to free the same memory allocation more than once. In Echo Go applications that use the AWS SDK for Go with DynamoDB, this typically arises through a mismatch between resource ownership and lifecycle management across the HTTP request handled by Echo. The vulnerability is exposed when developers inadvertently retain references to request-scoped objects after they have already been released, and those objects are later used in DynamoDB operations.

Consider an Echo handler that unmarshals a request into a struct, passes a pointer to that struct to a DynamoDB GetItem call, and then reuses or explicitly releases the struct in a deferred function before the DynamoDB operation completes or while another goroutine still references it. If the SDK performs retries or background processing on the request context and the underlying memory is freed prematurely, a subsequent access on the stale pointer can trigger a second free. This is more likely when using custom retry logic or context cancellation that interacts with the SDK’s internal HTTP client, because the same memory may be released both by the developer’s cleanup and by the SDK’s own resource management.

In practice, this often surfaces when integrating Echo middleware that manipulates request context or response writers concurrently with DynamoDB calls. For example, if you store a pointer to a request-bound object in the Echo context and later attempt to free that object in a deferred context.CancelFunc while an in-flight DynamoDB operation still holds a reference, the runtime can encounter a double free. The combination of Echo’s flexible request handling, Go’s memory model, and the asynchronous nature of some DynamoDB SDK interactions increases the likelihood of encountering this class of bug.

To prevent Double Free issues, ensure that memory is freed exactly once and that no concurrent operation holds a reference after free. In Echo Go with DynamoDB, this means careful handling of request-scoped objects, avoiding manual memory management where possible, and ensuring that context lifetimes and SDK calls are properly coordinated.

Below are concrete Go examples using the AWS SDK for Go v2 (github.com/aws/aws-sdk-go-v2) with DynamoDB. The first example shows a safe pattern where request-scoped data is copied and not manually freed, relying on Go’s garbage collector. The second demonstrates proper context management to avoid use-after-free without double frees.

Safe DynamoDB GetItem in an Echo handler

import (
    "context"
    "github.com/labstack/echo/v4"
    "github.com/aws/aws-sdk-go-v2/aws"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb/types"
)

type ItemInput struct {
    ID string `json:"id"`
}

func GetItemHandler(dbClient *dynamodb.Client) echo.HandlerFunc {
    return func(c echo.Context) error {
        var input ItemInput
        if err := c.Bind(&input); err != nil {
            return echo.NewHTTPError(400, "invalid input").SetInternal(err)
        }

        // Copy input into a new variable to avoid any reuse issues
        reqInput := &dynamodb.GetItemInput{
            TableName: aws.String("Items"),
            Key: map[string]types.AttributeValue{
                "ID": &types.AttributeValueMemberS{Value: input.ID},
            },
        }

        // Perform the DynamoDB call with a request-scoped context
        ctx, cancel := context.WithTimeout(c.Request().Context(), 10)
        defer cancel() // Safe: cancels after GetItem completes

        _, err := dbClient.GetItem(ctx, reqInput)
        if err != nil {
            return echo.NewHTTPError(500, "dynamodb error").SetInternal(err)
        }

        return c.JSON(200, map[string]string{"status": "ok"})
    }
}

Avoiding double free with context cancellation and deferred cleanup

func ProcessWithCleanup(dbClient *dynamodb.Client, c echo.Context) error {
    // Create a cancellable context tied to the request
    reqCtx, reqCancel := context.WithCancel(c.Request().Context())
    // Ensure cleanup happens after the DynamoDB operation
    defer func() {
        // Do NOT free SDK-managed resources manually; cancel the context only
        reqCancel()
    }()

    input := &dynamodb.GetItemInput{
        TableName: aws.String("Products"),
        Key: map[string]types.AttributeValue{
            "SKU": &types.AttributeValueMemberS{Value: "abc123"},
        },
    }

    // Use the copied context for the SDK call
    result, err := dbClient.GetItem(reqCtx, input)
    if err != nil {
        return err
    }

    // Process result safely; no manual memory free required
    _ = result
    return nil
}

Key remediation practices include: avoiding explicit memory deallocation in Go, ensuring that pointers passed to DynamoDB are not reused or freed before the operation completes, using context cancellation only to signal cancellation without manual free calls, and copying input data when necessary to prevent mutation or double-free scenarios across goroutines. These practices align with safe patterns for Echo handlers and DynamoDB integration.