Use After Free in Buffalo with Dynamodb

Use After Free (UAF) occurs when memory is deallocated but references to it remain in use, leading to unpredictable behavior or potential code execution. In the context of Buffalo applications using the AWS SDK for DynamoDB, UAF can arise through improper handling of request objects, response parsing, or shared buffers across goroutine boundaries.

Buffalo applications often interact with DynamoDB via the AWS SDK for Go (v2), where API calls return structs populated by the service. If a developer stores references to these response objects—such as a *dynamodb.GetItemOutput—beyond the lifecycle of the request or goroutine, and the underlying memory is reused or freed, a UAF condition can be triggered. This is especially likely when using context timeouts or cancellation, where a response arrives after the original request context has ended and memory has been released.

DynamoDB-specific patterns that increase risk include:

• Reusing response structs across multiple requests without deep copying, causing stale pointers to be accessed after the original memory is freed.
• Passing pointers to temporary objects into background workers or goroutines that execute after the parent request context is done.
• Using interface{} or type assertions on DynamoDB attribute values (e.g., from AttributeValue) where the underlying byte slices or strings reference transient buffers.

An example scenario: a Buffalo handler launches a goroutine to process a DynamoDB GetItem result. If the handler does not copy the output and the request context cancels before the goroutine reads the data, the goroutine may access freed memory, leading to data corruption or crashes. While Buffalo itself does not manage memory, the interaction between the framework’s request lifecycle and the AWS SDK’s response handling creates conditions where UAF can manifest.

Real-world attack patterns involving UAF in API services often lead to information disclosure or unstable behavior, which can be probed during a black-box scan. middleBrick’s checks for unsafe consumption and input validation help surface such risky patterns by correlating runtime behavior with expected schema constraints.

To prevent Use After Free in Buffalo applications using DynamoDB, ensure response objects are not shared across goroutines and that all data is fully copied before being used asynchronously. Below are concrete, working examples using the AWS SDK for Go (v2) with DynamoDB.

1. Avoid sharing response pointers across goroutines

Instead of passing a response object to a goroutine, copy the necessary data into new structures with value semantics.

import (
	"context"
	"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 ItemCopy struct {
	ID   string
	Name string
}

func getItemCopy(ctx context.Context, client *dynamodb.Client, key map[string]types.AttributeValue) (*ItemCopy, error) {
	out, err := client.GetItem(ctx, &dynamodb.GetItemInput{
		TableName: key("table"),
		Key:       key,
	})
	if err != nil {
		return nil, err
	}
	// Copy data out of the DynamoDB response into a plain struct
	copy := &ItemCopy{}
	if v, ok := out.Item["id"]; ok {
		copy.ID = *v.S
	}
	if v, ok := out.Item["name"]; ok {
		copy.Name = *v.S
	}
	return copy, nil
}

// Safe usage in a handler
item, err := getItemCopy(ctx, client, key)
if err != nil {
	// handle error
}
go func(data ItemCopy) {
	// data is a copy, safe to use after request ends
	process(data)
}(*item)

2. Deep-copy nested attribute values from DynamoDB responses

DynamoDB attribute values can contain nested structures. Use explicit copying rather than holding references to AttributeValue internals.

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

func copyAttributeValue(av types.AttributeValue) types.AttributeValue {
	switch v := av.(type) {
	case *types.AttributeValueMemberS:
		return &types.AttributeValueMemberS{Value: v.Value}
	case *types.AttributeValueMemberM:
		m := make(map[string]types.AttributeValue, len(v.Value))
		for k, val := range v.Value {
			m[k] = copyAttributeValue(val)
		}
		return &types.AttributeValueMemberM{Value: m}
	case *types.AttributeValueMemberL:
		l := make([]types.AttributeValue, len(v.Value))
		for i, val := range v.Value {
			l[i] = copyAttributeValue(val)
		}
		return &types.AttributeValueMemberL{Value: l}
	default:
		// handle other types as needed
		return av
	}
}

func safeProcessItem(item map[string]types.AttributeValue) {
	copied := make(map[string]types.AttributeValue, len(item))
	for k, v := range item {
		copied[k] = copyAttributeValue(v)
	}
	// Use copied map safely
}

3. Use context-aware timeouts without retaining stale references

Ensure that any background work completes or exits cleanly when the request context is canceled, avoiding attempts to read from freed memory.

import (
	"context"
	"time"
)

func processWithTimeout(ctx context.Context, data *ItemCopy) {
	ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
	defer cancel()
	select {
	case <-ctx.Done():
		// context canceled, do not access data if it may have been freed
		return
	case <-time.After(1 * time.Second):
		// safe to use data here if ownership is clear
		_ = data
	}
}

By copying data out of DynamoDB responses and avoiding pointer sharing, Buffalo applications can eliminate common UAF vectors. These practices align with broader secure coding patterns and are validated through runtime security checks that correlate spec definitions with observed behavior.