Use After Free in Chi with Dynamodb

Use After Free (UAF) in the context of a Chi application using Amazon DynamoDB occurs when memory that was previously allocated for an object or data structure is reclaimed while references to that memory still exist and are subsequently accessed. In Chi, a common Go HTTP router, this can manifest when request-scoped objects (such as parsed path parameters or middleware state) that reference a DynamoDB item or attribute are not properly invalidated between requests or after explicit deletion. DynamoDB itself does not expose raw memory, but the Go structures that model items retrieved from or prepared for DynamoDB can become unsafe if the application retains pointers to transient objects that are recycled by the runtime or by an object pool.

Consider a handler that decodes a DynamoDB item into a struct, then passes a pointer to that struct into middleware that mutates or logs it. If the handler returns and the struct is reused by an object pool (e.g., via chi.Context or a custom pool), a subsequent request may overwrite the underlying memory while the pointer from the previous request still exists. Accessing the stale pointer can lead to reading or writing arbitrary memory, which in the context of DynamoDB might expose item attributes, API keys, or other sensitive data that were present in the previous request’s payload. This is especially risky when combined with DynamoDB’s attribute value handling, where nested structures or binary fields can create complex object graphs that increase the chance of retaining stale references.

An example scenario: a Chi route extracts an id from the URL, retrieves an item from DynamoDB, and attaches the item struct to the request context. If the context is reused and the struct is not deep-copied, a later request that reuses the same context may observe a Use After Free when the original item struct’s backing memory is overwritten. This can lead to information disclosure (reading another user’s data) or, in more contrived situations, code execution if attacker-influenced data is interpreted as code pointers. The risk is not in DynamoDB’s service behavior but in how Chi-managed objects reference and reuse memory that DynamoDB data structures populate.

To reproduce the class of issue safely, consider a contrived handler that keeps a pointer to a DynamoDB item across requests. A scan using the middleBrick CLI (e.g., middlebrick scan https://api.example.com) may surface related input validation or improper data exposure findings, but the root cause is the mishandling of object lifetimes in Chi. Real-world parallels include issues flagged under CWE-416 (Use After Free) and can intersect with OWASP API Top 10’s Broken Object Level Authorization when stale references enable access to unauthorized data.

Key takeaways: UAF in Chi with DynamoDB is about Go object lifecycle management, not DynamoDB internals. Ensure that data extracted from DynamoDB is either copied before storing in long-lived contexts or explicitly cleared when no longer needed. Avoid attaching raw pointers to request-scoped objects that may be reused, and prefer immutable data representations where feasible to prevent inadvertent memory reuse.

Remediation focuses on ensuring that data from DynamoDB is handled with strict ownership semantics and that request contexts do not retain references to mutable or pooled objects. Below are concrete code examples for Chi that demonstrate safe patterns.

1. Avoid attaching raw pointers to request context

Instead of attaching a pointer to a DynamoDB item to chi.Context, create a copy or a DTO (data transfer object) that does not share memory with transient objects.

// Unsafe: attaching a pointer that may be reused
item := &DynamoItem{}
if err := dynamo.GetItem(context.Background(), &dynamodb.GetItemInput{
    TableName: aws.String("Items"),
    Key: map[string]types.AttributeValue{
        "ID": &types.AttributeValueMemberS{Value: id},
    },
}, item); err != nil {
    http.Error(w, err.Error(), http.StatusInternalServerError)
    return
}
chi.WithValue(ctx, "item", item) // Dangerous: item pointer may be reused

// Safe: deep copy into a new struct
safeItem := *item // shallow copy for structs; for nested slices/maps, use explicit copy
type SafeItem struct {
    ID   string
    Name string
    Data []byte
}
safe := SafeItem{
    ID:   safeItem.ID,
    Name: safeItem.Name,
    Data: append([]byte{}, safeItem.Data...),
}
chi.WithValue(ctx, "item", safe) // Safe: independent memory

2. Use value semantics and avoid reusing request-scoped objects

Chi allows middleware to mutate context values. Ensure that any values derived from DynamoDB are treated as immutable for the request lifetime, or re-created per request rather than pooled.

// Middleware that recomputes data per request instead of reusing pointers
func DynamoSafeMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        id := chi.URLParam(r, "id")
        var item DynamoItem
        if err := dynamo.GetItem(context.Background(), &dynamodb.GetItemInput{
            TableName: aws.String("Items"),
            Key: map[string]types.AttributeValue{
                "ID": &types.AttributeValueMemberS{Value: id},
            },
        }, &item); err != nil {
            http.Error(w, err.Error(), http.StatusInternalServerError)
            return
        }
        // Work with item directly; do not store pointer in context
        ctx := context.WithValue(r.Context(), "itemData", item)
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

3. Validate and sanitize outputs to prevent indirect exposure

Even with safe memory handling, ensure that DynamoDB data is validated before use, and avoid reflecting raw attribute values into outputs that could be misinterpreted (e.g., JSON injection that leads to script execution). Use structured marshalling and strict type checks.

// Example: safe JSON output using explicit structs
type APIItem struct {
    ID   string `json:"id"`
    Name string `json:"name"`
}
func handler(w http.ResponseWriter, r *http.Request) {
    id := chi.URLParam(r, "id")
    var dynamoItem struct {
        ID   string
        Name string
    }
    // ... fetch into dynamoItem ...
    out := APIItem{ID: dynamoItem.ID, Name: dynamoItem.Name}
    w.Header().Set("Content-Type", "application/json")
    json.NewEncoder(w).Encode(out) // Safe: no pointer exposure
}

4. Leverage middleware for cleanup if using object pools

If your Chi configuration or underlying runtime uses object pools (e.g., sync.Pool), provide middleware that resets or discards references to DynamoDB-derived data at the end of each request to prevent cross-request contamination.

func CleanupMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        defer func() {
            // Clear any request-local references that might point to pooled objects
            chi.Reset(r.Context())
        }()
        next.ServeHTTP(w, r)
    })
}

These patterns reduce the surface for Use After Free by ensuring that data from DynamoDB is copied, isolated, and not tied to mutable or reused memory. They align with secure coding practices for Go and help prevent CWE-416-type flaws in API services that rely on Chi and DynamoDB.