Type Confusion in Chi with Dynamodb

Type confusion in Chi when interacting with DynamoDB occurs when the runtime representation of data does not match the expected type defined in the application logic or schema. In Chi, a lightweight Go HTTP router, developers often bind URL parameters, query strings, or JSON payloads directly into structs. If these structs are used to construct DynamoDB expressions or conditionals without strict type validation, an attacker can supply a value that changes the interpreted type of a field during serialization or condition building.

For example, a DynamoDB condition expression may expect a numeric attribute for a quantity field, but if Chi binds the input into an interface{} or a loosely typed map, an attacker can provide a string that is later concatenated into the expression. This can cause the expression to resolve differently at runtime, potentially bypassing numeric checks or altering partition key routing. The DynamoDB SDK may serialize the value as a string instead of a number, leading to a condition that evaluates unexpectedly or exposes a different item than intended.

When using the AWS SDK for Go with Chi, a common pattern is to unmarshal JSON into a map[string]interface{} before building an UpdateItemInput. If the map is then used to construct an Update expression without type assertions, a string supplied as an ID can be misinterpreted when compared against a numeric attribute in condition expressions. This mismatch can lead to privilege escalation or information exposure when combined with BOLA/IDOR checks that rely on type-sensitive comparisons.

In the context of middleBrick’s 12 security checks, type confusion is flagged under Input Validation and Property Authorization. The scanner examines bindings in Chi routes, traces how user input flows into DynamoDB condition builders, and detects when type assertions are missing or when interface{} usage permits ambiguous serialization. This helps identify paths where an attacker can manipulate data types to bypass authorization logic or cause the DynamoDB service to interpret expressions in unintended ways.

To prevent type confusion in Chi with DynamoDB, enforce strict type binding and explicit type assertions before constructing any DynamoDB expressions. Use strongly-typed structs for request binding and validate numeric or boolean fields before using them in condition expressions.

Example 1: Safe binding with Chi and DynamoDB UpdateItem

import (
    "github.com/go-chi/chi/v5"
    "github.com/aws/aws-sdk-go/aws"
    "github.com/aws/aws-sdk-go/service/dynamodb"
    "github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute"
)

type UpdateItemRequest struct {
    Quantity int    `json:"quantity"`
    UserID   string `json:"userId" validate:"required"`
}

func updateHandler(db *dynamodb.DynamoDB) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        var req UpdateItemRequest
        if err := binding.Bind(r, &req); err != nil {
            http.Error(w, "invalid request", 400)
            return
        }

        av, err := dynamodbattribute.MarshalMap(map[string]interface{}{
            "Quantity": req.Quantity,
        })
        if err != nil {
            http.Error(w, "failed to marshal", 500)
            return
        }

        input := &dynamodb.UpdateItemInput{
            TableName: aws.String("Items"),
            Key: map[string]*dynamodb.AttributeValue{
                "UserID": {S: aws.String(req.UserID)},
            },
            UpdateExpression: aws.String("SET Quantity = :q"),
            ExpressionAttributeValues: map[string]*dynamodb.AttributeValue{
                ":q": {N: aws.String(fmt.Sprintf("%d", req.Quantity))},
            },
            ConditionExpression: aws.String("attribute_exists(UserID)"),
        }

        _, err = db.UpdateItem(input)
        if err != nil {
            http.Error(w, "update failed", 500)
            return
        }

        w.WriteHeader(200)
    }
}

Example 2: Validating numeric input before building a condition

func buildCondition(quantity interface{}) (*string, error) {
    q, ok := quantity.(int)
    if !ok {
        return nil, fmt.Errorf("quantity must be an integer")
    }
    if q <= 0 {
        return nil, fmt.Errorf("quantity must be positive")
    }
    expr := fmt.Sprintf("Quantity <= %d", q)
    return &expr, nil
}

Example 3: Using middleware in Chi to enforce types

func TypeCheck(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Validate query parameters before routing
        q := r.URL.Query().Get("limit")
        if _, err := strconv.Atoi(q); err != nil {
            http.Error(w, "limit must be an integer", 400)
            return
        }
        next.ServeHTTP(w, r)
    })
}

r := chi.NewRouter()
r.Use(TypeCheck)
r.Get("/items", func(w http.ResponseWriter, r *http.Request) {
    limitStr := r.URL.Query().Get("limit")
    limit, _ := strconv.Atoi(limitStr)
    // Use limit as int in DynamoDB scan input
    input := &dynamodb.ScanInput{
        TableName: aws.String("Items"),
        Limit:     aws.Int64(int64(limit)),
    }
    // proceed with scan
})

These patterns ensure that user input is explicitly typed before being used in DynamoDB expressions, reducing the risk of type confusion. middleBrick’s scans can validate that such checks are present in Chi routes and that DynamoDB inputs are not constructed from unchecked interface{} values.