Prototype Pollution in Buffalo with Dynamodb

Prototype pollution in JavaScript runtimes occurs when an attacker can modify the prototype of core objects such as Object or Array, causing properties to be shared across all objects. When this happens in a Buffalo application that interacts with Amazon DynamoDB, the risk pattern becomes twofold: server-side logic can be tricked into processing malicious input, and data retrieved from DynamoDB may be used to construct objects that inherit polluted prototypes.

Consider a Buffalo endpoint that accepts user-supplied query parameters to filter or tag resources. If the handler merges these parameters into a struct or a map without validation, an input like __proto__[role]=admin can mutate the shared prototype. In a typical Buffalo handler you might unmarshal incoming JSON into a map and later query DynamoDB using that map as a filter expression. Because the map carries the polluted property, any downstream object creation using that map may exhibit unexpected behavior, such as elevated privileges or bypassed checks.

When DynamoDB responses are deserialized into Go structs or interface maps, the resulting data can propagate the polluted prototype if the application reuses base objects or relies on shallow copies. For example, if you use a helper that copies response data into a new map using a naive assignment, the prototype chain can carry over injected properties. This is especially relevant when the application builds dynamic queries or constructs response objects by extending base prototypes, a pattern sometimes used to reduce boilerplate in handlers.

Moreover, Buffalo’s HTML template helpers can inadvertently expose injected properties if templates render user-controlled data without proper escaping. If a polluted property like __proto__ or constructor ends up in the template context, it may affect how other objects behave during rendering. Combined with DynamoDB-stored user data, this creates a chain where an attacker can influence both backend logic and frontend output, even though the storage layer itself remains unchanged.

To detect this vector, scans that combine runtime testing with spec-aware analysis are valuable. Because middleBrick cross-references OpenAPI/Swagger definitions with runtime findings, it can highlight endpoints that accept user input used in DynamoDB operations and flag missing input validation. This does not prevent the issue, but it clarifies where prototype pollution risks intersect with database interactions in Buffalo services.

Remediation focuses on strict input validation, avoiding prototype-mutating operations, and isolating DynamoDB data from object inheritance chains. Below are concrete code examples for a Buffalo handler that safely works with DynamoDB.

First, define explicit input structs instead of using raw maps. This prevents __proto__ keys from being interpreted as prototype modifiers.

type FilterParams struct {
	Status string `json:"status" validate:"required,oneof=active inactive"`
	Tag    string `json:"tag" validate:"omitempty,maxlength=64"`
}

Next, validate and sanitize before using the data in DynamoDB expressions. Use a dedicated library to clean field names and values, ensuring no keys like __proto__ or constructor are forwarded.

import "github.com/go-playground/validator/v10"

func (v *Validation) ValidateFilterParams(i interface{}) error {
	validate := validator.New()
	return validate.Struct(i)
}

When building DynamoDB queries, prefer the AWS SDK’s expression builder and avoid string concatenation with user input. Here’s how to construct a safe filter expression using the github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute package:

params := &dynamodb.ScanInput{
	TableName: aws.String("Resources"),
	FilterExpression: aws.String("#status = :status"),
	ExpressionAttributeNames: map[string]*string{
		"#status": aws.String("status"),
	},
	ExpressionAttributeValues: map[string]*dynamodb.AttributeValue{
		":status": {S: aws.String("active")},
	},
}

When unmarshaling DynamoDB responses, use explicit structs rather than interface maps to break prototype linkage. For example:

type ResourceItem struct {
	ID     string `json:"id"`
	Status string `json:"status"`
	Tag    string `json:"tag"`
}

var items []ResourceItem
if err := dynamodbattribute.UnmarshalListOfMaps(records, &items); err != nil {
	// handle error
}

Finally, ensure template rendering does not expose polluted properties. Buffalo’s c.Data should only contain explicitly assigned safe data:

c.Data["resources"] = safeItems // items of type []ResourceItem, not raw maps

These steps reduce the attack surface by eliminating implicit prototype behavior and ensuring DynamoDB interactions remain strictly typed and validated.