Formula Injection in Buffalo with Dynamodb

Formula Injection occurs when untrusted input is interpreted as code or a formula by the application or an integrated service. In a Buffalo application using Amazon DynamoDB, this typically happens when user-controlled data is used to construct DynamoDB expression attribute values or expression names without proper validation or escaping. While DynamoDB itself does not evaluate arbitrary code, unsafe construction of KeyConditionExpressions or FilterExpressions can lead to unintended behavior, including data leakage or bypass of access controls.

Buffalo provides request handling and view rendering but does not inherently sanitize inputs destined for DynamoDB. If a developer directly interpolates parameters from params into DynamoDB SDK calls, an attacker may supply values such as :userId that contain malicious expressions or operators. For example, a search endpoint that builds an expression like KeyConditionExpression: "user_id = :uid" with :uid taken directly from a query parameter can be abused if the parameter includes reserved words or crafted payloads intended to probe schema information.

In a real-world scenario, an endpoint accepting ?user_id=123 might build a condition user_id = :user_id with :user_id set to 123. If input validation is weak, an attacker could provide :user_id = "123 OR begins_with(email, 'a')" in a context where the string is parsed as part of expression construction, potentially altering the logical intent. While DynamoDB does not support arbitrary string evaluation, such inputs can still cause malformed expressions or expose metadata through error messages or timing differences when reserved keywords are used as attribute names.

The risk is amplified when combined with other checks. For instance, if the application also uses the output in other contexts (e.g., logging or passing to other services), injection vectors may extend beyond DynamoDB. The scanner checks related to Input Validation and Property Authorization can surface these issues by identifying that user input reaches sensitive query constructs without canonicalization or strict allowlists.

To detect this pattern, middleBrick runs checks that examine how user input flows into DynamoDB expression construction during unauthenticated scans. It does not modify code but reports when potentially unsafe concatenation is observed, providing remediation guidance to use expression attribute values correctly and validate inputs against strict patterns.

Remediation focuses on ensuring that user input never directly alters the structure of DynamoDB expressions. Always use expression attribute values for data and expression attribute names only for trusted, schema-defined identifiers. Validate and sanitize all inputs before they are used in query construction.

Example of vulnerable code

// Unsafe: directly using user input in expression components
func (r UserResource) Show(c buffalo.Context) error {
    userID := c.Param("user_id")
    svc := dynamodb.New(session.New())
    input := &dynamodb.GetItemInput{
        Key: map[string]*dynamodb.AttributeValue{
            "user_id": {S: aws.String(userID)},
        },
        TableName: aws.String("Users"),
    }
    result, err := svc.GetItem(input)
    // handle result
    return nil
}

While this example uses parameterized key values safely, a vulnerability arises if the developer builds expression strings using concatenation. The following demonstrates a problematic pattern that should be avoided.

// Unsafe expression construction (illustrative)
expr := "user_id = :uid"
// If expr is built from user input, it can be manipulated

Secure implementation using expression attribute values

Always use the built-in expression builders or ensure values are passed as attribute values, not expression text.

// Safe: using expression attribute values correctly
func (r UserResource) Show(c buffalo.Context) error {
    userID := c.Param("user_id")
    // Validate userID format, e.g., UUID or integer pattern
    if !isValidUserID(userID) {
        return c.Error(400, errors.New("invalid user id"))
    }
    svc := dynamodb.New(session.New())
    input := &dynamodb.GetItemInput{
        Key: map[string]*dynamodb.AttributeValue{
            "user_id": {S: aws.String(userID)},
        },
        TableName: aws.String("Users"),
    }
    result, err := svc.GetItem(input)
    if err != nil {
        return c.Error(500, err)
    }
    // process result
    return nil
}

When constructing query expressions that require dynamic attribute names (rare), use a strict allowlist to map incoming keys to known safe values. Never pass raw user input into expression name placeholders.

// Safe: using expression attribute names only for trusted schema fields
allowedSortFields := map[string]string{
    "created_at": "created_at",
    "status":     "status",
}
sortKey := c.Param("sort")
fieldName, ok := allowedSortFields[sortKey]
if !ok {
    sortKey = "created_at" // default
    fieldName = "created_at"
}
expr := aws.String("user_id = :uid AND " + fieldName + " = :val")
// Use attribute values for :uid and :val, expression name is trusted

These patterns ensure that user input is treated strictly as data, preventing injection risks. middleBrick can validate these practices by scanning your Buffalo endpoints and reporting when DynamoDB expression construction lacks proper isolation between code and data.