Cors Wildcard in Buffalo with Dynamodb

A CORS wildcard (Access-Control-Allow-Origin: *) combined with a Buffalo application that uses Amazon DynamoDB can unintentionally expose sensitive data and enable unauthorized cross-origin requests. When the server sets a permissive CORS policy, any origin can make authenticated requests to the Buffalo app. If the Buffalo routes rely on DynamoDB to retrieve or modify records without enforcing per-request authorization, the wildcard allows malicious origins to invoke these endpoints on behalf of users.

DynamoDB does not enforce CORS; it is a backend data store. The risk arises at the application layer where Buffalo constructs responses after querying DynamoDB. For example, a route that fetches a user profile by ID and returns results to the client will leak data if the CORS policy permits any origin and the Buffalo handler does not validate that the requesting user is allowed to view the given record. Attackers can craft web pages that call the Buffalo endpoint, leveraging the victim’s credentials (e.g., session cookies) to read or modify DynamoDB items.

Consider a Buffalo app with a route /api/users/:id that queries DynamoDB for a user record. If CORS allows *, an attacker’s site can send authenticated requests to this route and enumerate user IDs, potentially exposing personal data stored in DynamoDB. This becomes more critical when DynamoDB items contain sensitive fields (e.g., email, role, or internal identifiers) and the Buffalo handler does not apply property-level or row-level authorization before returning the response.

The interaction between Buffalo, CORS configuration, and DynamoDB highlights the need to treat CORS as a first-class control. The database layer is not responsible for CORS; misconfiguration at the API layer can negate even tightly secured DynamoDB permissions. Proper remediation requires aligning CORS policies with authentication and authorization checks, ensuring that responses only include data the origin is explicitly permitted to access.

Secure remediation focuses on two areas: tightening CORS policies in Buffalo and ensuring DynamoDB access is constrained by per-request authorization. Below are concrete code examples for a Buffalo application using the official AWS SDK for Go v2.

1) Restrict CORS in Buffalo

Configure CORS to allow only specific origins and methods. Avoid wildcard origins when handling sensitive data. The example uses the cors middleware to set precise rules.

// main.go
package main

import (
    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/buffalo/middleware"
    "github.com/gobuffalo/buffalo/middleware/cors"
)

func app() *buffalo.App {
    app := buffalo.New(buffalo.Options{})
    app.Use(middleware.ParameterParser)

    corsOpts := cors.Options{
        AllowedOrigins:   []string{"https://trusted.example.com"},
        AllowedMethods:   []string{"GET", "POST", "PUT", "DELETE"},
        AllowedHeaders:   []string{"Authorization", "Content-Type"},
        ExposedHeaders:   []string{"X-Request-ID"},
        AllowCredentials: true,
        MaxAge:           3600,
    }
    app.Use(cors.Handler(corsOpts))

    // routes here
    return app
}

2) Authorize DynamoDB access per request

Ensure that each DynamoDB operation checks that the requesting user is permitted to access the target item. Use the AWS SDK to get item attributes and compare them with the authenticated user’s identity.

// handlers/user.go
package handlers

import (
    "context"
    "net/http"

    "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"
    "github.com/gobuffalo/buffalo"
)

func ShowUser(c buffalo.Context) error {
    userID := c.Param("id")
    authUser := c.Value("current_user").(string) // authenticated user identity

    svc := dynamodb.NewFromConfig(aws.Config{})
    out, err := svc.GetItem(context.Background(), &dynamodb.GetItemInput{
        TableName: aws.String("Users"),
        Key: map[string]types.AttributeValue{
            "id": &types.AttributeValueMemberS{Value: userID},
        },
    })
    if err != nil {
        return c.Render(http.StatusInternalServerError, r.JSON(err.Error()))
    }
    if out.Item == nil {
        return c.Render(http.StatusNotFound, nil)
    }

    // Enforce ownership: only allow access if the requesting user matches the item
    owner, ok := out.Item["owner_id"].(*types.AttributeValueMemberS)
    if !ok || owner.Value != authUser {
        return c.Render(http.StatusForbidden, nil)
    }

    // Return only allowed fields
    email, _ := out.Item["email"].(*types.AttributeValueMemberS)
    resp := map[string]string{"email": email.Value}
    return c.Render(http.StatusOK, r.JSON(resp))
}

3) Apply row-level authorization at the query layer

For queries that return multiple items (e.g., a list), filter on the server side using DynamoDB filters and ensure the Buffalo handler validates ownership before returning results.

// handlers/posts.go
package handlers

import (
    "context"
    "net/http"

    "github.com/aws/aws-sdk-go-v2/service/dynamodb"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb/types"
    "github.com/gobuffalo/buffalo"
)

func ListPosts(c buffalo.Context) error {
    authUser := c.Value("current_user").(string)

    svc := dynamodb.NewFromConfig(aws.Config{})
    out, err := svc.Query(context.Background(), &dynamodb.QueryInput{
        TableName:              aws.String("Posts"),
        IndexName:              aws.String("OwnerIndex"),
        KeyConditionExpression: aws.String("owner_id = :uid"),
        ExpressionAttributeValues: map[string]types.AttributeValue{
            ":uid": &types.AttributeValueMemberS{Value: authUser},
        },
    })
    if err != nil {
        return c.Render(http.StatusInternalServerError, r.JSON(err.Error()))
    }

    var posts []map[string]string
    for _, item := range out.Items {
        title, _ := item["title"].(*types.AttributeValueMemberS)
        posts = append(posts, map[string]string{"title": title.Value})
    }
    return c.Render(http.StatusOK, r.JSON(posts))
}

By combining specific CORS rules with per-request DynamoDB authorization, a Buffalo application reduces the risk of data exposure via cross-origin requests. These patterns align with checks across multiple security categories in a middleBrick scan, which can surface misconfigured CORS and overly permissive DynamoDB policies.