Cors Wildcard in Gin with Dynamodb

A CORS wildcard (Access-Control-Allow-Origin: *) combined with a Gin backend that uses Amazon DynamoDB can unintentionally expose data and amplify authorization issues. When * is set in the CORS configuration, any origin in a browser can make authenticated requests and read responses. If the Gin routes also rely on broad identity-based IAM permissions on DynamoDB (for example, using a shared API key or an IAM role assigned to the host), the risk is that a malicious webpage served from any origin can query DynamoDB endpoints through the Gin service.

In practice, this often occurs when developers configure CORS at the Gin level with cors.AllowAll() while the DynamoDB client in the service uses permissive resource-level permissions (e.g., dynamodb:Scan or dynamodb:GetItem on * resources). An attacker can craft a web page that issues cross-origin requests to the Gin endpoints, leveraging any session or API credentials attached to the browser. Because the Gin service does not validate the origin and returns data from DynamoDB, sensitive records may be exfiltrated via the browser to an external domain.

Another exposure scenario involves preflight requests. When CORS headers are too permissive, browsers automatically send OPTIONS requests to verify allowed methods and headers. If the Gin handler responds with * for Access-Control-Allow-Origin, the subsequent actual request can include credentials. This can lead to unintended data exposure when the response contains DynamoDB query results, such as user PII or configuration data, that should not be readable by external origins.

Additionally, if the Gin application uses DynamoDB condition expressions or partial key queries without validating the requester’s tenant or ownership context, a wildcard CORS policy can facilitate Insecure Direct Object Reference (IDOR) patterns. An attacker may iterate over known identifiers and read records across partitions because the CORS layer does not enforce origin-based isolation and the DynamoDB permissions are not scoped to the authenticated principal.

Remediation centers on two layers: tightening CORS in Gin and scoping DynamoDB permissions and requests to the authenticated principal. Below are concrete, realistic examples for a Go Gin service that uses the AWS SDK for DynamoDB.

1. Configure CORS to disallow wildcards

Replace cors.AllowAll() with a policy that enumerates trusted origins and does not allow credentials for wildcard origins.

import (
    "github.com/gin-gonic/gin"
    "github.com/gin-contrib/cors"
)

func SetupRouter() *gin.Engine {
    r := gin.Default()
    config := cors.Config{
        AllowOrigins:     []string{"https://app.example.com", "https://api.example.com"},
        AllowMethods:     []string{"GET", "POST", "OPTIONS"},
        AllowHeaders:     []string{"Origin", "Content-Type"},
        ExposeHeaders:    []string{"Content-Length"},
        AllowCredentials: false, // Important: do not allow credentials with a restricted set
        MaxAge:           600,
    }
    r.Use(cors.New(config))
    // routes...
    return r
}

2. Scope DynamoDB access per request

Use the authenticated identity (e.g., from JWT or session) to constrain DynamoDB operations. For example, enforce a user_id partition key filter on queries.

import (
    "context"
    "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/gin-gonic/gin"
)

type UserService struct {
    DynamoClient *dynamodb.Client
    TableName    string
}

func (svc *UserService) GetUserProfile(c *gin.Context) {
    userID := c.MustGet("userID").(string) // injected by auth middleware
    input := &dynamodb.GetItemInput{
        TableName: aws.String(svc.TableName),
        Key: map[string]types.AttributeValue{
            "user_id": &types.AttributeValueMemberS{Value: userID},
        },
    }
    out, err := svc.DynamoClient.GetItem(c, input)
    if err != nil {
        c.JSON(500, gin.H{"error": "unable to fetch profile"})
        return
    }
    c.JSON(200, out.Item)
}

3. Enforce ownership checks before write/delete

When modifying items, validate that the request’s subject matches the item’s owner attribute in DynamoDB, avoiding relying solely on CORS for isolation.

func (svc *UserService) UpdateUserEmail(c *gin.Context) {
    userID := c.MustGet("userID").(string)
    var req struct {
        Email string `json:"email"`
    }
    if err := c.BindJSON(&req); err != nil {
        c.JSON(400, gin.H{"error": "invalid payload"})
        return
    }
    // Conditional update to ensure ownership
    input := &dynamodb.UpdateItemInput{
        TableName: aws.String(svc.TableName),
        Key: map[string]types.AttributeValue{
            "user_id": &types.AttributeValueMemberS{Value: userID},
        },
        UpdateExpression: aws.String("set email = :e"),
        ConditionExpression: aws.String("attribute_exists(user_id)"),
        ExpressionAttributeValues: map[string]types.AttributeValue{
            ":e": &types.AttributeValueMemberS{Value: req.Email},
        },
    }
    _, err := svc.DynamoClient.UpdateItem(c, input)
    if err != nil {
        c.JSON(409, gin.H{"error": "conflict or unauthorized"})
        return
        // Handle ConditionalCheckFailedException separately if needed
    }
    c.Status(204)
}

4. Apply least-privilege IAM for the service role

Ensure the IAM role/user associated with the Gin service uses policies that restrict DynamoDB actions to specific table ARNs and required operations, avoiding wildcards on resources.

5. Validate and encode output to prevent injection

Even with scoped permissions, ensure response data is validated and encoded to mitigate injection via malicious DynamoDB content.

import "html"

func SafeProfile(c *gin.Context) {
    // after retrieving item...
    email, _ := out.Item["email"].(*types.AttributeValueMemberS)
    c.HTML(200, "profile.tmpl", gin.H{
        "email": html.EscapeString(email.Value),
    })
}