Dangling Dns in Echo Go with Dynamodb

A dangling DNS configuration in an Echo Go service that interacts with DynamoDB can expose unintended internal endpoints or external resolvers, creating a pathway for SSRF or data exposure. When the Go application uses the standard library or a framework to resolve hostnames for DynamoDB client calls, a missing or misconfigured DNS search list can cause resolution to fall back to public or unintended DNS servers. If the application resolves a hostname such as dynamodb.internal.corp and the DNS query returns an external IP due to a missing internal zone, the client may send database requests to a host controlled by an attacker or to an unintended cloud endpoint.

In an Echo Go app, this often occurs when custom DNS settings are used in a containerized environment or when service discovery relies on unmanaged DNS. The DynamoDB client, typically configured with a region and endpoint resolver, will use the system resolver unless explicitly overridden. An attacker can observe or manipulate DNS responses to redirect the client, enabling SSRF against the DynamoDB endpoint or leaking metadata service calls. This becomes a chain: the Echo server performs unauthenticated endpoint scanning as part of its security checks, and the misrouted DNS resolution leads the DynamoDB client to a malicious listener that returns crafted responses.

Because middleBrick tests unauthenticated attack surfaces and includes SSRF and DNS-related checks among its 12 parallel security checks, such a configuration is detectable. The scanner does not fix the issue but provides findings with remediation guidance, emphasizing the need to pin internal endpoints or enforce strict DNS policies. The combination of Echo Go, DynamoDB, and permissive DNS behavior increases risk because it widens the unauthenticated attack surface that middleBrick is designed to assess.

To mitigate dangling DNS risks when using DynamoDB in Echo Go, explicitly configure the AWS SDK to use a controlled resolver and avoid reliance on system DNS for sensitive endpoints. Below are concrete, working examples that you can adopt to reduce exposure.

1. Use a custom HTTP client with a fixed endpoint and DNS override

Force the DynamoDB client to use a specific IP or internal hostname by providing a custom HTTP client with a transport that overrides DNS resolution. This approach isolates DynamoDB traffic from the system resolver used by Echo Go.

// go
package main

import (
    "context"
    "net/http"
    "net/netip"
    "github.com/labstack/echo/v4"
    "github.com/aws/aws-sdk-go-v2/aws"
    "github.com/aws/aws-sdk-go-v2/config"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb"
)

func main() {
    e := echo.New()

    // Custom resolver that returns a fixed IP for a specific hostname
    resolver := &net.Resolver{PreferGo: true, Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
        d := &net.Dialer{}
        return d.DialContext(ctx, "udp", "10.0.0.1:53") // internal DNS resolver
    }}

    // Create a custom HTTP transport with DNS control
    transport := http.Transport{
        DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
            // Replace with your DynamoDB endpoint host
            host, port := "dynamodb.usw2.amazonaws.com", "443"
            ip, _ := resolver.LookupIPAddr(ctx, host)
            if len(ip) == 0 {
                return nil, fmt.Errorf("no IP resolved")
            }
            return net.DialTCP("tcp", nil, &net.TCPAddr{IP: ip[0].IP, Port: atoi(port)})
        },
        TLSHandshakeTimeout: 5 * time.Second,
    }

    cfg, err := config.LoadDefaultConfig(context.TODO(),
        config.WithHTTPClient(&http.Client{Transport: &transport}),
        config.WithRegion("us-west-2"),
    )
    if err != nil {
        log.Fatalf("unable to load SDK config: %v", err)
    }

    client := dynamodb.NewFromConfig(cfg)

    // Use client in an Echo handler
    e.GET('/items/:id', func(c echo.Context) error {
        id := c.Param("id")
        out, err := client.GetItem(context.TODO(), &dynamodb.GetItemInput{
            TableName: aws.String("Items"),
            Key: map[string]types.AttributeValue{
                "ID": &types.AttributeValueMemberS{Value: id},
            },
        })
        if err != nil {
            return c.String(http.StatusInternalServerError, err.Error())
        }
        return c.JSON(out.Item)
    })

    e.Start(":8080")
}

2. Enforce AWS SDK endpoint configuration and disable default resolver fallback

Explicitly set the DynamoDB endpoint and disable automatic endpoint discovery that may fall back to public DNS. This ensures the client communicates only with the intended AWS regional endpoint or an internal proxy.

// go
package main

import (
    "context"
    "github.com/labstack/echo/v4"
    "github.com/aws/aws-sdk-go-v2/aws"
    "github.com/aws/aws-sdk-go-v2/config"
    "github.com/aws/aws-sdk-go-v2/credentials/stscreds"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb"
)

func getDynamoDBClient() (*dynamodb.Client, error) {
    // Explicit endpoint; avoid relying on SDK's default resolver chain
    cfg, err := config.LoadDefaultConfig(context.TODO(),
        config.WithEndpointResolver(aws.EndpointResolverFunc(func(service, region string) (aws.Endpoint, error) {
            return aws.Endpoint{
                URL: "https://dynamodb.usw2.amazonaws.com",
            }, nil
        })),
        config.WithRegion("us-west-2"),
        // Optional: use IAM roles or static credentials as appropriate
    )
    if err != nil {
        return nil, err
    }
    return dynamodb.NewFromConfig(cfg), nil
}

func main() {
    e := echo.New()
    client, err := getDynamoDBClient()
    if err != nil {
        panic(err)
    }

    e.GET('/scan/:pk', func(c echo.Context) error {
        pk := c.Param("pk")
        result, err := client.GetItem(context.TODO(), &dynamodb.GetItemInput{
            TableName: aws.String("AuditTable"),
            Key: map[string]types.AttributeValue{
                "PK": &types.AttributeValueMemberS{Value: pk},
            },
        })
        if err != nil {
            return c.String(http.StatusInternalServerError, err.Error())
        }
        return c.JSON(result.Item)
    })

    e.Logger.Fatal(e.Start(":8080"))
}

3. Apply middleware to validate and log outbound calls

Use Echo Go middleware to inspect outgoing requests and ensure they target approved hosts. Log anomalies to support monitoring without altering the remediation behavior of the scanner, which only detects and reports.

// go
package main

import (
    "net/http"
    "github.com/labstack/echo/v4"
)

func outboundAudit(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        // Example: ensure requests go only to approved host
        req := c.Request()
        if req.URL.Hostname() != "dynamodb.usw2.amazonaws.com" {
            return c.String(http.StatusForbidden, "outbound host not allowed")
        }
        return next(c)
    }
}

func main() {
    e := echo.New()
    e.Use(outboundAudit)

    e.GET('/data/:key', func(c echo.Context) error {
        // handler using pre-configured client
        return c.String(http.StatusOK, "ok")
    })

    e.Logger.Fatal(e.Start(":8080"))
}

These examples demonstrate how to reduce DNS-related risks by controlling resolution and endpoint configuration in Echo Go applications that use DynamoDB. They align with remediation guidance that middleBrick provides in its findings, emphasizing explicit configuration and validation.