Dns Rebinding in Echo Go

Dns Rebinding attacks exploit the trust relationship between Echo Go applications and DNS-resolved hosts. In Echo Go applications, this vulnerability typically manifests when HTTP clients accept hostnames that can be dynamically resolved to different IP addresses over time.

Consider a common Echo Go pattern where an application proxies requests to user-supplied URLs:

func proxyHandler(c echo.Context) error {
    url := c.QueryParam("target")
    resp, err := http.Get(url)
    if err != nil {
        return err
    }
    defer resp.Body.Close()
    
    body, _ := ioutil.ReadAll(resp.Body)
    return c.JSON(http.StatusOK, map[string]string{
        "data": string(body),
    })
}

The vulnerability occurs when an attacker registers a domain that initially resolves to their own server, then rebinds it to a private IP address like 192.168.1.1. Echo Go's HTTP client will follow the DNS resolution without validating whether the final IP is public or private.

A typical attack sequence:

• Attacker registers evil.com pointing to their public server
• Echo Go application makes request to evil.com:1234
• Attacker rebinds DNS to 192.168.1.1:1234 (private IP)
• Echo Go client reconnects and sends request to internal service
• Internal service responds, believing the request came from the public internet

This pattern is especially dangerous in Echo Go applications that handle webhook verification, API proxying, or any feature that makes outbound HTTP requests based on user input.

The Echo Go context adds another layer of complexity. When using middleware for authentication or rate limiting, the attack might bypass these protections by targeting internal services that don't have the same security controls as the public-facing Echo Go application.

Detecting Dns Rebinding in Echo Go applications requires both runtime monitoring and proactive scanning. The middleBrick scanner specifically tests for this vulnerability by attempting controlled DNS rebinding attacks against your Echo Go endpoints.

middleBrick's approach includes:

• Testing endpoints that accept URL parameters for outbound requests
• Checking if the application makes requests to private IP ranges (10.x, 172.16-31.x, 192.168.x, 127.x, 169.254.x)
• Verifying if DNS TTL values are respected or if caching creates rebinding opportunities
• Scanning for endpoints that might proxy to internal services

For manual detection in Echo Go, implement request logging middleware that captures outbound connection attempts:

func dnsRebindingLogger() echo.MiddlewareFunc {
    return func(next echo.HandlerFunc) echo.HandlerFunc {
        return func(c echo.Context) error {
            // Log outbound requests for analysis
            start := time.Now()
            
            err := next(c)
            
            if c.Response().Status == http.StatusOK {
                // Check for any outbound requests made during this request
                // This would require instrumenting the HTTP client
            }
            
            return err
        }
    }
}

middleBrick's scanner also checks for Echo Go-specific patterns like:

• Handler functions that use c.QueryParam("url") or similar to accept URLs
• Middleware that doesn't validate DNS resolution results
• Echo Go's default HTTP client configuration that doesn't restrict IP ranges

The scanner provides a security score (A-F) and specific findings about whether your Echo Go application is vulnerable to Dns Rebinding, along with remediation guidance tailored to Echo Go's architecture.

Remediating Dns Rebinding in Echo Go requires a multi-layered approach. The most effective solution is implementing IP validation before making outbound requests.

First, create a validation function that checks if a resolved IP is public:

func isPublicIP(ip net.IP) bool {
    if ip.IsLoopback() || ip.IsLinkLocalUnicast() || ip.IsLinkLocalMulticast() {
        return false
    }
    
    // Check private IP ranges
    if ip4 := ip.To4(); ip4 != nil {
        switch {
        case ip4[0] == 10:
            return false
        case ip4[0] == 172 && ip4[1] >= 16 && ip4[1] <= 31:
            return false
        case ip4[0] == 192 && ip4[1] == 168:
            return false
        case ip4[0] == 100 && ip4[1] >= 64 && ip4[1] <= 127:
            return false
        case ip4[0] == 169 && ip4[1] == 254:
            return false
        }
    }
    
    return true
}

Then wrap your HTTP client calls with validation:

func safeGet(urlStr string) (*http.Response, error) {
    u, err := url.Parse(urlStr)
    if err != nil {
        return nil, err
    }
    
    // Resolve DNS and validate IP
    addrs, err := net.DefaultResolver.LookupIP(context.Background(), "ip", u.Hostname())
    if err != nil {
        return nil, err
    }
    
    if !isPublicIP(addrs[0]) {
        return nil, errors.New("resolved to private IP")
    }
    
    return http.Get(urlStr)
}

// Use in Echo handler
func secureProxyHandler(c echo.Context) error {
    url := c.QueryParam("target")
    
    resp, err := safeGet(url)
    if err != nil {
        return c.JSON(http.StatusBadRequest, map[string]string{
            "error": "invalid or unsafe URL",
        })
    }
    defer resp.Body.Close()
    
    body, _ := ioutil.ReadAll(resp.Body)
    return c.JSON(http.StatusOK, map[string]string{
        "data": string(body),
    })
}

For Echo Go applications that must proxy to internal services, implement a whitelist approach:

var allowedInternalIPs = map[string]bool{
    "192.168.1.100": true,  // Your internal API server
    "10.0.0.5": true,      // Another trusted service
}

func safeInternalGet(urlStr string) (*http.Response, error) {
    u, err := url.Parse(urlStr)
    if err != nil {
        return nil, err
    }
    
    addrs, err := net.DefaultResolver.LookupIP(context.Background(), "ip", u.Hostname())
    if err != nil {
        return nil, err
    }
    
    ipStr := addrs[0].String()
    if !allowedInternalIPs[ipStr] {
        return nil, errors.New("IP not in allowed list")
    }
    
    return http.Get(urlStr)
}

middleBrick's Pro plan includes continuous monitoring that can alert you if your Echo Go application's security score drops due to new Dns Rebinding vulnerabilities introduced in code changes. The GitHub Action integration can fail your CI/CD pipeline if the security score falls below your defined threshold.