Dns Rebinding in Fiber with Cockroachdb

DNS Rebinding is an application-layer attack where an attacker forces a victim’s browser to resolve a domain to an internal IP address that is not publicly routable. In a Fiber application that interacts with CockroachDB, this can occur when the server uses a hostname to connect to the database and that hostname resolves differently at connection time versus query time. If the initial DNS resolution points to a public address but is later rebounded to a private address such as 127.0.0.1 or an internal cluster IP, the application may inadvertently allow the browser or an authenticated client to reach internal services through the server’s network path.

Consider a Fiber API that accepts a database host parameter for CockroachDB connections. If the server does not validate or restrict the host to a whitelist of known endpoints and relies on DNS at connection time only, an attacker can set up a domain that initially resolves to the intended public CockroachDB proxy but then switches to an internal address. Because the server may cache the resolved IP incorrectly or maintain long-lived connections, subsequent queries could be routed to the internal target. This exposes administrative interfaces, unsecured SQL ports, or metadata services that should never be reachable from client-side contexts.

The attack flow typically involves luring a user or system to a malicious page that triggers requests to the vulnerable Fiber endpoint. The malicious page may use JavaScript to perform requests with the victim’s credentials if authentication is cookie-based and not properly protected against cross-origin misuse. Because DNS Rebinding does not require the attacker to compromise the server directly, it bypasses perimeter defenses and leverages the trust the application has in internal network addresses. CockroachDB’s HTTP Admin UI or SQL port exposed internally can become accessible when the server logic does not enforce network-level separation between public API surfaces and private database endpoints.

To detect this pattern with middleBrick, you can submit the Fiber endpoint URL for an unauthenticated scan. The tool runs checks including SSRF and Input Validation, which can surface unsafe host resolution and improper network segregation. Findings will highlight whether the application resolves hostnames at runtime without additional validation and whether internal services are reachable through the API path. middleBrick maps these observations to the OWASP API Top 10 and provides remediation guidance, helping you understand how DNS Rebinding could intersect with CockroachDB connectivity in your deployment.

Mitigating DNS Rebinding in a Fiber application that uses CockroachDB requires strict host validation, network segregation, and avoidance of dynamic hostname resolution for database connections. Below are concrete code examples for a Fiber service that connects to CockroachDB safely.

First, avoid accepting database host parameters from client input. Instead, define the CockroachDB connection string as a fixed environment variable and validate it at startup. Use the pgx driver with a static connection string that points only to approved endpoints.

package main

import (
    "context"
    "fmt"
    "log"
    "net/http"
    "os"

    "github.com/gofiber/fiber/v2"
    "github.com/jackc/pgx/v5/pgxpool"
)

func main() {
    // Define the CockroachDB connection string from a trusted source
    connStr := os.Getenv("COCKROACHDB_URL")
    if connStr == "" {
        log.Fatal("COCKROACHDB_URL environment variable is required")
    }

    // Validate that the connection string uses an allowed host
    allowedHosts := map[string]bool{
        "cockroachdb.example.com": true,
        "proxy.example.com":       true,
    }
    // Perform basic host validation (in production, use URL parsing and stricter checks)
    if !allowedHosts["cockroachdb.example.com"] {
        log.Fatal("database host not allowed")
    }

    pool, err := pgxpool.New(context.Background(), connStr)
    if err != nil {
        log.Fatalf("unable to connect to database: %v", err)
    }
    defer pool.Close()

    app := fiber.New()

    app.Get("/api/users/:id", func(c *fiber.Ctx) error {
        userID := c.Params("id")
        var username string
        // Use parameterized queries to avoid SQL injection
        err := pool.QueryRow(c.Context(), "SELECT username FROM users WHERE id = $1", userID).Scan(&username)
        if err != nil {
            return c.Status(http.StatusInternalServerError).JSON(fiber.Map{"error": "unable to fetch user"})
        }
        return c.JSON(fiber.Map{"username": username})
    })

    log.Fatal(app.Listen(":3000"))
}

Second, enforce network-level controls so that the Fiber process cannot reach unintended hosts. Run the application in a restricted network zone where egress to private IP ranges is blocked by firewall rules. Even if DNS Rebinding manipulates resolution, the underlying network policy prevents connections to internal CockroachDB nodes that are not part of the approved proxy or public endpoint.

Third, if you must use multiple CockroachDB endpoints, implement a strict allowlist at runtime and re-resolve hostnames only at startup. Do not perform fresh DNS lookups for each request. Combine this with middleware in Fiber that validates requested resources and ensures that no parameter can redirect the database connection to a different host.

// Example of safe host validation at startup
type dbConfig struct {
    Pool *pgxpool.Pool
}

func newDBConfig() (*dbConfig, error) {
    connStr := os.Getenv("COCKROACHDB_URL")
    parsed, err := pgxpool.ParseConfig(connStr)
    if err != nil {
        return nil, fmt.Errorf("invalid connection string: %w", err)
    }
    // Ensure the host matches an allowed pattern
    if parsed.ConnConfig.Host != "cockroachdb.example.com:26257" {
        return nil, fmt.Errorf("host not allowed")
    }
    pool, err := pgxpool.NewWithConfig(context.Background(), parsed)
    if err != nil {
        return nil, err
    }
    return &dbConfig{Pool: pool}, nil
}

By combining static configuration, strict hostname allowlisting, and network controls, you reduce the risk that DNS Rebinding can exploit the interaction between Fiber and CockroachDB. middleBrick can support this posture by scanning your API endpoints for input validation weaknesses and SSRF-related findings, ensuring that attacker-controlled data cannot influence internal network paths.