Api Rate Abuse in Fiber with Openid Connect

Rate abuse in a Fiber API that uses OpenID Connect (OIDC) for authentication can occur when rate limiting is applied after authentication rather than before it, or when tokens are accepted in a way that bypasses intended limits. In a typical Fiber service, unprotected entry points such as the token introspection or userinfo endpoints may be invoked for every request that carries an OIDC access token. If these endpoints do not enforce strict per-client or per-user rate limits, an attacker can flood them with repeated introspection requests to consume server resources or infer the presence of valid tokens.

Another scenario involves token validation endpoints exposed as public routes. With OIDC, the authorization server issues access tokens that are often validated locally using JWKS. If the JWKS endpoint or the token validation logic is not rate-limited, attackers can generate many malformed or signed-but-unauthorized tokens to trigger expensive cryptographic operations. This can lead to denial of service or allow brute-force attempts against token signatures to learn key material or validity windows. Because OIDC tokens commonly carry scopes and roles, abuse against endpoints that decode tokens to enforce authorization may reveal which tokens are considered valid, aiding further attacks such as BOLA/IDOR.

Furthermore, if rate limiting is applied only at the API gateway or middleware layer and not within the application logic in Fiber, authenticated sessions may be exploited differently. An attacker with a single valid OIDC token can open many concurrent sessions or use token replay across endpoints, saturating resources tied to user identity. For example, endpoints that fetch user profile or permissions from an upstream OIDC userinfo service may be hammered, causing high latency or crashes. The interplay between OIDC token lifecycle (issuance, refresh, revocation) and Fiber route handling means that without coordinated rate limiting at both the identity provider interaction and the API handler levels, abuse can persist even after initial defenses.

In practice, scanning an OIDC-protected Fiber endpoint with middleBrick can surface these risks by flagging missing rate limiting on authentication and token-related routes, excessive agency in token handling patterns, and unauthenticated LLM endpoints that may expose debug or introspection logic. The scanner evaluates whether token validation and introspection paths are subject to the same controls as protected resources, and whether per-client and per-user limits are enforced consistently. Findings highlight the need to align rate limits with OIDC flows, ensuring that token validation, userinfo, and token revocation paths are included in protection policies.

To mitigate rate abuse in Fiber with OpenID Connect, apply rate limits before expensive token validation and ensure that OIDC-specific routes are included in the same limits as regular API endpoints. Below are concrete, realistic examples for a Fiber-based Go service using the golang-jwt and go-oidc libraries.

Rate limiting token introspection and userinfo

Wrap your token validation and userinfo handlers with a per-client limiter. Use a map keyed by client ID or issuer plus subject, and enforce a sliding window or token bucket. For simplicity, this example uses a basic in-memory rate limiter applied before token parsing.

//go
package main

import (
	"context"
	"net/http"
	"time"

	"github.com/gofiber/fiber/v2"
	goidc "github.com/coreos/go-oidc/v3/oidc"
	"golang.org/x/oauth2"
)

// simpleLimiter allows up to N requests per window per key.
type simpleLimiter struct {
	limits map[string]int
	window time.Duration
}

func newLimiter(window time.Duration) *simpleLimiter {
	return &simpleLimiter{
		limits: make(map[string]int),
		window: window,
	}
}

func (rl *simpleLimiter) allow(key string) bool {
	// In production, use a more robust algorithm (e.g., token bucket).
	// This is illustrative.
	ticker := time.NewTicker(rl.window)
	defer ticker.Stop()
	select {
	case <-ticker.C:
		rl.limits[key] = 0
	default:
	}
	if rl.limits[key] >= 5 {
		return false
	}
	rl.limits[key]++
	return true
}

func introspectHandler(c *fiber.Ctx) error {
	key := c.Params("key") // e.g., client_id:issuer
	if !limiter.allow(key) {
		return c.Status(fiber.StatusTooManyRequests).JSON(fiber.Map{
			"error": "rate limit exceeded",
		})
	}
	// proceed with token introspection
	return c.JSON(fiber.Map{"active": true})
}

func userinfoHandler(c *fiber.Ctx) error {
	key := c.Params("key")
	if !limiter.allow(key) {
		return c.Status(fiber.StatusTooManyRequests).JSON(fiber.Map{
			"error": "rate limit exceeded",
		})
	}
	// fetch userinfo from OIDC provider
	return c.JSON(fiber.Map{"sub": "user-123"})
}

func main() {
	limiter = newLimiter(time.Minute)
	app := fiber.New()
	app.Post("/introspect/:key", introspectHandler)
	app.Get("/userinfo/:key", userinfoHandler)
	app.Listen(":3000")
}

In addition to protecting introspection and userinfo, apply rate limits around token validation and authorization checks inside your Fiber routes. This prevents an attacker from sending many crafted tokens that trigger expensive cryptographic verification. Use a per-key limiter on the token identifier (e.g., token jti or a hash of the key material) before calling VerifyToken.

//go
func verifyToken(tokenString string, key string) (*goidc.IDToken, error) {
	// key could be a combination of issuer and client id
	if !limiter.allow(key) {
		return nil, fmt.Errorf("rate limit exceeded")
	}
	ctx := context.Background()
	provider, err := oidc.NewProvider(ctx, "https://your-issuer")
	if err != nil {
		return nil, err
	}
	verifier := provider.Verifier(&oidc.Config{ClientID: "your-client-id"})
	return verifier.Verify(ctx, tokenString)
}

app.Post("/api/protected", func(c *fiber.Ctx) error {
	auth := c.Get("Authorization")
	if auth == "" {
		return c.SendStatus(fiber.StatusUnauthorized)
	}
	token := strings.TrimPrefix(auth, "Bearer ")
	key := deriveKey(c) // implement per-client/issuer key
	idToken, err := verifyToken(token, key)
	if err != nil {
		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": err.Error()})
	}
	// claims-based authorization
	var claims map[string]interface{}
	if err := idToken.Claims(&claims); err != nil {
		return c.SendStatus(fiber.StatusInternalServerError)
	}
	return c.JSON(fiber.Map{"claims": claims})
})"

// deriveKey returns a rate-limit key based on issuer and client.
func deriveKey(c *fiber.Ctx) string {
	// Simplified: extract issuer and client from request or token header.
	return c.Get("x-issuer") + ":" + c.Get("x-client-id")
}

Ensure that rate limits cover the full OIDC interaction path: authorization code exchange, token refresh, and revocation. For example, limit the number of token exchanges per client ID per minute within the /token endpoint if you expose it, and apply stricter limits on userinfo and introspection. Coupling these limits with logging and anomaly detection helps identify patterns of abuse. middleBrick’s scan can highlight whether these OIDC-specific routes are missing from your rate limiting policy and whether findings map to frameworks like OWASP API Top 10 and GDPR, giving you prioritized remediation guidance.