# GoFiber Flash Cookie DoS: Unbounded Memory Allocation Attack

> Critical GoFiber vulnerability allows denial of service through crafted flash cookies that trigger massive memory allocation.

**URL:** https://www.ciptadusa.com/blog/gofiber-flash-cookie-dos-unbounded-memory  
**Type:** blog  
**Author:** PT Cipta Dua Saudara  
**Category:** Application Security  
**Published:** 2026-05-30  
**Cover:** https://www.ciptadusa.com/media/defaults/blog-cover.svg  

## Article

# GoFiber Flash Cookie DoS: Unbounded Memory Allocation Attack

## Overview

A critical denial of service (DoS) vulnerability was discovered in GoFiber's flash cookie implementation. Tracked as GHSA-2mr3-m5q5-wgp6, this vulnerability allows attackers to force unbounded memory allocation on any GoFiber server using flash cookies, potentially exhausting server resources with a single crafted request.

## Technical Details

### Vulnerability Description

The vulnerability exists in GoFiber's flash cookie handling mechanism. A crafted 10-character cookie value can trigger an attempt to allocate up to 85GB of memory, causing the server to crash or become unresponsive.

### Root Cause

GoFiber's flash cookie implementation fails to properly validate cookie values before parsing. When processing specially crafted values, the decoder attempts to allocate excessive memory based on the input data.

### Affected Versions

- GoFiber v3.0.0 and all previous versions
- Applications using flash cookie functionality

### Attack Mechanism

```go
// Attack payload (simplified)
// A 10-character cookie value that triggers excessive allocation
cookieValue := "AAAAAAAAAA" // Crafted value

// When GoFiber processes this flash cookie:
// 1. Parser reads the cookie value
// 2. Attempts to allocate memory based on parsed data
// 3. Allocation size reaches 85GB
// 4. Server crashes due to OOM (Out of Memory)
```

## Exploitation

### Attack Scenario

1. **Craft Payload**: Attacker creates malicious flash cookie value
2. **Send Request**: Single HTTP request with crafted cookie
3. **Resource Exhaustion**: Server attempts massive memory allocation
4. **Service Crash**: Application crashes or becomes unresponsive

### Example Attack

```http
GET / HTTP/1.1
Host: vulnerable-app.com
Cookie: flash=AAAAAAAAAA
```

## Impact Assessment

### Severity: High

- **Availability**: High - Complete service disruption
- **Confidentiality**: None
- **Integrity**: None

### Business Impact

- Service downtime
- Revenue loss
- Reputation damage
- Customer dissatisfaction

## Mitigation Strategies

### Immediate Actions

1. **Update GoFiber** to version 3.1.0 or later
2. **Disable flash cookies** if not needed:
   ```go
   // Remove or comment out flash cookie middleware
   // app.Use(flash.New())
   ```

### Alternative Implementation

```go
// Use secure cookie handling instead of flash
import (
    "github.com/gofiber/fiber/v3"
    "github.com/gofiber/fiber/v3/middleware/cookie"
)

app := fiber.New()

// Use standard cookie middleware with validation
app.Use(cookie.New(cookie.Config{
    // Set appropriate limits
    CookieMaxAge:   3600,
    CookieSecure:   true,
    CookieHTTPOnly: true,
}))
```

### Defense in Depth

```go
// Custom middleware to validate cookie sizes
func CookieSizeValidator(maxSize int) fiber.Handler {
    return func(c *fiber.Ctx) error {
        // Check all cookies
        c.Request().Header.VisitAllCookie(func(key, value []byte) {
            if len(value) > maxSize {
                // Log and reject oversized cookies
                log.Printf("Oversized cookie detected: %s (%d bytes)", 
                    string(key), len(value))
                c.ClearCookie(string(key))
            }
        })
        return c.Next()
    }
}

// Apply middleware
app.Use(CookieSizeValidator(4096)) // 4KB max cookie size
```

## Related Vulnerabilities in GoFiber

### CVE-2025-54801: BodyParser Slice Overflow

A vulnerability in GoFiber's `BodyParser` where large numeric keys in form data can cause out-of-bounds slice allocation.

```go
// Vulnerable pattern
type FormData struct {
    Items []string `form:"items"`
}

// Attack: POST with test.18446744073704=value
// Causes slice allocation of 18446744073705 elements
```

### CVE-2024-38513: Session Fixation

Session middleware vulnerability allowing predetermined session IDs.

## GoFiber Security Configuration

### Recommended Settings

```go
// Secure GoFiber configuration
app := fiber.New(fiber.Config{
    // Limit request body size
    BodyLimit: 10 * 1024 * 1024, // 10MB
    
    // Set read/write timeouts
    ReadTimeout:  10 * time.Second,
    WriteTimeout: 10 * time.Second,
    IdleTimeout:  120 * time.Second,
    
    // Disable server header
    ServerHeader: "",
    
    // Enable case-sensitive routing
    CaseSensitive: true,
    
    // Enable strict routing
    StrictRouting: true,
})
```

### Security Middleware Stack

```go
// Complete security middleware setup
import (
    "github.com/gofiber/fiber/v3"
    "github.com/gofiber/fiber/v3/middleware/helmet"
    "github.com/gofiber/fiber/v3/middleware/cors"
    "github.com/gofiber/fiber/v3/middleware/limiter"
    "github.com/gofiber/fiber/v3/middleware/csrf"
)

func SetupSecurityMiddleware(app *fiber.App) {
    // Security headers
    app.Use(helmet.New())
    
    // CORS configuration
    app.Use(cors.New(cors.Config{
        AllowOrigins:     "https://trusted-domain.com",
        AllowMethods:     "GET,POST,PUT,DELETE",
        AllowCredentials: true,
    }))
    
    // Rate limiting
    app.Use(limiter.New(limiter.Config{
        Max:        100,
        Expiration: 1 * time.Minute,
    }))
    
    // CSRF protection
    app.Use(csrf.New(csrf.Config{
        KeyLookup:      "header:X-CSRF-Token",
        CookieSecure:   true,
        CookieHTTPOnly: true,
    }))
}
```

## Monitoring and Detection

### Resource Monitoring

```go
// Monitor memory usage
import "runtime"

func MonitorMemory() {
    var stats runtime.MemStats
    runtime.ReadMemStats(&stats)
    
    log.Printf("Memory Usage: Alloc=%d MB, TotalAlloc=%d MB, Sys=%d MB",
        stats.Alloc/1024/1024,
        stats.TotalAlloc/1024/1024,
        stats.Sys/1024/1024)
    
    // Alert if memory usage exceeds threshold
    if stats.Alloc > 1024*1024*1024 { // 1GB
        log.Println("WARNING: High memory usage detected!")
        alertOpsTeam()
    }
}
```

### Request Monitoring

```go
// Monitor for suspicious cookie patterns
func CookieMonitor() fiber.Handler {
    return func(c *fiber.Ctx) error {
        // Check for suspicious cookie values
        cookies := c.Cookies()
        for key, value := range cookies {
            if len(value) > 1000 {
                log.Printf("Suspicious cookie: %s (size: %d)", key, len(value))
            }
        }
        return c.Next()
    }
}
```

## Best Practices for GoFiber Security

### Input Validation

1. **Validate all input**: Never trust client-supplied data
2. **Set size limits**: Configure appropriate limits for cookies, headers, and body
3. **Use middleware**: Leverage GoFiber's built-in security middleware

### Regular Updates

1. **Monitor security advisories**: Subscribe to GoFiber security announcements
2. **Update dependencies**: Keep GoFiber and dependencies current
3. **Security audits**: Conduct regular security reviews

## Conclusion

GHSA-2mr3-m5q5-wgp6 demonstrates how seemingly innocuous features like flash cookies can introduce critical vulnerabilities. By updating GoFiber and implementing proper input validation, developers can protect their applications from denial-of-service attacks.

## References

- [GoFiber Security Advisory GHSA-2mr3-m5q5-wgp6](https://github.com/gofiber/fiber/security/advisories/GHSA-2mr3-m5q5-wgp6)
- [GoFiber Security Documentation](https://docs.gofiber.io/security/)
- [OWASP - Denial of Service](https://owasp.org/www-community/attacks/Denial_of_Service)

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*Markdown version of https://www.ciptadusa.com/blog/gofiber-flash-cookie-dos-unbounded-memory — generated for AI agents and LLM crawlers.*