Stack Overflow in Fiber

Stack overflow vulnerabilities in Fiber applications typically occur when recursive function calls or unbounded data structures consume the Go runtime stack beyond its limits. In Fiber, this often manifests through recursive middleware chains, deeply nested JSON parsing, or infinite template rendering loops.

A common Fiber-specific pattern involves recursive middleware that creates unbounded call stacks. Consider this vulnerable middleware chain:

func recursiveMiddleware(c *fiber.Ctx) error {
    // No base case to terminate recursion
    return recursiveMiddleware(c)
}

app.Use(recursiveMiddleware)

When this middleware is registered, each request triggers an infinite recursive call chain until the Go runtime stack overflows, crashing the application with a runtime: goroutine stack exceeds 1000000000-byte limit error.

Another Fiber-specific scenario involves unbounded template rendering with recursive includes. Fiber's default templating engine can enter infinite loops:

// templates/main.tmpl
{{ template "nested" }}

// templates/nested.tmpl
{{ template "main" }}

This mutual recursion between templates causes stack overflow during rendering, potentially exposing sensitive error information to attackers.

JSON parsing with deeply nested structures also poses risks. Fiber's default JSON unmarshaler doesn't limit nesting depth:

type DeepStruct struct {
    Next *DeepStruct `json:"next"`
}

// Attacker sends: {"next": {"next": {"next": ... }}}

Unbounded recursion during JSON decoding can exhaust stack space, leading to denial of service.

Detecting stack overflow vulnerabilities in Fiber requires both static analysis and runtime monitoring. middleBrick's black-box scanning approach identifies these issues without requiring source code access.

middleBrick tests for stack overflow vulnerabilities by sending requests with intentionally recursive patterns. For middleware detection, it analyzes response patterns for stack trace leaks and monitors for application crashes under controlled recursion attempts.

The scanner specifically tests Fiber's JSON parsing by sending deeply nested JSON payloads with varying depths, measuring response times and error patterns. It also attempts template recursion by requesting endpoints that render templates with circular references.

For middleware analysis, middleBrick examines the application's middleware chain for potential recursion points. It identifies patterns like:

func vulnerableMiddleware(c *fiber.Ctx) error {
    // No depth limit or base case
    return c.Next()
}

The scanner flags middleware that could create unbounded call chains, especially when combined with other recursive middleware components.

middleBrick's LLM security module also detects stack-related issues in AI-powered Fiber applications. It tests for excessive agency patterns where recursive tool calls could create infinite loops, and scans for system prompt leakage that might reveal internal stack traces or error handling logic.

Continuous monitoring in the Pro plan tracks stack usage patterns over time, alerting when unusual recursion or stack growth is detected in production APIs.

Preventing stack overflow in Fiber applications requires implementing depth limits and safe recursion patterns. Here are Fiber-specific remediation strategies:

For middleware recursion, always implement depth counters and base cases:

func safeMiddleware(depth int) fiber.Handler {
    return func(c *fiber.Ctx) error {
        if depth > 10 {
            return c.Next()
        }
        return safeMiddleware(depth + 1)(c)
    }
}

app.Use(safeMiddleware(0))

This limits recursion to 10 levels, preventing stack overflow while maintaining functionality.

For JSON parsing, configure Fiber to limit nesting depth:

app.Use(func(c *fiber.Ctx) error {
    // Custom JSON decoder with depth limit
    decoder := json.NewDecoder(c.BodyStream)
    decoder.UseNumber()
    decoder.DisallowUnknownFields()
    
    // Set maximum depth (adjust based on requirements)
    decoder.MaxDepth(32)
    
    var data interface{}
    if err := decoder.Decode(&data); err != nil {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{
            "error": "Invalid JSON structure",
        })
    }
    
    c.Locals("parsedData", data)
    return c.Next()
})

This prevents attackers from sending arbitrarily nested JSON structures.

For template rendering, implement safe template includes with depth tracking:

func safeRender(c *fiber.Ctx, name string, data interface{}, depth int) error {
    if depth > 10 {
        return c.Status(fiber.StatusInternalServerError).SendString("Template recursion limit exceeded")
    }
    
    // Render template with depth tracking
    return c.Render(name, data)
}

middleBrick's scanning results provide specific findings about which endpoints are vulnerable to stack overflow, allowing you to prioritize these fixes in your remediation workflow.

For production deployments, combine these fixes with middleBrick's continuous monitoring to detect any new stack overflow vulnerabilities introduced through code changes or third-party dependencies.