Stack Overflow in Chi

Stack overflow vulnerabilities in Chi applications typically arise when handling unbounded user input in HTTP request contexts. Chi's router middleware stack processes requests through multiple layers, creating several potential overflow surfaces.

The most common manifestation occurs in path parameter handling. Consider this vulnerable pattern:

func main() {
    r := chi.NewRouter()
    r.Get("/api/users/{id}", func(w http.ResponseWriter, r *http.Request) {
        id := chi.URLParam(r, "id")
        // No validation on id length
        processUser(id) // processUser may have unsafe CGO calls
    })
}

Chi's URLParam extracts parameters without bounds checking, which becomes dangerous when these values flow into native code or memory-unsafe operations. The vulnerability amplifies when combined with Chi's context middleware:

r.Use(middleware.Timeout(60 * time.Second))
r.Get("/api/unsafe/{data}", func(w http.ResponseWriter, r *http.Request) {
    data := chi.URLParam(r, "data")
    ctx := context.WithValue(r.Context(), "user_data", data)
    processWithContext(ctx) // May overflow internal buffers
})

Another Chi-specific vector involves multipart form handling. Chi's default behavior doesn't enforce size limits:

r.Post("/upload", func(w http.ResponseWriter, r *http.Request) {
    err := r.ParseMultipartForm(10 << 20) // 10MB limit
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }
    files := r.MultipartForm.File["files[]"]
    for _, file := range files {
        // No validation on file content or size
        processFileUpload(file) // May cause stack overflow
    }
})

The issue compounds when Chi applications integrate with CGO or use unsafe pointers for performance optimizations, creating direct pathways for stack corruption.

Detecting stack overflow vulnerabilities in Chi applications requires examining both the Go code structure and the runtime behavior of the router middleware.

Static analysis should focus on these Chi-specific patterns:

# Look for unbounded parameter extraction
grep -r "chi\.URLParam" . --include="*.go" | 
grep -v "len(" | grep -v "validate"

# Find missing size limits in multipart handling
grep -r "ParseMultipartForm" . --include="*.go" | 
grep -v "10 <<" | grep -v "1 <<"

# Check for unsafe context value usage
grep -r "context\.WithValue" . --include="*.go" | 
grep -v "string" | grep -v "int"

Dynamic scanning with middleBrick reveals runtime stack overflow risks through its black-box testing methodology. The scanner sends progressively larger payloads to Chi endpoints and monitors for stack corruption indicators:

{
  "endpoint": "/api/users/{id}",
  "test_cases": [
    {"payload": "A", "response": "200 OK"},
    {"payload": "A".repeat(1000), "response": "200 OK"},
    {"payload": "A".repeat(100000), "response": "500 Internal Server Error"}
  ],
  "vulnerability": "Potential stack overflow in parameter handling",
  "severity": "high"
}

middleBrick's LLM security module also detects when Chi applications use AI features that might be vulnerable to prompt injection attacks, which can indirectly trigger stack overflows in LLM processing pipelines.

Chi provides several native mechanisms to prevent stack overflow vulnerabilities. The most effective approach combines input validation with Chi's built-in middleware.

For path parameter validation, use Chi's routing constraints:

func main() {
    r := chi.NewRouter()
    
    // Only allow numeric IDs up to 10 digits
    r.Get("/api/users/{id:[0-9]{1,10}}", func(w http.ResponseWriter, r *http.Request) {
        id := chi.URLParam(r, "id")
        // Safe to use - validated by router
        processUser(id)
    })
    
    // For string parameters with length limits
    r.Get("/api/search/{query:[a-zA-Z0-9]{1,50}}", func(w http.ResponseWriter, r *http.Request) {
        query := chi.URLParam(r, "query")
        search(query)
    })
}

Chi's middleware ecosystem provides stack protection:

func main() {
    r := chi.NewRouter()
    
    // Set maximum request size
    r.Use(middleware.DefaultCompress)
    r.Use(middleware.Recoverer)
    r.Use(middleware.Timeout(30 * time.Second))
    
    // Custom size limiter
    r.Use(func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            r.Body = http.MaxBytesReader(w, r.Body, 1<<20) // 1MB limit
            next.ServeHTTP(w, r)
        })
    })
    
    r.Post("/upload", func(w http.ResponseWriter, r *http.Request) {
        // Safe - body size already limited
        processUpload(r)
    })
}

For applications using CGO or unsafe operations, implement additional stack canaries:

import "runtime/debug"

func safeHandler(w http.ResponseWriter, r *http.Request) {
    defer func() {
        if err := recover(); err != nil {
            log.Printf("stack overflow recovered: %v", err)
            http.Error(w, "Internal server error", http.StatusInternalServerError)
            debug.PrintStack()
        }
    }()
    
    // Handler logic
    processPotentiallyUnsafe(r)
}

Chi's context handling should use typed values instead of raw strings:

type userData struct {
    ID   string
    Name string
}

r.Get("/api/profile", func(w http.ResponseWriter, r *http.Request) {
    ctx := context.WithValue(r.Context(), "user_data", &userData{ID: "123"})
    processWithTypedContext(ctx)
})