Symlink Attack in Chi

Symlink attacks in Chi applications typically exploit the framework's file handling capabilities when serving static assets or processing uploads. Chi, being a lightweight router for Go applications, doesn't provide built-in protections against symbolic link traversal, leaving this responsibility to developers.

The most common manifestation occurs when Chi serves files from directories that contain symlinks. An attacker can create a symlink pointing to sensitive files outside the intended directory, then request those files through Chi's file serving mechanisms. For example:

router := chi.NewRouter()
router.Handle("/static/*", http.StripPrefix("/static/", http.FileServer(http.Dir("./public"))))

If the ./public directory contains a symlink to /etc/passwd, an attacker can access it via /static/passwd. This works because http.FileServer follows symlinks by default, and Chi passes the request directly to the underlying file server without validation.

Another attack vector involves upload processing. When Chi applications handle file uploads and store them based on user-provided paths, attackers can upload files with symlinks or use symlinks in the target path. The application might then serve or process these symlinks, exposing sensitive data or creating unexpected behavior.

Path traversal combined with symlinks is particularly dangerous. An attacker might request /static/../../etc/passwd, which the file server resolves to /etc/passwd if symlinks are followed. Chi's routing doesn't prevent this because it delegates to the standard library's file server implementation.

Detecting symlink vulnerabilities in Chi applications requires both static analysis and runtime scanning. For static analysis, examine your Chi routes that serve files or handle uploads. Look for patterns like:

router.Handle("/static/*", http.FileServer(http.Dir("./static")))

Check if any of these routes use http.Dir() with paths that might contain symlinks or are writable by untrusted users. Also examine upload handlers that store files based on user input.

Runtime detection with middleBrick can identify these vulnerabilities by scanning your API endpoints. The scanner tests for path traversal and symlink following by attempting to access files outside the intended directories. For Chi applications, middleBrick specifically checks:

• Whether static file routes follow symlinks outside the served directory
• If upload endpoints allow path traversal through symlinks
• Whether configuration files or sensitive data can be accessed through symlink manipulation
• If the application properly validates file paths before serving or processing

The scanner generates a security score (A–F) and provides specific findings about symlink vulnerabilities, including the exact endpoints affected and recommended remediation steps. This is particularly valuable for Chi applications because the framework's minimal nature means developers must implement these protections manually.

middleBrick's LLM security scanning also checks if your Chi application serves AI/ML endpoints that might be vulnerable to symlink-based attacks on model files or training data.

Remediating symlink attacks in Chi applications requires implementing proper file system validation and using safer file serving alternatives. The most effective approach is to avoid using http.FileServer directly and instead implement custom file serving with symlink validation.

func safeFileHandler(root string) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        path := filepath.Join(root, r.URL.Path)
        
        // Resolve symlinks and check if path is within root
        realPath, err := filepath.Abs(path)
        if err != nil || !strings.HasPrefix(realPath, filepath.Clean(root)+string(filepath.Separator)) {
            http.Error(w, "Forbidden", http.StatusForbidden)
            return
        }
        
        // Check if path is a symlink
        fileInfo, err := os.Lstat(realPath)
        if err != nil || fileInfo.Mode()&os.ModeSymlink != 0 {
            http.Error(w, "Forbidden", http.StatusForbidden)
            return
        }
        
        http.ServeFile(w, r, realPath)
    }
}

router := chi.NewRouter()
router.Handle("/static/*", safeFileHandler("./public"))

This handler validates that the resolved path stays within the intended directory and rejects any symlinks. For upload handling, validate file paths before saving:

func uploadHandler(w http.ResponseWriter, r *http.Request) {
    file, header, err := r.FormFile("file")
    if err != nil {
        http.Error(w, err.Error(), http.StatusBadRequest)
        return
    }
    defer file.Close()
    
    // Sanitize and validate filename
    filename := filepath.Base(header.Filename)
    targetPath := filepath.Join("./uploads", filename)
    
    // Check for symlink attacks
    realPath, err := filepath.Abs(targetPath)
    if err != nil || !strings.HasPrefix(realPath, filepath.Clean("./uploads")) {
        http.Error(w, "Invalid file path", http.StatusBadRequest)
        return
    }
    
    out, err := os.Create(targetPath)
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }
    defer out.Close()
    
    io.Copy(out, file)
    w.Write([]byte("Upload successful"))
}

For production deployments, consider using a reverse proxy like nginx with proper symlink restrictions, or cloud storage services that don't follow symlinks. middleBrick's continuous monitoring can verify these fixes by periodically scanning your endpoints for symlink vulnerabilities.