CWE-787 in Chi

CWE-787: Out-of-bounds Write occurs when an application writes data past the end, or before the beginning, of the intended buffer. In Chi, this vulnerability can manifest through several API-specific patterns that are particularly dangerous given Chi's high-performance, concurrent architecture.

Chi's buffer management for request payloads creates specific attack vectors. When handling multipart form data or streaming requests, Chi's internal buffer allocation can be exploited if size validation is insufficient. Attackers can craft requests with oversized content-length headers or malformed multipart boundaries to trigger buffer overflows.

func (h *Handler) handleUpload(w http.ResponseWriter, r *http.Request) {
    // Vulnerable: No validation of Content-Length
    file, header, err := r.FormFile("file")
    if err != nil {
        http.Error(w, err.Error(), http.StatusBadRequest)
        return
    }
    defer file.Close()
    
    // Chi allocates buffer based on Content-Length
    // Attacker can specify huge Content-Length to overflow buffer
    buf := make([]byte, header.Size)
    io.ReadFull(file, buf)
    
    // Process file...
}

Another manifestation occurs in Chi's context value storage. Chi uses a map[string]interface{} for storing request-scoped values, and improper bounds checking when serializing these values can lead to out-of-bounds writes:

func (c *Context) SerializeValues() ([]byte, error) {
    var buf bytes.Buffer
    for k, v := range c.values {
        // Vulnerable: No bounds checking on serialized data
        _, err := fmt.Fprintf(&buf, "%s:%v;", k, v)
        if err != nil {
            return nil, err
        }
    }
    return buf.Bytes(), nil
}

Chi's middleware chain also presents opportunities for CWE-787. When middleware modifies request bodies or headers without proper validation, out-of-bounds writes can occur during buffer resizing operations:

func (mw *BodyModifier) Middleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        body, _ := ioutil.ReadAll(r.Body)
        
        // Vulnerable: Appending without bounds checking
        modified := append(body, []byte("{additional: data}")...)
        
        r.Body = ioutil.NopCloser(bytes.NewReader(modified))
        next.ServeHTTP(w, r)
    })
}

Detecting CWE-787 in Chi applications requires a combination of static analysis, dynamic testing, and runtime monitoring. middleBrick's API security scanner includes specific checks for buffer overflow vulnerabilities in Chi applications, analyzing both the OpenAPI specification and runtime behavior.

middleBrick's Chi-specific detection includes:

• Content-Length header validation checks
• Buffer allocation analysis for multipart form handling
• Context value serialization boundary checks
• Middleware chain analysis for unsafe buffer operations
• Runtime fuzzing with oversized payloads

Here's how to use middleBrick to scan a Chi API for CWE-787 vulnerabilities:

# Install middleBrick CLI
npm install -g middlebrick

# Scan your Chi API endpoint
middlebrick scan https://api.example.com/upload

# Or scan with OpenAPI spec
middlebrick scan --spec openapi.json

middleBrick's scanner specifically looks for Chi's buffer allocation patterns and tests them with oversized payloads. The scanner identifies endpoints that accept file uploads, form data, or have middleware that modifies request bodies.

For manual detection, monitor your Chi application's memory usage and crash patterns. Use tools like go tool pprof to identify memory corruption issues, and implement runtime bounds checking in critical paths.

// Runtime bounds checking in Chi middleware
func SafeMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        if r.ContentLength > 10*1024*1024 { // 10MB limit
            http.Error(w, "Request too large", http.StatusRequestEntityTooLarge)
            return
        }
        next.ServeHTTP(w, r)
    })
}

// Add to Chi router
router.Use(SafeMiddleware)

Remediating CWE-787 in Chi applications requires implementing strict bounds checking and safe buffer handling patterns. Chi provides several mechanisms for secure buffer management that developers should leverage.

First, always validate Content-Length headers and implement proper size limits:

func (h *Handler) handleUpload(w http.ResponseWriter, r *http.Request) {
    // Validate Content-Length before processing
    if r.ContentLength > 5*1024*1024 { // 5MB max
        http.Error(w, "File too large", http.StatusRequestEntityTooLarge)
        return
    }
    
    // Use Chi's context for size validation
    ctx := chi.NewRouteContext()
    ctx.Req = r
    
    // Process file with bounds checking
    file, header, err := r.FormFile("file")
    if err != nil {
        http.Error(w, err.Error(), http.StatusBadRequest)
        return
    }
    defer file.Close()
    
    // Read with size limit
    buf := make([]byte, header.Size)
    n, err := io.ReadFull(file, buf)
    if err != nil && err != io.EOF {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }
    
    if int64(n) != header.Size {
        http.Error(w, "Incomplete file read", http.StatusInternalServerError)
        return
    }
    
    // Process file safely...
}

For context value serialization, implement safe bounds checking:

func (c *Context) SerializeValues() ([]byte, error) {
    var buf bytes.Buffer
    maxBufferSize := 1024 * 1024 // 1MB limit
    
    for k, v := range c.values {
        kvStr := fmt.Sprintf("%s:%v;", k, v)
        if buf.Len()+len(kvStr) > maxBufferSize {
            return nil, errors.New("context values exceed buffer size")
        }
        buf.WriteString(kvStr)
    }
    
    return buf.Bytes(), nil
}

Chi's middleware system should include bounds checking for all buffer operations:

func SafeBodyModifier(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        body, err := ioutil.ReadAll(io.LimitReader(r.Body, 10*1024*1024))
        if err != nil {
            http.Error(w, err.Error(), http.StatusInternalServerError)
            return
        }
        
        // Safe append with bounds checking
        additional := []byte("{additional: data}")
        if len(body)+len(additional) > 10*1024*1024 {
            http.Error(w, "Modified body too large", http.StatusRequestEntityTooLarge)
            return
        }
        
        modified := append(body, additional...)
        r.Body = ioutil.NopCloser(bytes.NewReader(modified))
        next.ServeHTTP(w, r)
    })
}

// Add to Chi router with proper error handling
router.Use(middleware.Recoverer)
router.Use(SafeBodyModifier)

Integrate middleBrick's continuous monitoring to catch new CWE-787 vulnerabilities as they're introduced:

# GitHub Action for CI/CD integration
name: API Security Scan

on: [push, pull_request]

jobs:
  security:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - name: Run middleBrick Scan
        run: |
          npm install -g middlebrick
          middlebrick scan https://staging-api.example.com --fail-below B
        env:
          MIDDLEBRICK_API_KEY: ${{ secrets.MIDDLEBRICK_API_KEY }}