Format String in Chi

Format string vulnerabilities in Chi arise when user-controlled data is passed directly to formatting functions without proper sanitization. In Chi, this typically occurs in logging, error handling, and response generation contexts where string interpolation is used dynamically.

A common manifestation appears in error response formatting where status codes or user input are interpolated directly:

func handleRequest(c *chi.Context) {
    statusCode := c.URLParam("status")
    message := c.URLParam("message")
    
    // Vulnerable: user input in format string
    c.JSON(http.StatusBadRequest, fmt.Sprintf("%s: %s", statusCode, message))
}

This becomes exploitable when an attacker supplies format specifiers like %n or %x in the message parameter. The %n specifier writes the number of bytes written so far to an attacker-controlled memory address, potentially enabling arbitrary memory writes.

Another Chi-specific pattern occurs in middleware logging:

func loggingMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Vulnerable: user input in log format string
        log.Printf("[%s] %s", r.Header.Get("X-Forwarded-For"), r.URL.Path)
        next.ServeHTTP(w, r)
    })
}

Attackers can inject format specifiers via the X-Forwarded-For header to manipulate log output or trigger undefined behavior in the logging system.

Detecting format string vulnerabilities in Chi applications requires both static analysis and runtime scanning. middleBrick's black-box scanner identifies these issues by testing endpoints with format string payloads and analyzing responses for anomalies.

middleBrick tests Chi APIs using these specific payloads:

%n          // Memory write
%12345678x   // Stack consumption
%42$s       // Arbitrary memory read
%1$hn       // Half-word write

The scanner monitors for indicators including application crashes, unusual response patterns, or log entries that reveal format string processing. middleBrick's Chi-specific detection includes:

• Testing chi.Context parameter handling for format string injection
• Analyzing middleware chains for vulnerable logging patterns
• Checking route parameter processing for unsafe interpolation
• Scanning error handlers for format string exposure

For development-time detection, middleBrick's CLI can be integrated into your CI/CD pipeline:

middlebrick scan https://api.example.com --format=json --fail-below=B

This provides immediate feedback on format string vulnerabilities before deployment. The GitHub Action integration enables automated scanning:

- name: Scan for Format String Vulnerabilities
  uses: middlebrick/middlebrick-action@v1
  with:
    target: https://staging-api.example.com
    fail-on-severity: HIGH

Remediating format string vulnerabilities in Chi requires replacing unsafe interpolation with safe alternatives. The primary approach is using explicit argument passing instead of format string interpolation.

Vulnerable logging middleware should be rewritten as:

func safeLoggingMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        clientIP := r.Header.Get("X-Forwarded-For")
        path := r.URL.Path
        
        // Safe: explicit arguments, no format string injection
        log.Printf("[%s] %s", clientIP, path)
        next.ServeHTTP(w, r)
    })
}

For error responses, use structured JSON instead of formatted strings:

func handleRequest(c *chi.Context) {
    statusCode := c.URLParam("status")
    message := c.URLParam("message")
    
    // Safe: no format string interpolation
    c.JSON(http.StatusBadRequest, map[string]string{
        "status": statusCode,
        "message": message,
    })
}

Chi's built-in context parameter handling provides safe alternatives:

func safeHandler(c *chi.Context) {
    // Safe: URLParam returns string without format processing
    id := c.URLParam("id")
    
    // Safe: use structured logging
    logrus.WithFields(logrus.Fields{
        "request_id": c.RequestID,
        "user_id": id,
    }).Info("Processing request")
}

middleBrick's Pro plan includes continuous monitoring that scans your APIs on a configurable schedule, alerting you if new format string vulnerabilities are introduced through code changes or dependency updates.