Heap Overflow in Chi with Jwt Tokens

A heap-based buffer overflow in a Chi application that handles JWT tokens typically occurs when token payloads or header fields are copied into fixed-size buffers without proper length checks. Chi is a lightweight HTTP routing library for Go, and while the framework does not manage memory directly, unsafe use of token strings in handlers can expose a program to memory corruption. For example, if a developer reads a JWT token from an Authorization header and passes it to C-based code via cgo, or stores token segments in a fixed-size byte array, an oversized token can write past allocated memory boundaries.

Attackers can supply a JWT with an extremely long payload or header, such as a deeply nested JSON structure or a very long string in the sub or jti claim, to overflow the heap. This can corrupt adjacent memory, potentially leading to arbitrary code execution or denial of service. Because JWTs are often processed before authentication decisions are made, an unchecked token can affect unauthenticated attack surfaces, aligning with checks such as Input Validation and Unsafe Consumption that middleBrick scans for.

Consider a handler that copies a token into a fixed buffer:

buf := make([]byte, 256)
copy(buf, []byte(tokenString)) // tokenString may be much larger than 256 bytes

If tokenString exceeds 256 bytes, the copy will only write 256 bytes, but if the surrounding code uses unsafe operations or Cgo with smaller buffers, a heap overflow may occur. MiddleBrick’s checks for Input Validation and Unsafe Consumption help flag such risky patterns by analyzing the unauthenticated attack surface and scanning for improper handling of external inputs like JWTs.

To remediate heap overflow risks when handling JWT tokens in Chi, enforce strict length checks and avoid fixed-size buffers for token data. Use Go’s built-in types and the standard encoding/base64 package to decode tokens safely. Always validate token size before processing and prefer streaming or chunked processing for large payloads.

Below is a secure example of JWT handling in a Chi route without fixed buffers:

package main

import (
	"fmt"
	"net/http"

	"github.com/go-chi/chi/v5"
	"github.com/golang-jwt/jwt/v5"
)

func main() {
	r := chi.NewRouter()
	r.Use(func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			auth := r.Header.Get("Authorization")
			if auth == "" {
				http.Error(w, "missing token", http.StatusUnauthorized)
				return
			}
			const bearerPrefix = "Bearer "
			if len(auth) < len(bearerPrefix) || auth[:len(bearerPrefix)] != bearerPrefix {
				http.Error(w, "invalid authorization header", http.StatusUnauthorized)
				return
			}
			tokenString := auth[len(bearerPrefix):]
			// Validate token length before any processing
			if len(tokenString) == 0 || len(tokenString) > 4096 {
				http.Error(w, "invalid token length", http.StatusBadRequest)
				return
			}
			// Parse and validate token using the standard library
			_, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
				if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
					return nil, fmt.Errorf("unexpected signing method")
				}
				return []byte("your-secret-key"), nil
			})
			if err != nil {
				http.Error(w, "invalid token", http.StatusUnauthorized)
				return
			}
			next.ServeHTTP(w, r)
		})
	})
	r.Get("/secure", func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte("protected resource"))
	})
	http.ListenAndServe(":8080", r)
}

In this example, token length is validated before parsing, and no fixed-size buffers are used. The JWT library handles memory safely, and the Chi middleware ensures that only tokens of acceptable size proceed. For continuous monitoring of such vulnerabilities across your APIs, consider the Pro plan, which provides continuous scanning and integrates checks like Input Validation and Unsafe Consumption into your workflow.

Additionally, using the CLI tool, you can scan endpoints from the terminal:

middlebrick scan https://api.example.com/secure

This helps detect risky endpoints that handle JWTs unsafely, supporting compliance with frameworks such as OWASP API Top 10 and SOC2.