Out Of Bounds Read in Chi with Jwt Tokens

An Out Of Bounds Read occurs when a request accesses memory locations outside the intended buffer. In Chi, this risk can emerge when handling JWT tokens if the token parsing or validation logic does not properly enforce length and boundary checks. Because Chi is a lightweight, expressive router for Go, developers often integrate JWT validation directly into chi middleware without adding strict bounds checks on token strings or decoded payloads.

When a JWT token is supplied in an Authorization header, the application typically decodes the token to inspect claims. If the token is unusually large or malformed, and the decoder reads into adjacent memory, an Out Of Bounds Read can occur. This may expose sensitive data from the process memory or lead to instability. The scanner’s Authentication and Input Validation checks will flag cases where JWT handling lacks length validation or does not reject tokens that exceed reasonable size limits, indicating a potential boundary violation.

Additionally, if the application uses reflection or unsafe operations to process JWT claims, the risk of reading beyond intended structures increases. For example, binding token claims into a loosely typed map without validating field sizes can expose underlying memory. The LLM/AI Security checks further highlight scenarios where token processing logic might leak system patterns or be probed for prompt or data leakage in AI-assisted tooling, though the core issue remains memory safety around JWT handling.

middleBrick identifies these concerns by testing the unauthenticated attack surface, including malformed JWT inputs and oversized tokens, without requiring credentials. The findings include severity levels and remediation guidance, helping developers address the root cause before an attacker can exploit the read boundary.

To mitigate Out Of Bounds Read risks with JWT tokens in Chi, enforce strict validation on token size and structure before decoding. Always set reasonable length limits on the raw token string and validate claims after decoding. Use a well-maintained JWT library with safe memory handling and avoid unsafe operations or unchecked reflection.

Below are concrete, working code examples for secure JWT handling in Chi.

Example 1: Validating token length and using a standard JWT parser

//go
package main

import (
	"context"
	"net/http"
	"strings"

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

const maxTokenLength = 4096

func jwtValidator(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, "authorization header required", http.StatusUnauthorized)
			return
		}
		scheme := strings.Split(auth, " ")
		if len(scheme) != 2 || strings.ToLower(scheme[0]) != "bearer" {
			http.Error(w, "invalid authorization format", http.StatusUnauthorized)
			return
		}
		token := scheme[1]
		if len(token) > maxTokenLength {
			http.Error(w, "token too large", http.StatusBadRequest)
			return
		}

		claims := jwt.MapClaims{}
		_, err := jwt.ParseWithClaims(token, claims, func(token *jwt.Token) (interface{}, error) {
			// TODO: use your key or public key here
			return []byte("your-secret-key"), nil
		})
		if err != nil {
			http.Error(w, "invalid token", http.StatusUnauthorized)
			return
		}

		ctx := context.WithValue(r.Context(), "claims", claims)
		next.ServeHTTP(w, r.WithContext(ctx))
	})
}

func main() {
	r := chi.NewRouter()
	r.Use(jwtValidator)
	r.Get("/secure", func(w http.ResponseWriter, r *http.Request) {
		claims, ok := r.Context().Value("claims").(jwt.MapClaims)
		if !ok {
			http.Error(w, "claims not found", http.StatusInternalServerError)
			return
		}
		w.Write([]byte("token claims: " + claims["sub"].(string)))
	})

	http.ListenAndServe(":8080", r)
}

Example 2: Rejecting tokens with unexpected or excessive claims

//go
package main

import (
	"context"
	"fmt"
	"net/http"
	"reflect"

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

func safeClaimsValidator(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, "authorization header required", http.StatusUnauthorized)
			return
		}
		scheme := strings.Split(auth, " ")
		if len(scheme) != 2 || strings.ToLower(scheme[0]) != "bearer" {
			http.Error(w, "invalid authorization format", http.StatusUnauthorized)
			return
		}
		token := scheme[1]
		if len(token) > maxTokenLength {
			http.Error(w, "token too large", http.StatusBadRequest)
			return
		}

		claims := jwt.MapClaims{}
		_, err := jwt.ParseWithClaims(token, claims, func(token *jwt.Token) (interface{}, error) {
			return []byte("your-secret-key"), nil
		})
		if err != nil {
			http.Error(w, "invalid token", http.StatusUnauthorized)
			return
		}

		// Enforce expected claim types and bounds
		if sub, ok := claims["sub"].(string); !ok || len(sub) == 0 || len(sub) > 256 {
			http.Error(w, "invalid subject claim", http.StatusBadRequest)
			return
		}
		if exp, ok := claims["exp"].(float64); !ok || exp == 0 {
			http.Error(w, "missing or invalid exp claim", http.StatusBadRequest)
			return
		}

		// Ensure no unexpected or dynamically added claims via reflection safety
		if reflect.ValueOf(claims).MapIndex(reflect.ValueOf("unsafe_custom")）。IsValid() {
			http.Error(w, "disallowed claim detected", http.StatusBadRequest)
			return
		}

		ctx := context.WithValue(r.Context(), "claims", claims)
		next.ServeHTTP(w, r.WithContext(ctx))
	})
}

func main() {
	r := chi.NewRouter()
	r.Use(safeClaimsValidator)
	r.Get("/profile", func(w http.ResponseWriter, r *http.Request) {
		claims := r.Context().Value("claims").(jwt.MapClaims)
		fmt.Fprintf(w, "user: %s, exp: %v", claims["sub"], claims["exp"])
	})

	http.ListenAndServe(":8080", r)
}