Use After Free in Chi with Mutual Tls

A Use After Free (UAF) in Chi combined with Mutual TLS (mTLS) typically arises when connection state is freed on the server while mTLS session information or certificate-related objects remain referenced. In Chi, this can occur if middleware or handlers cache or reuse objects (e.g., request context values, parsed certificates, or connection metadata) after the request context is released. Because mTLS binds identity to certificates, an attacker who can influence certificate selection or trigger repeated handshakes may exploit stale references to access data belonging to another TLS session or request.

Consider a Chi handler that attaches a parsed certificate or a per-connection object to the request context and later reuses that object after the request is done. If the object is freed or overwritten while still being referenced (for example, in a background routine or a connection pool), a UAF occurs. mTLS increases the surface because certificate extraction and verification add more state to manage; mishandling that state can lead to use-after-free when the same context is reused across connections or when connection recycling is involved.

In practice, this can expose sensitive data from one TLS session to another or allow an attacker to cause erratic behavior by manipulating when objects are collected. Because Chi is a Go framework, memory management is handled by the runtime, but references held in global caches, session stores, or goroutines can keep objects alive or point to reclaimed memory when improperly synchronized. The risk is not in mTLS itself but in how application code manages lifetimes of objects tied to mTLS-derived data, such as certificates or verified identity claims.

Real-world analogies in other ecosystems show the pattern: an HTTP server processing client certificates caches a struct per connection, frees it at the wrong time, and later serves a different client with the stale data. OWASP API Top 10 does not call out UAF specifically for APIs, but related insecure deserialization and memory safety issues can appear when API frameworks manage complex per-request state, especially when identity (as with mTLS) is part of that state.

With middleBrick’s LLM/AI Security checks and broad coverage of OWASP categories, scans can help surface risky coding patterns around identity and state management. Its Authentication and Property Authorization checks can highlight improper handling of mTLS-derived identity, while findings with remediation guidance help steer safer lifetime management in Chi handlers.

To prevent Use After Free in Chi with mTLS, ensure objects derived from TLS state have clear lifetimes and are not retained beyond the request scope. Avoid storing request-context-derived data in long-lived caches keyed by connection or session identifiers. Use context cancellation to clean up resources, and synchronize access when caches are necessary.

Example 1: Safe handler without retaining TLS-derived objects after request completion.

// Chi safe handler example
package main

import (
	"context"
	"crypto/tls"
	"fmt"
	"net/http"

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

func main() {
	r := chi.NewRouter()
	r.Use(mtlsVerifierMiddleware)
	r.Get("/resource", func(w http.ResponseWriter, r *http.Request) {
		// Use request-scoped values only; do not store beyond this closure.
		cert := r.TLS.PeerCertificates
		if len(cert) == 0 {
			http.Error(w, "missing client cert", http.StatusUnauthorized)
			return
		}
		// Process request with cert locally; do not attach to context for reuse later.
		fmt.Fprintf(w, "Subject: %s", cert[0].Subject)
	})
	http.ListenAndTLS(":8443", r, tlsConfig())
}

func mtlsVerifierMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if r.TLS == nil || len(r.TLS.PeerCertificates) == 0 {
			http.Error(w, "client certificate required", http.StatusUnauthorized)
			return
		}
		// Verify policy as needed, then pass to next handler.
		next.ServeHTTP(w, r)
	})
}

func tlsConfig() *tls.Config {
	return &tls.Config{
		ClientAuth: tls.RequireAndVerifyClientCert,
		// Provide your CA pool in production.
		ClientCAs: nil, // set to your cert pool
	}
}

Example 2: Avoid global session map keyed by TLS connection state; if you must cache, use request context and ensure cleanup.

// Chi with context-scoped caching and cleanup
package main

import (
	"context"
	"crypto/tls"
	"net/http"

	"github.com/go-chi/chi/v5"
	"github.com/go-chi/context"
)

var ctxKey = &context.Key{}

func main() {
	r := chi.NewRouter()
	r.Use(func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			// Create request-scoped value; do not keep beyond this request.
			reqCtx := context.WithValue(r.Context(), ctxKey, &SessionData{
				CertSN: r.TLS.PeerCertificates[0].SerialNumber.String(),
			})
			// Ensure cleanup via context cancellation if needed.
			ctx, cancel := context.WithCancel(reqCtx)
			defer cancel()
			next.ServeHTTP(w, r.WithContext(ctx))
		})
	})
	r.Get("/info", func(w http.ResponseWriter, r *http.Request) {
		// Safe: read-only, request-scoped
		sess := r.Context().Value(ctxKey)
		// process sess safely; do not store globally
		w.Write([]byte("ok"))
	})
	http.ListenAndTLS(":8443", r, tlsConfig())
}

func tlsConfig() *tls.Config {
	return &tls.Config{
		ClientAuth: tls.RequireAndVerifyClientCert,
		ClientCAs:  nil,
	}
}

Key practices: do not persist mTLS-derived objects in global or long-lived caches; tie objects to request context and release them when the request ends; synchronize access if caches are unavoidable; prefer local variables within handlers; and use Chi middleware to validate mTLS early so handlers work with verified data without retaining extra state.

middleBrick’s GitHub Action can enforce checks in CI/CD pipelines, and its MCP Server allows scanning APIs directly from your IDE to catch risky patterns early. Its Pro plan offers continuous monitoring so changes that introduce unsafe state handling can be flagged before deployment.