Fix lint errors

Use VerifyConnection instead of VerifyPeerCertificate; the other 2 fixes are "meh" not really a big deal or an issue at all.

cmd/commandfuncs.go

@@ -697,7 +697,7 @@ func cmdFmt(fl Flags) (int, error) {
 	output := caddyfile.Format(input)
 
 	if fl.Bool("overwrite") {
-		if err := os.WriteFile(configFile, output, 0o600); err != nil {
+		if err := os.WriteFile(configFile, output, 0o600); err != nil { //nolint:gosec // path traversal is not really a thing here, this is either "Caddyfile" or admin-controlled
 			return caddy.ExitCodeFailedStartup, fmt.Errorf("overwriting formatted file: %v", err)
 		}
 		return caddy.ExitCodeSuccess, nil

modules/caddytls/connpolicy.go

@@ -885,24 +885,29 @@ func (clientauth *ClientAuthentication) ConfigureTLSConfig(cfg *tls.Config) erro
 
 	// if a custom verification function already exists, wrap it
 	clientauth.existingVerifyPeerCert = cfg.VerifyPeerCertificate
-	cfg.VerifyPeerCertificate = clientauth.verifyPeerCertificate
+	cfg.VerifyConnection = clientauth.verifyConnection
 	return nil
 }
 
-// verifyPeerCertificate is for use as a tls.Config.VerifyPeerCertificate
-// callback to do custom client certificate verification. It is intended
-// for installation only by clientauth.ConfigureTLSConfig().
-func (clientauth *ClientAuthentication) verifyPeerCertificate(rawCerts [][]byte, verifiedChains [][]*x509.Certificate) error {
+// verifyConnection is for use as a tls.Config.VerifyConnection callback
+// to do custom client certificate verification. It is intended for
+// installation only by clientauth.ConfigureTLSConfig().
+//
+// Unlike VerifyPeerCertificate, VerifyConnection is called on every
+// connection including resumed sessions, preventing session-resumption bypass.
+func (clientauth *ClientAuthentication) verifyConnection(cs tls.ConnectionState) error {
 	// first use any pre-existing custom verification function
 	if clientauth.existingVerifyPeerCert != nil {
-		err := clientauth.existingVerifyPeerCert(rawCerts, verifiedChains)
-		if err != nil {
+		rawCerts := make([][]byte, len(cs.PeerCertificates))
+		for i, cert := range cs.PeerCertificates {
+			rawCerts[i] = cert.Raw
+		}
+		if err := clientauth.existingVerifyPeerCert(rawCerts, cs.VerifiedChains); err != nil {
 			return err
 		}
 	}
 	for _, verifier := range clientauth.verifiers {
-		err := verifier.VerifyClientCertificate(rawCerts, verifiedChains)
-		if err != nil {
+		if err := verifier.VerifyClientCertificate(nil, cs.VerifiedChains); err != nil {
 			return err
 		}
 	}

modules/caddytls/folderloader.go

@@ -19,6 +19,7 @@ import (
 	"crypto/tls"
 	"encoding/pem"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"strings"
@@ -62,18 +63,27 @@ func (fl FolderLoader) Provision(ctx caddy.Context) error {
 func (fl FolderLoader) LoadCertificates() ([]Certificate, error) {
 	var certs []Certificate
 	for _, dir := range fl {
-		err := filepath.Walk(dir, func(fpath string, info os.FileInfo, err error) error {
+		root, err := os.OpenRoot(dir)
+		if err != nil {
+			return nil, fmt.Errorf("unable to open root directory %s: %w", dir, err)
+		}
+		err = filepath.WalkDir(dir, func(fpath string, d fs.DirEntry, err error) error {
 			if err != nil {
 				return fmt.Errorf("unable to traverse into path: %s", fpath)
 			}
-			if info.IsDir() {
+			if d.IsDir() {
 				return nil
 			}
-			if !strings.HasSuffix(strings.ToLower(info.Name()), ".pem") {
+			if !strings.HasSuffix(strings.ToLower(d.Name()), ".pem") {
 				return nil
 			}
 
-			bundle, err := os.ReadFile(fpath)
+			rel, err := filepath.Rel(dir, fpath)
+			if err != nil {
+				return fmt.Errorf("unable to get relative path for %s: %w", fpath, err)
+			}
+
+			bundle, err := root.ReadFile(rel)
 			if err != nil {
 				return err
 			}
@@ -83,11 +93,11 @@ func (fl FolderLoader) LoadCertificates() ([]Certificate, error) {
 			}
 
 			certs = append(certs, Certificate{Certificate: cert})
-
 			return nil
 		})
+		_ = root.Close()
 		if err != nil {
-			return nil, err
+			return nil, fmt.Errorf("walking certificates directory %s: %w", dir, err)
 		}
 	}
 	return certs, nil