diff --git a/internal/device/certs/gsma-rsp2-root-ci1.pem b/internal/device/certs/gsma-rsp2-root-ci1.pem
new file mode 100644
index 0000000..d9b7113
--- /dev/null
+++ b/internal/device/certs/gsma-rsp2-root-ci1.pem
@@ -0,0 +1,15 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
diff --git a/internal/device/es9p.go b/internal/device/es9p.go
index ec0d878..cf77037 100644
--- a/internal/device/es9p.go
+++ b/internal/device/es9p.go
@@ -3,6 +3,7 @@ package device
 import (
 	"bytes"
 	"context"
+	"crypto/x509"
 	"encoding/base64"
 	"encoding/json"
 	"errors"
@@ -14,9 +15,24 @@ import (
 	"strings"
 	"time"
 
+	_ "embed"
+
 	"vocat/internal/netguard"
 )
 
+// GSM Association RSP2 Root CI1; SHA-256 fingerprint:
+// 5E:3E:91:FD:45:43:27:C3:AF:5D:32:A7:A7:3B:BC:59:FE:43:AA:7D:85:FD:32:D5:DB:44:42:3F:80:A5:6B:B3.
+//
+//go:embed certs/gsma-rsp2-root-ci1.pem
+var gsmaRSP2RootCI1PEM []byte
+
+var gsmaRSP2RootCI1SHA256 = [32]byte{
+	0x5e, 0x3e, 0x91, 0xfd, 0x45, 0x43, 0x27, 0xc3,
+	0xaf, 0x5d, 0x32, 0xa7, 0xa7, 0x3b, 0xbc, 0x59,
+	0xfe, 0x43, 0xaa, 0x7d, 0x85, 0xfd, 0x32, 0xd5,
+	0xdb, 0x44, 0x42, 0x3f, 0x80, 0xa5, 0x6b, 0xb3,
+}
+
 // es9pClient speaks SGP.22 ES9+ — JSON over HTTPS — to one SM-DP+. It is the
 // network half of the LPA download flow: the host authenticates nothing itself
 // (the eUICC does all certificate verification on-card); it only shuttles the
@@ -50,13 +66,28 @@ func newES9PClient(ctx context.Context, smdp string) (*es9pClient, error) {
 	if err != nil {
 		return nil, fmt.Errorf("esim: unsafe SM-DP+ address: %w", err)
 	}
+	roots, err := es9pRootCAs()
+	if err != nil {
+		return nil, err
+	}
 	return &es9pClient{
 		smdp:     validated.Host,
 		endpoint: validated,
-		http:     netguard.NewPublicHTTPClient(90*time.Second, true),
+		http:     netguard.NewPublicHTTPClientWithRootCAs(90*time.Second, true, roots),
 	}, nil
 }
 
+func es9pRootCAs() (*x509.CertPool, error) {
+	roots, err := x509.SystemCertPool()
+	if err != nil || roots == nil {
+		roots = x509.NewCertPool()
+	}
+	if !roots.AppendCertsFromPEM(gsmaRSP2RootCI1PEM) {
+		return nil, errors.New("esim: load GSMA RSP2 Root CI1 certificate")
+	}
+	return roots, nil
+}
+
 // es9pError is a failed ES9+ functionExecutionStatus. Message is the SM-DP+'s
 // own explanation (surfaced verbatim, as the reference implementation does).
 type es9pError struct {
diff --git a/internal/device/es9p_test.go b/internal/device/es9p_test.go
index 9999cfa..5d939e9 100644
--- a/internal/device/es9p_test.go
+++ b/internal/device/es9p_test.go
@@ -3,8 +3,11 @@ package device
 import (
 	"bytes"
 	"context"
+	"crypto/sha256"
+	"crypto/x509"
 	"encoding/base64"
 	"encoding/json"
+	"encoding/pem"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
@@ -12,6 +15,30 @@ import (
 	"testing"
 )
 
+func TestES9PRootCAsIncludeGSMARSP2RootCI1(t *testing.T) {
+	roots, err := es9pRootCAs()
+	if err != nil {
+		t.Fatal(err)
+	}
+	block, _ := pem.Decode(gsmaRSP2RootCI1PEM)
+	if block == nil {
+		t.Fatal("GSMA Root CI1 PEM did not decode")
+	}
+	certificate, err := x509.ParseCertificate(block.Bytes)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if actual := sha256.Sum256(certificate.Raw); actual != gsmaRSP2RootCI1SHA256 {
+		t.Fatalf("GSMA root SHA-256 = %X, want %X", actual, gsmaRSP2RootCI1SHA256)
+	}
+	if certificate.Subject.CommonName != "GSM Association - RSP2 Root CI1" || !certificate.IsCA {
+		t.Fatalf("unexpected GSMA root certificate: subject=%q ca=%v", certificate.Subject.CommonName, certificate.IsCA)
+	}
+	if _, err := certificate.Verify(x509.VerifyOptions{Roots: roots}); err != nil {
+		t.Fatalf("GSMA root is not trusted by the ES9+ pool: %v", err)
+	}
+}
+
 // newTestES9P routes an es9pClient at a throwaway TLS server.
 func newTestES9P(t *testing.T, handler http.HandlerFunc) *es9pClient {
 	t.Helper()
diff --git a/internal/netguard/public.go b/internal/netguard/public.go
index 4b1a50a..71707c9 100644
--- a/internal/netguard/public.go
+++ b/internal/netguard/public.go
@@ -3,6 +3,7 @@ package netguard
 import (
 	"context"
 	"crypto/tls"
+	"crypto/x509"
 	"errors"
 	"fmt"
 	"net"
@@ -48,6 +49,13 @@ func ValidatePublicURL(ctx context.Context, raw string, requireHTTPS bool) (*url
 // rejects private/special-use destinations at dial time, and validates every
 // redirect before following it.
 func NewPublicHTTPClient(timeout time.Duration, requireHTTPS bool) *http.Client {
+	return NewPublicHTTPClientWithRootCAs(timeout, requireHTTPS, nil)
+}
+
+// NewPublicHTTPClientWithRootCAs creates the same guarded client while using
+// the supplied trust pool for protocols whose standards define additional
+// public roots beyond the host operating system's CA bundle.
+func NewPublicHTTPClientWithRootCAs(timeout time.Duration, requireHTTPS bool, roots *x509.CertPool) *http.Client {
 	if timeout <= 0 {
 		timeout = 30 * time.Second
 	}
@@ -60,6 +68,7 @@ func NewPublicHTTPClient(timeout time.Duration, requireHTTPS bool) *http.Client
 		ExpectContinueTimeout: time.Second,
 		TLSClientConfig: &tls.Config{
 			MinVersion: tls.VersionTLS12,
+			RootCAs:    roots,
 		},
 	}
 	return &http.Client{