diff --git a/acme/api/certificate_test.go b/acme/api/certificate_test.go
index 4ff8fec47..f9e92dc6c 100644
--- a/acme/api/certificate_test.go
+++ b/acme/api/certificate_test.go
@@ -3,7 +3,6 @@ package api
 import (
 	"crypto/rand"
 	"crypto/rsa"
-	"net/http"
 	"testing"
 
 	"github.com/go-acme/lego/v4/platform/tester"
@@ -74,14 +73,14 @@ rzFL1KZfz+HZdnFwFW2T2gVW8L3ii1l9AJDuKzlvjUH3p6bgihVq02sjT8mx+GM2
 `
 
 func TestCertificateService_Get_issuerRelUp(t *testing.T) {
-	apiURL := tester.MockACMEServer().
+	apiURL, client := tester.MockACMEServer().
 		Route("POST /certificate", servermock.RawStringResponse(certResponseMock)).
-		Build(t)
+		BuildHTTPS(t)
 
 	key, err := rsa.GenerateKey(rand.Reader, 2048)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := New(http.DefaultClient, "lego-test", apiURL+"/dir", "", key)
+	core, err := New(client, "lego-test", apiURL+"/dir", "", key)
 	require.NoError(t, err)
 
 	cert, issuer, err := core.Certificates.Get(apiURL+"/certificate", true)
@@ -91,14 +90,14 @@ func TestCertificateService_Get_issuerRelUp(t *testing.T) {
 }
 
 func TestCertificateService_Get_embeddedIssuer(t *testing.T) {
-	apiURL := tester.MockACMEServer().
+	apiURL, client := tester.MockACMEServer().
 		Route("POST /certificate", servermock.RawStringResponse(certResponseMock)).
-		Build(t)
+		BuildHTTPS(t)
 
 	key, err := rsa.GenerateKey(rand.Reader, 2048)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := New(http.DefaultClient, "lego-test", apiURL+"/dir", "", key)
+	core, err := New(client, "lego-test", apiURL+"/dir", "", key)
 	require.NoError(t, err)
 
 	cert, issuer, err := core.Certificates.Get(apiURL+"/certificate", true)
diff --git a/acme/api/internal/nonces/nonce_manager_test.go b/acme/api/internal/nonces/nonce_manager_test.go
index a172a0b69..097e57587 100644
--- a/acme/api/internal/nonces/nonce_manager_test.go
+++ b/acme/api/internal/nonces/nonce_manager_test.go
@@ -8,35 +8,37 @@ import (
 
 	"github.com/go-acme/lego/v4/acme"
 	"github.com/go-acme/lego/v4/acme/api/internal/sender"
-	"github.com/go-acme/lego/v4/platform/tester"
+	"github.com/go-acme/lego/v4/platform/tester/servermock"
 )
 
 func TestNotHoldingLockWhileMakingHTTPRequests(t *testing.T) {
-	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
-		time.Sleep(250 * time.Millisecond)
-		w.Header().Set("Replay-Nonce", "12345")
-		w.Header().Set("Retry-After", "0")
-		err := tester.WriteJSONResponse(w, &acme.Challenge{Type: "http-01", Status: "Valid", URL: "http://example.com/", Token: "token"})
-		if err != nil {
-			http.Error(w, err.Error(), http.StatusInternalServerError)
-			return
-		}
-	}))
-	t.Cleanup(server.Close)
+	manager, _ := servermock.NewBuilder(
+		func(server *httptest.Server) (*Manager, error) {
+			doer := sender.NewDoer(server.Client(), "lego-test")
+
+			return NewManager(doer, server.URL), nil
+		}).
+		Route("HEAD /", http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+			time.Sleep(250 * time.Millisecond)
+
+			rw.Header().Set("Replay-Nonce", "12345")
+			rw.Header().Set("Retry-After", "0")
+
+			servermock.JSONEncode(&acme.Challenge{Type: "http-01", Status: "Valid", URL: "https://example.com/", Token: "token"}).ServeHTTP(rw, req)
+		})).
+		BuildHTTPS(t)
 
-	doer := sender.NewDoer(http.DefaultClient, "lego-test")
-	j := NewManager(doer, server.URL)
 	ch := make(chan bool)
 	resultCh := make(chan bool)
 	go func() {
-		_, errN := j.Nonce()
+		_, errN := manager.Nonce()
 		if errN != nil {
 			t.Log(errN)
 		}
 		ch <- true
 	}()
 	go func() {
-		_, errN := j.Nonce()
+		_, errN := manager.Nonce()
 		if errN != nil {
 			t.Log(errN)
 		}
diff --git a/acme/api/internal/secure/jws_test.go b/acme/api/internal/secure/jws_test.go
index 2e625f24f..07761fb5e 100644
--- a/acme/api/internal/secure/jws_test.go
+++ b/acme/api/internal/secure/jws_test.go
@@ -9,35 +9,37 @@ import (
 	"github.com/go-acme/lego/v4/acme"
 	"github.com/go-acme/lego/v4/acme/api/internal/nonces"
 	"github.com/go-acme/lego/v4/acme/api/internal/sender"
-	"github.com/go-acme/lego/v4/platform/tester"
+	"github.com/go-acme/lego/v4/platform/tester/servermock"
 )
 
 func TestNotHoldingLockWhileMakingHTTPRequests(t *testing.T) {
-	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
-		time.Sleep(250 * time.Millisecond)
-		w.Header().Set("Replay-Nonce", "12345")
-		w.Header().Set("Retry-After", "0")
-		err := tester.WriteJSONResponse(w, &acme.Challenge{Type: "http-01", Status: "Valid", URL: "http://example.com/", Token: "token"})
-		if err != nil {
-			http.Error(w, err.Error(), http.StatusInternalServerError)
-			return
-		}
-	}))
-	t.Cleanup(server.Close)
+	manager, _ := servermock.NewBuilder(
+		func(server *httptest.Server) (*nonces.Manager, error) {
+			doer := sender.NewDoer(server.Client(), "lego-test")
+
+			return nonces.NewManager(doer, server.URL), nil
+		}).
+		Route("HEAD /", http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+			time.Sleep(250 * time.Millisecond)
+
+			rw.Header().Set("Replay-Nonce", "12345")
+			rw.Header().Set("Retry-After", "0")
+
+			servermock.JSONEncode(&acme.Challenge{Type: "http-01", Status: "Valid", URL: "https://example.com/", Token: "token"}).ServeHTTP(rw, req)
+		})).
+		BuildHTTPS(t)
 
-	doer := sender.NewDoer(http.DefaultClient, "lego-test")
-	j := nonces.NewManager(doer, server.URL)
 	ch := make(chan bool)
 	resultCh := make(chan bool)
 	go func() {
-		_, errN := j.Nonce()
+		_, errN := manager.Nonce()
 		if errN != nil {
 			t.Log(errN)
 		}
 		ch <- true
 	}()
 	go func() {
-		_, errN := j.Nonce()
+		_, errN := manager.Nonce()
 		if errN != nil {
 			t.Log(errN)
 		}
diff --git a/acme/api/internal/sender/sender.go b/acme/api/internal/sender/sender.go
index 2e1bbec8d..e70a4bd9c 100644
--- a/acme/api/internal/sender/sender.go
+++ b/acme/api/internal/sender/sender.go
@@ -27,6 +27,8 @@ type Doer struct {
 
 // NewDoer Creates a new Doer.
 func NewDoer(client *http.Client, userAgent string) *Doer {
+	client.Transport = newHTTPSOnly(client)
+
 	return &Doer{
 		httpClient: client,
 		userAgent:  userAgent,
@@ -150,3 +152,28 @@ func checkError(req *http.Request, resp *http.Response) error {
 	}
 	return nil
 }
+
+type httpsOnly struct {
+	rt http.RoundTripper
+}
+
+func newHTTPSOnly(client *http.Client) *httpsOnly {
+	if client.Transport == nil {
+		return &httpsOnly{rt: http.DefaultTransport}
+	}
+
+	return &httpsOnly{rt: client.Transport}
+}
+
+// RoundTrip ensure HTTPS is used.
+// Each ACME function is accomplished by the client sending a sequence of HTTPS requests to the server [RFC2818],
+// carrying JSON messages [RFC8259].
+// Use of HTTPS is REQUIRED.
+// https://datatracker.ietf.org/doc/html/rfc8555#section-6.1
+func (r *httpsOnly) RoundTrip(req *http.Request) (*http.Response, error) {
+	if req.URL.Scheme != "https" {
+		return nil, fmt.Errorf("HTTPS is required: %s", req.URL)
+	}
+
+	return r.rt.RoundTrip(req)
+}
diff --git a/acme/api/internal/sender/sender_test.go b/acme/api/internal/sender/sender_test.go
index 2fd43c878..69bb8ae4e 100644
--- a/acme/api/internal/sender/sender_test.go
+++ b/acme/api/internal/sender/sender_test.go
@@ -12,13 +12,13 @@ import (
 
 func TestDo_UserAgentOnAllHTTPMethod(t *testing.T) {
 	var ua, method string
-	server := httptest.NewServer(http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
+	server := httptest.NewTLSServer(http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
 		ua = r.Header.Get("User-Agent")
 		method = r.Method
 	}))
 	t.Cleanup(server.Close)
 
-	doer := NewDoer(http.DefaultClient, "")
+	doer := NewDoer(server.Client(), "")
 
 	testCases := []struct {
 		method string
@@ -65,3 +65,13 @@ func TestDo_CustomUserAgent(t *testing.T) {
 	}
 	assert.Len(t, strings.Split(ua, " "), 5)
 }
+
+func TestDo_failWithHTTP(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(_ http.ResponseWriter, _ *http.Request) {}))
+	t.Cleanup(server.Close)
+
+	sender := NewDoer(server.Client(), "test")
+
+	_, err := sender.Post(server.URL, strings.NewReader("data"), "text/plain", nil)
+	require.ErrorContains(t, err, "HTTPS is required: http://")
+}
diff --git a/acme/api/order_test.go b/acme/api/order_test.go
index ebbb7d7ce..da98f4714 100644
--- a/acme/api/order_test.go
+++ b/acme/api/order_test.go
@@ -22,7 +22,7 @@ func TestOrderService_NewWithOptions(t *testing.T) {
 	privateKey, errK := rsa.GenerateKey(rand.Reader, 1024)
 	require.NoError(t, errK, "Could not generate test key")
 
-	apiURL := tester.MockACMEServer().
+	apiURL, client := tester.MockACMEServer().
 		Route("POST /newOrder",
 			http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
 				body, err := readSignedBody(req, privateKey)
@@ -52,9 +52,9 @@ func TestOrderService_NewWithOptions(t *testing.T) {
 					Replaces:       order.Replaces,
 				}).ServeHTTP(rw, req)
 			})).
-		Build(t)
+		BuildHTTPS(t)
 
-	core, err := New(http.DefaultClient, "lego-test", apiURL+"/dir", "", privateKey)
+	core, err := New(client, "lego-test", apiURL+"/dir", "", privateKey)
 	require.NoError(t, err)
 
 	testCases := []struct {
diff --git a/certificate/certificates_test.go b/certificate/certificates_test.go
index ddccd3541..a016e3b6b 100644
--- a/certificate/certificates_test.go
+++ b/certificate/certificates_test.go
@@ -175,14 +175,14 @@ Ob8VZRzI9neWagqNdwvYkQsEjgfbKbYK7p2CNTUQ
 `
 
 func Test_checkResponse(t *testing.T) {
-	apiURL := tester.MockACMEServer().
+	apiURL, client := tester.MockACMEServer().
 		Route("POST /certificate", servermock.RawStringResponse(certResponseMock)).
-		Build(t)
+		BuildHTTPS(t)
 
 	key, err := rsa.GenerateKey(rand.Reader, 2048)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", key)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", key)
 	require.NoError(t, err)
 
 	certifier := NewCertifier(core, &resolverMock{}, CertifierOptions{KeyType: certcrypto.RSA2048})
@@ -209,14 +209,14 @@ func Test_checkResponse(t *testing.T) {
 }
 
 func Test_checkResponse_issuerRelUp(t *testing.T) {
-	apiURL := tester.MockACMEServer().
+	apiURL, client := tester.MockACMEServer().
 		Route("POST /certificate", servermock.RawStringResponse(certResponseMock)).
-		Build(t)
+		BuildHTTPS(t)
 
 	key, err := rsa.GenerateKey(rand.Reader, 2048)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", key)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", key)
 	require.NoError(t, err)
 
 	certifier := NewCertifier(core, &resolverMock{}, CertifierOptions{KeyType: certcrypto.RSA2048})
@@ -243,14 +243,14 @@ func Test_checkResponse_issuerRelUp(t *testing.T) {
 }
 
 func Test_checkResponse_no_bundle(t *testing.T) {
-	apiURL := tester.MockACMEServer().
+	apiURL, client := tester.MockACMEServer().
 		Route("POST /certificate", servermock.RawStringResponse(certResponseMock)).
-		Build(t)
+		BuildHTTPS(t)
 
 	key, err := rsa.GenerateKey(rand.Reader, 2048)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", key)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", key)
 	require.NoError(t, err)
 
 	certifier := NewCertifier(core, &resolverMock{}, CertifierOptions{KeyType: certcrypto.RSA2048})
@@ -277,21 +277,21 @@ func Test_checkResponse_no_bundle(t *testing.T) {
 }
 
 func Test_checkResponse_alternate(t *testing.T) {
-	apiURL := tester.MockACMEServer().
+	apiURL, client := tester.MockACMEServer().
 		Route("POST /certificate",
 			http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
 				rw.Header().Add("Link",
-					fmt.Sprintf(`<http://%s/certificate/1>;title="foo";rel="alternate"`, req.Context().Value(http.LocalAddrContextKey)))
+					fmt.Sprintf(`<https://%s/certificate/1>;title="foo";rel="alternate"`, req.Context().Value(http.LocalAddrContextKey)))
 
 				servermock.RawStringResponse(certResponseMock).ServeHTTP(rw, req)
 			})).
 		Route("/certificate/1", servermock.RawStringResponse(certResponseMock2)).
-		Build(t)
+		BuildHTTPS(t)
 
 	key, err := rsa.GenerateKey(rand.Reader, 2048)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", key)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", key)
 	require.NoError(t, err)
 
 	certifier := NewCertifier(core, &resolverMock{}, CertifierOptions{KeyType: certcrypto.RSA2048})
@@ -321,14 +321,14 @@ func Test_checkResponse_alternate(t *testing.T) {
 }
 
 func Test_Get(t *testing.T) {
-	apiURL := tester.MockACMEServer().
+	apiURL, client := tester.MockACMEServer().
 		Route("POST /acme/cert/test-cert", servermock.RawStringResponse(certResponseMock)).
-		Build(t)
+		BuildHTTPS(t)
 
 	key, err := rsa.GenerateKey(rand.Reader, 2048)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", key)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", key)
 	require.NoError(t, err)
 
 	certifier := NewCertifier(core, &resolverMock{}, CertifierOptions{KeyType: certcrypto.RSA2048})
diff --git a/certificate/renewal_test.go b/certificate/renewal_test.go
index 9cf478600..7280f839f 100644
--- a/certificate/renewal_test.go
+++ b/certificate/renewal_test.go
@@ -43,7 +43,7 @@ func TestCertifier_GetRenewalInfo(t *testing.T) {
 	require.NoError(t, err)
 
 	// Test with a fake API.
-	apiURL := tester.MockACMEServer().
+	apiURL, client := tester.MockACMEServer().
 		Route("GET /renewalInfo/"+ariLeafCertID,
 			servermock.RawStringResponse(`{
 				"suggestedWindow": {
@@ -55,12 +55,12 @@ func TestCertifier_GetRenewalInfo(t *testing.T) {
 		}`).
 				WithHeader("Content-Type", "application/json").
 				WithHeader("Retry-After", "21600")).
-		Build(t)
+		BuildHTTPS(t)
 
 	key, err := rsa.GenerateKey(rand.Reader, 2048)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", key)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", key)
 	require.NoError(t, err)
 
 	certifier := NewCertifier(core, &resolverMock{}, CertifierOptions{KeyType: certcrypto.RSA2048})
@@ -82,24 +82,23 @@ func TestCertifier_GetRenewalInfo_errors(t *testing.T) {
 	require.NoError(t, err, "Could not generate test key")
 
 	testCases := []struct {
-		desc       string
-		httpClient *http.Client
-		request    RenewalInfoRequest
-		handler    http.HandlerFunc
+		desc    string
+		timeout time.Duration
+		request RenewalInfoRequest
+		handler http.HandlerFunc
 	}{
 		{
-			desc:       "API timeout",
-			httpClient: &http.Client{Timeout: 500 * time.Millisecond}, // HTTP client that times out after 500ms.
-			request:    RenewalInfoRequest{leaf},
+			desc:    "API timeout",
+			timeout: 500 * time.Millisecond, // HTTP client that times out after 500ms.
+			request: RenewalInfoRequest{leaf},
 			handler: func(w http.ResponseWriter, r *http.Request) {
 				// API that takes 2ms to respond.
 				time.Sleep(2 * time.Millisecond)
 			},
 		},
 		{
-			desc:       "API error",
-			httpClient: http.DefaultClient,
-			request:    RenewalInfoRequest{leaf},
+			desc:    "API error",
+			request: RenewalInfoRequest{leaf},
 			handler: func(w http.ResponseWriter, r *http.Request) {
 				// API that responds with error instead of renewal info.
 				http.Error(w, http.StatusText(http.StatusBadRequest), http.StatusBadRequest)
@@ -111,11 +110,15 @@ func TestCertifier_GetRenewalInfo_errors(t *testing.T) {
 		t.Run(test.desc, func(t *testing.T) {
 			t.Parallel()
 
-			apiURL := tester.MockACMEServer().
+			apiURL, client := tester.MockACMEServer().
 				Route("GET /renewalInfo/"+ariLeafCertID, test.handler).
-				Build(t)
+				BuildHTTPS(t)
 
-			core, err := api.New(test.httpClient, "lego-test", apiURL+"/dir", "", key)
+			if test.timeout != 0 {
+				client.Timeout = test.timeout
+			}
+
+			core, err := api.New(client, "lego-test", apiURL+"/dir", "", key)
 			require.NoError(t, err)
 
 			certifier := NewCertifier(core, &resolverMock{}, CertifierOptions{KeyType: certcrypto.RSA2048})
diff --git a/challenge/dns01/dns_challenge_test.go b/challenge/dns01/dns_challenge_test.go
index bb0564a81..6bc88ca23 100644
--- a/challenge/dns01/dns_challenge_test.go
+++ b/challenge/dns01/dns_challenge_test.go
@@ -4,7 +4,6 @@ import (
 	"crypto/rand"
 	"crypto/rsa"
 	"errors"
-	"net/http"
 	"testing"
 	"time"
 
@@ -32,12 +31,12 @@ func (p *providerTimeoutMock) CleanUp(domain, token, keyAuth string) error { ret
 func (p *providerTimeoutMock) Timeout() (time.Duration, time.Duration)     { return p.timeout, p.interval }
 
 func TestChallenge_PreSolve(t *testing.T) {
-	apiURL := tester.MockACMEServer().Build(t)
+	apiURL, client := tester.MockACMEServer().BuildHTTPS(t)
 
 	privateKey, err := rsa.GenerateKey(rand.Reader, 1024)
 	require.NoError(t, err)
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", privateKey)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", privateKey)
 	require.NoError(t, err)
 
 	testCases := []struct {
@@ -114,12 +113,12 @@ func TestChallenge_PreSolve(t *testing.T) {
 }
 
 func TestChallenge_Solve(t *testing.T) {
-	apiURL := tester.MockACMEServer().Build(t)
+	apiURL, client := tester.MockACMEServer().BuildHTTPS(t)
 
 	privateKey, err := rsa.GenerateKey(rand.Reader, 1024)
 	require.NoError(t, err)
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", privateKey)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", privateKey)
 	require.NoError(t, err)
 
 	testCases := []struct {
@@ -201,12 +200,12 @@ func TestChallenge_Solve(t *testing.T) {
 }
 
 func TestChallenge_CleanUp(t *testing.T) {
-	apiURL := tester.MockACMEServer().Build(t)
+	apiURL, client := tester.MockACMEServer().BuildHTTPS(t)
 
 	privateKey, err := rsa.GenerateKey(rand.Reader, 1024)
 	require.NoError(t, err)
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", privateKey)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", privateKey)
 	require.NoError(t, err)
 
 	testCases := []struct {
diff --git a/challenge/http01/http_challenge_test.go b/challenge/http01/http_challenge_test.go
index a5e9cc62f..4693a7f47 100644
--- a/challenge/http01/http_challenge_test.go
+++ b/challenge/http01/http_challenge_test.go
@@ -67,7 +67,7 @@ func TestProviderServer_GetAddress(t *testing.T) {
 }
 
 func TestChallenge(t *testing.T) {
-	apiURL := tester.MockACMEServer().Build(t)
+	apiURL, client := tester.MockACMEServer().BuildHTTPS(t)
 
 	providerServer := NewProviderServer("", "23457")
 
@@ -100,7 +100,7 @@ func TestChallenge(t *testing.T) {
 	privateKey, err := rsa.GenerateKey(rand.Reader, 1024)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", privateKey)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", privateKey)
 	require.NoError(t, err)
 
 	solver := NewChallenge(core, validate, providerServer)
@@ -123,7 +123,7 @@ func TestChallengeUnix(t *testing.T) {
 		t.Skip("only for UNIX systems")
 	}
 
-	apiURL := tester.MockACMEServer().Build(t)
+	apiURL, httpsClient := tester.MockACMEServer().BuildHTTPS(t)
 
 	dir := t.TempDir()
 	t.Cleanup(func() { _ = os.RemoveAll(dir) })
@@ -169,7 +169,7 @@ func TestChallengeUnix(t *testing.T) {
 	privateKey, err := rsa.GenerateKey(rand.Reader, 1024)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", privateKey)
+	core, err := api.New(httpsClient, "lego-test", apiURL+"/dir", "", privateKey)
 	require.NoError(t, err)
 
 	solver := NewChallenge(core, validate, providerServer)
@@ -188,12 +188,12 @@ func TestChallengeUnix(t *testing.T) {
 }
 
 func TestChallengeInvalidPort(t *testing.T) {
-	apiURL := tester.MockACMEServer().Build(t)
+	apiURL, client := tester.MockACMEServer().BuildHTTPS(t)
 
 	privateKey, err := rsa.GenerateKey(rand.Reader, 1024)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", privateKey)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", privateKey)
 	require.NoError(t, err)
 
 	validate := func(_ *api.Core, _ string, _ acme.Challenge) error { return nil }
@@ -371,7 +371,7 @@ func TestChallengeWithProxy(t *testing.T) {
 func testServeWithProxy(t *testing.T, header, extra *testProxyHeader, expectError bool) {
 	t.Helper()
 
-	apiURL := tester.MockACMEServer().Build(t)
+	apiURL, client := tester.MockACMEServer().BuildHTTPS(t)
 
 	providerServer := NewProviderServer("localhost", "23457")
 	if header != nil {
@@ -414,7 +414,7 @@ func testServeWithProxy(t *testing.T, header, extra *testProxyHeader, expectErro
 	privateKey, err := rsa.GenerateKey(rand.Reader, 1024)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", privateKey)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", privateKey)
 	require.NoError(t, err)
 
 	solver := NewChallenge(core, validate, providerServer)
diff --git a/challenge/resolver/solver_manager_test.go b/challenge/resolver/solver_manager_test.go
index a0f4630ed..8337463fd 100644
--- a/challenge/resolver/solver_manager_test.go
+++ b/challenge/resolver/solver_manager_test.go
@@ -37,7 +37,7 @@ func TestValidate(t *testing.T) {
 
 	privateKey, _ := rsa.GenerateKey(rand.Reader, 1024)
 
-	apiURL := tester.MockACMEServer().
+	apiURL, client := tester.MockACMEServer().
 		Route("POST /chlg",
 			http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
 				if err := validateNoBody(privateKey, req); err != nil {
@@ -46,7 +46,7 @@ func TestValidate(t *testing.T) {
 				}
 
 				rw.Header().Set("Link",
-					fmt.Sprintf(`<http://%s/my-authz>; rel="up"`, req.Context().Value(http.LocalAddrContextKey)))
+					fmt.Sprintf(`<https://%s/my-authz>; rel="up"`, req.Context().Value(http.LocalAddrContextKey)))
 
 				st := statuses[0]
 				statuses = statuses[1:]
@@ -74,9 +74,9 @@ func TestValidate(t *testing.T) {
 
 				servermock.JSONEncode(authorization).ServeHTTP(rw, req)
 			})).
-		Build(t)
+		BuildHTTPS(t)
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", privateKey)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", privateKey)
 	require.NoError(t, err)
 
 	testCases := []struct {
diff --git a/challenge/tlsalpn01/tls_alpn_challenge_test.go b/challenge/tlsalpn01/tls_alpn_challenge_test.go
index 5e0469d8c..72ccae603 100644
--- a/challenge/tlsalpn01/tls_alpn_challenge_test.go
+++ b/challenge/tlsalpn01/tls_alpn_challenge_test.go
@@ -8,7 +8,6 @@ import (
 	"crypto/tls"
 	"encoding/asn1"
 	"net"
-	"net/http"
 	"testing"
 
 	"github.com/go-acme/lego/v4/acme"
@@ -21,7 +20,7 @@ import (
 )
 
 func TestChallenge(t *testing.T) {
-	apiURL := tester.MockACMEServer().Build(t)
+	apiURL, client := tester.MockACMEServer().BuildHTTPS(t)
 
 	domain := "localhost"
 	port := "24457"
@@ -69,7 +68,7 @@ func TestChallenge(t *testing.T) {
 	privateKey, err := rsa.GenerateKey(rand.Reader, 1024)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", privateKey)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", privateKey)
 	require.NoError(t, err)
 
 	solver := NewChallenge(
@@ -93,12 +92,12 @@ func TestChallenge(t *testing.T) {
 }
 
 func TestChallengeInvalidPort(t *testing.T) {
-	apiURL := tester.MockACMEServer().Build(t)
+	apiURL, client := tester.MockACMEServer().BuildHTTPS(t)
 
 	privateKey, err := rsa.GenerateKey(rand.Reader, 1024)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", privateKey)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", privateKey)
 	require.NoError(t, err)
 
 	solver := NewChallenge(
@@ -123,7 +122,7 @@ func TestChallengeInvalidPort(t *testing.T) {
 }
 
 func TestChallengeIPaddress(t *testing.T) {
-	apiURL := tester.MockACMEServer().Build(t)
+	apiURL, client := tester.MockACMEServer().BuildHTTPS(t)
 
 	domain := "127.0.0.1"
 	port := "24457"
@@ -170,7 +169,7 @@ func TestChallengeIPaddress(t *testing.T) {
 	privateKey, err := rsa.GenerateKey(rand.Reader, 1024)
 	require.NoError(t, err, "Could not generate test key")
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", privateKey)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", privateKey)
 	require.NoError(t, err)
 
 	solver := NewChallenge(
diff --git a/lego/client_test.go b/lego/client_test.go
index 8a18c47c7..4b8a38f5f 100644
--- a/lego/client_test.go
+++ b/lego/client_test.go
@@ -13,7 +13,7 @@ import (
 )
 
 func TestNewClient(t *testing.T) {
-	apiURL := tester.MockACMEServer().Build(t)
+	apiURL, httpsClient := tester.MockACMEServer().BuildHTTPS(t)
 
 	key, err := rsa.GenerateKey(rand.Reader, 1024)
 	require.NoError(t, err, "Could not generate test key")
@@ -26,6 +26,7 @@ func TestNewClient(t *testing.T) {
 
 	config := NewConfig(user)
 	config.CADirURL = apiURL + "/dir"
+	config.HTTPClient = httpsClient
 
 	client, err := NewClient(config)
 	require.NoError(t, err, "Could not create client")
diff --git a/platform/tester/api.go b/platform/tester/api.go
index 4ffb62a50..5a7c52038 100644
--- a/platform/tester/api.go
+++ b/platform/tester/api.go
@@ -17,7 +17,7 @@ func MockACMEServer() *servermock.Builder[string] {
 			return server.URL, nil
 		}).
 		Route("GET /dir", http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
-			serverURL := fmt.Sprintf("http://%s", req.Context().Value(http.LocalAddrContextKey))
+			serverURL := fmt.Sprintf("https://%s", req.Context().Value(http.LocalAddrContextKey))
 
 			servermock.JSONEncode(acme.Directory{
 				NewNonceURL:   serverURL + "/nonce",
diff --git a/platform/tester/servermock/builder.go b/platform/tester/servermock/builder.go
index e3b41e5c3..d62cec14d 100644
--- a/platform/tester/servermock/builder.go
+++ b/platform/tester/servermock/builder.go
@@ -70,3 +70,15 @@ func (b *Builder[T]) Build(t *testing.T) T {
 
 	return client
 }
+
+func (b *Builder[T]) BuildHTTPS(t *testing.T) (T, *http.Client) {
+	t.Helper()
+
+	server := httptest.NewTLSServer(b.mux)
+	t.Cleanup(server.Close)
+
+	client, err := b.clientBuilder(server)
+	require.NoError(t, err)
+
+	return client, server.Client()
+}
diff --git a/registration/registar_test.go b/registration/registar_test.go
index 0b3f88f98..2074b8f0f 100644
--- a/registration/registar_test.go
+++ b/registration/registar_test.go
@@ -16,7 +16,7 @@ import (
 )
 
 func TestRegistrar_ResolveAccountByKey(t *testing.T) {
-	apiURL := tester.MockACMEServer().
+	apiURL, client := tester.MockACMEServer().
 		Route("/account",
 			http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
 				rw.Header().Set("Location",
@@ -24,7 +24,7 @@ func TestRegistrar_ResolveAccountByKey(t *testing.T) {
 
 				servermock.JSONEncode(acme.Account{Status: "valid"}).ServeHTTP(rw, req)
 			})).
-		Build(t)
+		BuildHTTPS(t)
 
 	key, err := rsa.GenerateKey(rand.Reader, 1024)
 	require.NoError(t, err, "Could not generate test key")
@@ -35,7 +35,7 @@ func TestRegistrar_ResolveAccountByKey(t *testing.T) {
 		privatekey: key,
 	}
 
-	core, err := api.New(http.DefaultClient, "lego-test", apiURL+"/dir", "", key)
+	core, err := api.New(client, "lego-test", apiURL+"/dir", "", key)
 	require.NoError(t, err)
 
 	registrar := NewRegistrar(core, user)