Nezha's authenticated DDNS webhook configuration allows blind SSRF from the dashboard host

@@ -1,14 +1,10 @@
 package model
 
 import (
-	"context"
-	"crypto/tls"
 	"errors"
 	"fmt"
 	"io"
-	"net"
 	"net/http"
-	"net/netip"
 	"net/url"
 	"strings"
 	"time"
@@ -29,35 +25,6 @@ const (
 	NotificationRequestMethodPOST
 )
 
-var errNotificationURLNotAllowed = errors.New("notification URL target is not allowed")
-
-var notificationBlockedCIDRs = mustParseNotificationCIDRs([]string{
-	"0.0.0.0/8",
-	"10.0.0.0/8",
-	"100.64.0.0/10",
-	"127.0.0.0/8",
-	"169.254.0.0/16",
-	"172.16.0.0/12",
-	"192.0.0.0/24",
-	"192.0.2.0/24",
-	"192.168.0.0/16",
-	"198.18.0.0/15",
-	"198.51.100.0/24",
-	"203.0.113.0/24",
-	"224.0.0.0/4",
-	"240.0.0.0/4",
-	"::/128",
-	"::1/128",
-	"::ffff:0:0/96",
-	"64:ff9b::/96",
-	"100::/64",
-	"2001::/23",
-	"2001:db8::/32",
-	"fc00::/7",
-	"fe80::/10",
-	"ff00::/8",
-})
-
 type NotificationServerBundle struct {
 	Notification *Notification
 	Server       *Server
@@ -191,105 +158,13 @@ func (ns *NotificationServerBundle) Send(message string) error {
 	return nil
 }
 
-func newNotificationHTTPClient(rawURL string, verifyTLS bool) (*http.Client, error) {
-	parsedURL, ip, err := resolveNotificationTarget(rawURL)
-	if err != nil {
-		return nil, err
-	}
-
-	port := parsedURL.Port()
-	if port == "" {
-		if parsedURL.Scheme == "https" {
-			port = "443"
-		} else {
-			port = "80"
-		}
-	}
-	targetAddress := net.JoinHostPort(ip.String(), port)
-	dialer := &net.Dialer{}
-
-	return &http.Client{
-		Transport: &http.Transport{
-			DialContext: func(ctx context.Context, network, address string) (net.Conn, error) {
-				return dialer.DialContext(ctx, network, targetAddress)
-			},
-			TLSClientConfig: &tls.Config{InsecureSkipVerify: !verifyTLS, ServerName: parsedURL.Hostname()},
-		},
-		CheckRedirect: func(req *http.Request, via []*http.Request) error {
-			return http.ErrUseLastResponse
-		},
-		Timeout: time.Minute * 10,
-	}, nil
-}
-
 func notificationResponseError(resp *http.Response) error {
 	_, _ = io.CopyN(io.Discard, resp.Body, 4096)
 	return fmt.Errorf("%d@%s", resp.StatusCode, resp.Status)
 }
 
-func resolveNotificationTarget(rawURL string) (*url.URL, net.IP, error) {
-	parsedURL, err := url.Parse(rawURL)
-	if err != nil {
-		return nil, nil, err
-	}
-	if parsedURL.Scheme != "http" && parsedURL.Scheme != "https" {
-		return nil, nil, errNotificationURLNotAllowed
-	}
-
-	host := parsedURL.Hostname()
-	if host == "" {
-		return nil, nil, errNotificationURLNotAllowed
-	}
-	if ip := net.ParseIP(host); ip != nil {
-		if !notificationIPAllowed(ip) {
-			return nil, nil, errNotificationURLNotAllowed
-		}
-		return parsedURL, ip, nil
-	}
-
-	ips, err := net.LookupIP(host)
-	if err != nil {
-		return nil, nil, err
-	}
-	if len(ips) == 0 {
-		return nil, nil, errNotificationURLNotAllowed
-	}
-	for _, ip := range ips {
-		if !notificationIPAllowed(ip) {
-			return nil, nil, errNotificationURLNotAllowed
-		}
-	}
-
-	return parsedURL, ips[0], nil
-}
-
-func notificationIPAllowed(ip net.IP) bool {
-	parsedIP, ok := netipFromIP(ip)
-	if !ok {
-		return false
-	}
-	for _, cidr := range notificationBlockedCIDRs {
-		if cidr.Contains(parsedIP) {
-			return false
-		}
-	}
-	return parsedIP.IsGlobalUnicast()
-}
-
-func netipFromIP(ip net.IP) (netip.Addr, bool) {
-	parsedIP, ok := netip.AddrFromSlice(ip)
-	if !ok {
-		return netip.Addr{}, false
-	}
-	return parsedIP.Unmap(), true
-}
-
-func mustParseNotificationCIDRs(cidrs []string) []netip.Prefix {
-	prefixes := make([]netip.Prefix, 0, len(cidrs))
-	for _, cidr := range cidrs {
-		prefixes = append(prefixes, netip.MustParsePrefix(cidr))
-	}
-	return prefixes
+func newNotificationHTTPClient(rawURL string, verifyTLS bool) (*http.Client, error) {
+	return utils.NewRestrictedHTTPClient(rawURL, !verifyTLS)
 }
 
 // replaceParamInString 替换字符串中的占位符

@@ -6,6 +6,8 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/nezhahq/nezha/pkg/utils"
 )
 
 var (
@@ -307,7 +309,7 @@ func TestNotificationTargetRejectsBlockedRanges(t *testing.T) {
 
 	for _, rawURL := range cases {
 		t.Run(rawURL, func(t *testing.T) {
-			if _, _, err := resolveNotificationTarget(rawURL); err == nil {
+			if _, _, err := utils.ResolveAllowedHTTPURL(rawURL); err == nil {
 				t.Fatalf("expected %s to be rejected", rawURL)
 			}
 		})
@@ -323,7 +325,7 @@ func TestNotificationTargetAllowsPublicAddresses(t *testing.T) {
 
 	for _, rawURL := range cases {
 		t.Run(rawURL, func(t *testing.T) {
-			parsedURL, _, err := resolveNotificationTarget(rawURL)
+			parsedURL, _, err := utils.ResolveAllowedHTTPURL(rawURL)
 			if err != nil {
 				t.Fatalf("expected %s to be allowed, got %v", rawURL, err)
 			}
@@ -334,31 +336,36 @@ func TestNotificationTargetAllowsPublicAddresses(t *testing.T) {
 	}
 }
 
-func TestNotificationHTTPClientPreservesTLSServerName(t *testing.T) {
-	client, err := newNotificationHTTPClient("https://1.1.1.1/webhook", true)
-	if err != nil {
-		t.Fatalf("expected public HTTPS URL to create client: %v", err)
-	}
-	transport, ok := client.Transport.(*http.Transport)
-	if !ok {
-		t.Fatalf("expected http.Transport, got %T", client.Transport)
-	}
-	if transport.TLSClientConfig == nil || transport.TLSClientConfig.ServerName != "1.1.1.1" {
-		t.Fatalf("expected TLS ServerName 1.1.1.1, got %#v", transport.TLSClientConfig)
-	}
-}
-
-func TestNotificationHTTPClientRejectsRedirects(t *testing.T) {
-	client, err := newNotificationHTTPClient("https://1.1.1.1/webhook", true)
-	if err != nil {
-		t.Fatalf("expected client construction: %v", err)
+func TestNotificationHTTPClientInvertsVerifyTLSFlag(t *testing.T) {
+	// newNotificationHTTPClient takes verifyTLS, utils.NewRestrictedHTTPClient
+	// takes skipVerifyTLS. The wrapper must invert the boolean; if a future
+	// refactor drops the negation, TLS verification silently turns off.
+	// SNI / redirect / IP-pinning are covered by pkg/utils/http_test.go.
+	cases := []struct {
+		name             string
+		verifyTLS        bool
+		wantSkipVerifyOn bool
+	}{
+		{"verifyTLS_true_means_skipVerify_false", true, false},
+		{"verifyTLS_false_means_skipVerify_true", false, true},
 	}
-	req, err := http.NewRequest(http.MethodGet, "https://1.1.1.1/start", nil)
-	if err != nil {
-		t.Fatalf("new request: %v", err)
-	}
-	via := []*http.Request{req}
-	if err := client.CheckRedirect(req, via); err != http.ErrUseLastResponse {
-		t.Fatalf("expected ErrUseLastResponse, got %v", err)
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			client, err := newNotificationHTTPClient("https://1.1.1.1/webhook", tc.verifyTLS)
+			if err != nil {
+				t.Fatalf("expected client construction: %v", err)
+			}
+			transport, ok := client.Transport.(*http.Transport)
+			if !ok {
+				t.Fatalf("expected *http.Transport, got %T", client.Transport)
+			}
+			if transport.TLSClientConfig == nil {
+				t.Fatalf("expected TLSClientConfig to be set")
+			}
+			if got := transport.TLSClientConfig.InsecureSkipVerify; got != tc.wantSkipVerifyOn {
+				t.Fatalf("verifyTLS=%v: expected InsecureSkipVerify=%v, got %v",
+					tc.verifyTLS, tc.wantSkipVerifyOn, got)
+			}
+		})
 	}
 }

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io"
 	"net/http"
 	"net/url"
 	"strings"
@@ -57,13 +58,19 @@ func (provider *Provider) SetRecords(ctx context.Context, zone string,
 			provider.ipAddr = rr.Data
 			provider.domain = fmt.Sprintf("%s.%s", rr.Name, strings.TrimSuffix(zone, "."))
 
-			req, err := provider.prepareRequest(ctx)
+			// WebhookURL is attacker-controlled (GHSA-6x26-5727-rrm9); the request and
+			// the client are paired so URL validation and DialContext pinning are driven
+			// by a single DNS resolution. Do not swap the client for utils.HttpClient.
+			req, client, err := provider.prepareRequest(ctx)
 			if err != nil {
 				return nil, fmt.Errorf("failed to update a domain: %s. Cause by: %v", provider.domain, err)
 			}
-			if _, err := utils.HttpClient.Do(req); err != nil {
+			resp, err := client.Do(req)
+			if err != nil {
 				return nil, fmt.Errorf("failed to update a domain: %s. Cause by: %v", provider.domain, err)
 			}
+			_, _ = io.Copy(io.Discard, resp.Body)
+			resp.Body.Close()
 		default:
 			return nil, fmt.Errorf("unsupported record type: %T", rec)
 		}
@@ -72,26 +79,32 @@ func (provider *Provider) SetRecords(ctx context.Context, zone string,
 	return recs, nil
 }
 
-func (provider *Provider) prepareRequest(ctx context.Context) (*http.Request, error) {
+func (provider *Provider) prepareRequest(ctx context.Context) (*http.Request, *http.Client, error) {
 	u, err := provider.reqUrl()
 	if err != nil {
-		return nil, err
+		return nil, nil, err
+	}
+	// Single SSRF check + dial pin; the returned client must be used by callers
+	// so the dialer's pinned IP and the validated URL stay in sync.
+	client, err := utils.NewRestrictedHTTPClient(u.String(), false)
+	if err != nil {
+		return nil, nil, err
 	}
 
 	body, err := provider.reqBody()
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	headers, err := utils.GjsonIter(
 		provider.formatWebhookString(provider.DDNSProfile.WebhookHeaders))
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	req, err := http.NewRequestWithContext(ctx, requestTypes[provider.DDNSProfile.WebhookMethod], u.String(), strings.NewReader(body))
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	provider.setContentType(req)
@@ -100,7 +113,7 @@ func (provider *Provider) prepareRequest(ctx context.Context) (*http.Request, er
 		req.Header.Set(k, v)
 	}
 
-	return req, nil
+	return req, client, nil
 }
 
 func (provider *Provider) setContentType(req *http.Request) {

@@ -2,6 +2,7 @@ package webhook
 
 import (
 	"context"
+	"strings"
 	"testing"
 
 	"github.com/nezhahq/nezha/model"
@@ -44,7 +45,7 @@ func execCase(t *testing.T, item testSt) {
 		t.Fatalf("Expected %s, but got %s", item.expectBody, reqBody)
 	}
 
-	req, err := pw.prepareRequest(context.Background())
+	req, _, err := pw.prepareRequest(context.Background())
 	if err != nil {
 		t.Fatalf("Error: %s", err)
 	}
@@ -69,11 +70,11 @@ func TestWebhookRequest(t *testing.T) {
 				Domains:        []string{"www.example.com"},
 				MaxRetries:     1,
 				EnableIPv4:     &ipv4,
-				WebhookURL:     "http://ddns.example.com/?ip=#ip#",
+				WebhookURL:     "http://1.1.1.1/?ip=#ip#",
 				WebhookMethod:  methodGET,
 				WebhookHeaders: `{"ip":"#ip#","record":"#record#"}`,
 			},
-			expectURL:         "http://ddns.example.com/?ip=1.1.1.1",
+			expectURL:         "http://1.1.1.1/?ip=1.1.1.1",
 			expectContentType: "",
 			expectHeader: map[string]string{
 				"ip":     "1.1.1.1",
@@ -85,12 +86,12 @@ func TestWebhookRequest(t *testing.T) {
 				Domains:            []string{"www.example.com"},
 				MaxRetries:         1,
 				EnableIPv4:         &ipv4,
-				WebhookURL:         "http://ddns.example.com/api",
+				WebhookURL:         "http://1.1.1.1/api",
 				WebhookMethod:      methodPOST,
 				WebhookRequestType: requestTypeJSON,
 				WebhookRequestBody: `{"ip":"#ip#","record":"#record#"}`,
 			},
-			expectURL:         "http://ddns.example.com/api",
+			expectURL:         "http://1.1.1.1/api",
 			expectContentType: reqTypeJSON,
 			expectBody:        `{"ip":"1.1.1.1","record":"A"}`,
 		},
@@ -99,12 +100,12 @@ func TestWebhookRequest(t *testing.T) {
 				Domains:            []string{"www.example.com"},
 				MaxRetries:         1,
 				EnableIPv4:         &ipv4,
-				WebhookURL:         "http://ddns.example.com/api",
+				WebhookURL:         "http://1.1.1.1/api",
 				WebhookMethod:      methodPOST,
 				WebhookRequestType: requestTypeForm,
 				WebhookRequestBody: `{"ip":"#ip#","record":"#record#"}`,
 			},
-			expectURL:         "http://ddns.example.com/api",
+			expectURL:         "http://1.1.1.1/api",
 			expectContentType: reqTypeForm,
 			expectBody:        "ip=1.1.1.1&record=A",
 		},
@@ -114,3 +115,58 @@ func TestWebhookRequest(t *testing.T) {
 		execCase(t, c)
 	}
 }
+
+func TestWebhookTargetRejectsBlockedRanges(t *testing.T) {
+	cases := []string{
+		"http://0.0.0.0/",
+		"http://10.1.2.3/",
+		"http://100.64.0.1/",
+		"http://127.0.0.1/",
+		"http://127.255.255.254/",
+		"http://169.254.169.254/",
+		"http://172.16.0.1/",
+		"http://192.0.0.1/",
+		"http://192.0.2.1/",
+		"http://192.168.1.1/",
+		"http://198.18.0.1/",
+		"http://198.51.100.1/",
+		"http://203.0.113.1/",
+		"http://224.0.0.1/",
+		"http://240.0.0.1/",
+		"http://[::]/",
+		"http://[::1]/",
+		"http://[::ffff:127.0.0.1]/",
+		"http://[64:ff9b::1]/",
+		"http://[100::1]/",
+		"http://[2001:db8::1]/",
+		"http://[fc00::1]/",
+		"http://[fe80::1]/",
+		"http://[ff00::1]/",
+		"ftp://example.com/",
+		"file:///etc/passwd",
+		"http:///path",
+	}
+
+	for _, rawURL := range cases {
+		t.Run(rawURL, func(t *testing.T) {
+			provider := Provider{DDNSProfile: &model.DDNSProfile{
+				Domains:        []string{"www.example.com"},
+				WebhookURL:     rawURL,
+				WebhookMethod:  methodGET,
+				WebhookHeaders: `{}`,
+			}}
+			provider.ipAddr = "1.1.1.1"
+			provider.domain = provider.DDNSProfile.Domains[0]
+			provider.ipType = "ipv4"
+			provider.recordType = "A"
+
+			_, _, err := provider.prepareRequest(context.Background())
+			if err == nil {
+				t.Fatalf("expected %s to be rejected", rawURL)
+			}
+			if !strings.Contains(err.Error(), "not allowed") {
+				t.Fatalf("expected not allowed error, got %q", err.Error())
+			}
+		})
+	}
+}

@@ -1,16 +1,53 @@
 package utils
 
 import (
+	"context"
 	"crypto/tls"
+	"errors"
+	"net"
 	"net/http"
+	"net/netip"
+	"net/url"
 	"time"
 )
 
+// HttpClient / HttpClientSkipTlsVerify must not be used to dispatch
+// requests to user-controlled URLs (SSRF risk, GHSA-6x26-5727-rrm9).
+// For any attacker-controlled URL use NewRestrictedHTTPClient instead.
 var (
 	HttpClientSkipTlsVerify *http.Client
 	HttpClient              *http.Client
 )
 
+var ErrHTTPURLTargetNotAllowed = errors.New("HTTP URL target is not allowed")
+
+var blockedHTTPClientCIDRs = mustParseHTTPClientCIDRs([]string{
+	"0.0.0.0/8",
+	"10.0.0.0/8",
+	"100.64.0.0/10",
+	"127.0.0.0/8",
+	"169.254.0.0/16",
+	"172.16.0.0/12",
+	"192.0.0.0/24",
+	"192.0.2.0/24",
+	"192.168.0.0/16",
+	"198.18.0.0/15",
+	"198.51.100.0/24",
+	"203.0.113.0/24",
+	"224.0.0.0/4",
+	"240.0.0.0/4",
+	"::/128",
+	"::1/128",
+	"::ffff:0:0/96",
+	"64:ff9b::/96",
+	"100::/64",
+	"2001::/23",
+	"2001:db8::/32",
+	"fc00::/7",
+	"fe80::/10",
+	"ff00::/8",
+})
+
 func init() {
 	HttpClientSkipTlsVerify = httpClient(_httpClient{
 		Transport: httpTransport(_httpTransport{
@@ -47,3 +84,106 @@ func httpClient(conf _httpClient) *http.Client {
 		Timeout:   time.Minute * 10,
 	}
 }
+
+func NewRestrictedHTTPClient(rawURL string, skipVerifyTLS bool) (*http.Client, error) {
+	parsedURL, ip, err := ResolveAllowedHTTPURL(rawURL)
+	if err != nil {
+		return nil, err
+	}
+	return buildRestrictedHTTPClient(parsedURL, ip, skipVerifyTLS), nil
+}
+
+// buildRestrictedHTTPClient assembles a client whose DialContext is pinned to
+// the already-vetted IP. Separated from NewRestrictedHTTPClient so tests can
+// exercise the SNI / redirect behavior without relying on live DNS.
+func buildRestrictedHTTPClient(parsedURL *url.URL, ip net.IP, skipVerifyTLS bool) *http.Client {
+	port := parsedURL.Port()
+	if port == "" {
+		if parsedURL.Scheme == "https" {
+			port = "443"
+		} else {
+			port = "80"
+		}
+	}
+	// Pin outbound webhooks to the vetted IP so DNS changes cannot retarget private hosts.
+	targetAddress := net.JoinHostPort(ip.String(), port)
+	dialer := &net.Dialer{}
+
+	return &http.Client{
+		Transport: &http.Transport{
+			DialContext: func(ctx context.Context, network, address string) (net.Conn, error) {
+				return dialer.DialContext(ctx, network, targetAddress)
+			},
+			TLSClientConfig: &tls.Config{InsecureSkipVerify: skipVerifyTLS, ServerName: parsedURL.Hostname()},
+		},
+		CheckRedirect: func(req *http.Request, via []*http.Request) error {
+			return http.ErrUseLastResponse
+		},
+		Timeout: time.Minute * 10,
+	}
+}
+
+func ResolveAllowedHTTPURL(rawURL string) (*url.URL, net.IP, error) {
+	parsedURL, err := url.Parse(rawURL)
+	if err != nil {
+		return nil, nil, err
+	}
+	if parsedURL.Scheme != "http" && parsedURL.Scheme != "https" {
+		return nil, nil, ErrHTTPURLTargetNotAllowed
+	}
+
+	host := parsedURL.Hostname()
+	if host == "" {
+		return nil, nil, ErrHTTPURLTargetNotAllowed
+	}
+	if ip := net.ParseIP(host); ip != nil {
+		if !HTTPURLTargetIPAllowed(ip) {
+			return nil, nil, ErrHTTPURLTargetNotAllowed
+		}
+		return parsedURL, ip, nil
+	}
+
+	ips, err := net.LookupIP(host)
+	if err != nil {
+		return nil, nil, err
+	}
+	if len(ips) == 0 {
+		return nil, nil, ErrHTTPURLTargetNotAllowed
+	}
+	for _, ip := range ips {
+		if !HTTPURLTargetIPAllowed(ip) {
+			return nil, nil, ErrHTTPURLTargetNotAllowed
+		}
+	}
+
+	return parsedURL, ips[0], nil
+}
+
+func HTTPURLTargetIPAllowed(ip net.IP) bool {
+	parsedIP, ok := netipFromIP(ip)
+	if !ok {
+		return false
+	}
+	for _, cidr := range blockedHTTPClientCIDRs {
+		if cidr.Contains(parsedIP) {
+			return false
+		}
+	}
+	return parsedIP.IsGlobalUnicast()
+}
+
+func netipFromIP(ip net.IP) (netip.Addr, bool) {
+	parsedIP, ok := netip.AddrFromSlice(ip)
+	if !ok {
+		return netip.Addr{}, false
+	}
+	return parsedIP.Unmap(), true
+}
+
+func mustParseHTTPClientCIDRs(cidrs []string) []netip.Prefix {
+	prefixes := make([]netip.Prefix, 0, len(cidrs))
+	for _, cidr := range cidrs {
+		prefixes = append(prefixes, netip.MustParsePrefix(cidr))
+	}
+	return prefixes
+}

@@ -0,0 +1,110 @@
+package utils
+
+import (
+	"net"
+	"net/http"
+	"net/url"
+	"testing"
+	"time"
+)
+
+func TestBuildRestrictedHTTPClientPreservesHostnameAsTLSServerName(t *testing.T) {
+	// Construct a hostname URL paired with an arbitrary public IP so we exercise
+	// the SNI preservation path without depending on live DNS in unit tests.
+	parsed, err := url.Parse("https://example.com/webhook")
+	if err != nil {
+		t.Fatalf("parse url: %v", err)
+	}
+	pinnedIP := net.ParseIP("1.1.1.1")
+	if pinnedIP == nil {
+		t.Fatalf("expected valid pinned IP")
+	}
+
+	client := buildRestrictedHTTPClient(parsed, pinnedIP, false)
+	transport, ok := client.Transport.(*http.Transport)
+	if !ok {
+		t.Fatalf("expected *http.Transport, got %T", client.Transport)
+	}
+	if transport.TLSClientConfig == nil {
+		t.Fatalf("expected TLSClientConfig to be set")
+	}
+	// SNI must come from the original URL hostname so the certificate validates
+	// the intended hostname, not the pinned dial IP.
+	if got := transport.TLSClientConfig.ServerName; got != "example.com" {
+		t.Fatalf("expected ServerName example.com, got %q", got)
+	}
+	if transport.TLSClientConfig.ServerName == pinnedIP.String() {
+		t.Fatalf("ServerName must not be the pinned IP, got %q", transport.TLSClientConfig.ServerName)
+	}
+	if transport.TLSClientConfig.InsecureSkipVerify {
+		t.Fatalf("expected verifyTLS path (InsecureSkipVerify=false)")
+	}
+}
+
+func TestBuildRestrictedHTTPClientHonorsSkipVerifyTLS(t *testing.T) {
+	parsed, _ := url.Parse("https://example.com/webhook")
+	client := buildRestrictedHTTPClient(parsed, net.ParseIP("1.1.1.1"), true)
+	transport := client.Transport.(*http.Transport)
+	if !transport.TLSClientConfig.InsecureSkipVerify {
+		t.Fatalf("expected InsecureSkipVerify=true when skipVerifyTLS=true")
+	}
+}
+
+func TestBuildRestrictedHTTPClientRejectsRedirects(t *testing.T) {
+	parsed, _ := url.Parse("https://example.com/start")
+	client := buildRestrictedHTTPClient(parsed, net.ParseIP("1.1.1.1"), false)
+	req, err := http.NewRequest(http.MethodGet, "https://example.com/start", nil)
+	if err != nil {
+		t.Fatalf("new request: %v", err)
+	}
+	if err := client.CheckRedirect(req, []*http.Request{req}); err != http.ErrUseLastResponse {
+		t.Fatalf("expected ErrUseLastResponse, got %v", err)
+	}
+}
+
+// TestBuildRestrictedHTTPClientPinsDialToVettedIP confirms DialContext routes
+// to the pinned IP even when the request URL uses a different hostname,
+// preventing DNS rebinding from retargeting traffic.
+func TestBuildRestrictedHTTPClientPinsDialToVettedIP(t *testing.T) {
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+	defer listener.Close()
+	_, port, err := net.SplitHostPort(listener.Addr().String())
+	if err != nil {
+		t.Fatalf("split host port: %v", err)
+	}
+
+	accepted := make(chan string, 1)
+	go func() {
+		conn, err := listener.Accept()
+		if err != nil {
+			accepted <- ""
+			return
+		}
+		accepted <- conn.LocalAddr().String()
+		conn.Close()
+	}()
+
+	requestURL := "http://example.com:" + port + "/"
+	parsed, _ := url.Parse(requestURL)
+	pinned := net.ParseIP("127.0.0.1")
+	client := buildRestrictedHTTPClient(parsed, pinned, false)
+	client.Timeout = 2 * time.Second
+
+	req, _ := http.NewRequest(http.MethodGet, requestURL, nil)
+	resp, _ := client.Do(req)
+	if resp != nil {
+		resp.Body.Close()
+	}
+
+	select {
+	case addr := <-accepted:
+		if addr == "" {
+			t.Fatalf("listener accept failed")
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatalf("expected dial to reach pinned IP 127.0.0.1:%s, listener did not accept", port)
+	}
+}