client/web: deprecate gorilla/csrfpatrickod/webui-sec-fetch-site

Replace gorilla/csrf with a handler that requires the Sec-Fetch-Site
header to be set to same-origin preventing CSRF attacks.

Ref: https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Sec-Fetch-Site
Ref: https://caniuse.com/mdn-http_headers_sec-fetch-site

Browser support should be now sufficiently broad to minimize
false-positive rejections.

Updates corp#25340

Signed-off-by: Patrick O'Doherty <patrick@tailscale.com>

5 files changed, 104 insertions, 152 deletions

diff --git a/client/web/web.go b/client/web/web.go
index 6eccdadcf..0cbf32c5f 100644
--- a/client/web/web.go
+++ b/client/web/web.go
@@ -6,7 +6,6 @@ package web
 
 import (
 	"context"
-	"crypto/rand"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -14,14 +13,11 @@ import (
 	"log"
 	"net/http"
 	"net/netip"
-	"os"
 	"path"
-	"path/filepath"
 	"strings"
 	"sync"
 	"time"
 
-	"github.com/gorilla/csrf"
 	"tailscale.com/client/local"
 	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/clientupdate"
@@ -205,7 +201,7 @@ func NewServer(opts ServerOpts) (s *Server, err error) {
 
 	var metric string
 	s.apiHandler, metric = s.modeAPIHandler(s.mode)
-	s.apiHandler = s.withCSRF(s.apiHandler)
+	s.apiHandler = s.withSecFetchSite(s.apiHandler)
 
 	// Don't block startup on reporting metric.
 	// Report in separate go routine with 5 second timeout.
@@ -218,23 +214,30 @@ func NewServer(opts ServerOpts) (s *Server, err error) {
 	return s, nil
 }
 
-func (s *Server) withCSRF(h http.Handler) http.Handler {
-	csrfProtect := csrf.Protect(s.csrfKey(), csrf.Secure(false))
-
-	// ref https://github.com/tailscale/tailscale/pull/14822
-	// signal to the CSRF middleware that the request is being served over
-	// plaintext HTTP to skip TLS-only header checks.
-	withSetPlaintext := func(h http.Handler) http.Handler {
-		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-			r = csrf.PlaintextHTTPRequest(r)
-			h.ServeHTTP(w, r)
-		})
-	}
+// withSecFetchSite wraps a HTTP handler and ensures requests made to it have
+// the `Sec-Fetch-Site` header set to same-origin to prevent CSRF attacks.
+func (s *Server) withSecFetchSite(h http.Handler) http.Handler {
+	// Ref https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Sec-Fetch-Site
+	// Sent by all modern browsers to indicate the relationship between the
+	// origin from which the request was made and the requested resource.
+	// Require that the Sec-Fetch-Site header is set to "same-origin"
+	// for all requests to the web client, preventing CSRF attacks.
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.Header.Get("Sec-Fetch-Site") {
+		case "same-origin":
+			// valid request
+		case "cross-site", "same-site":
+			// consider anything other than the same-origin to be invalid.
+			http.Error(w, "forbidden non `same-origin` request", http.StatusForbidden)
+			return
+		case "":
+			http.Error(w, "missing required Sec-Fetch-Site request header. You may need to update your browser", http.StatusForbidden)
+			return
+		}
+		// serve all same-site requests.
+		h.ServeHTTP(w, r)
+	})
 
-	// NB: the order of the withSetPlaintext and csrfProtect calls is important
-	// to ensure that we signal to the CSRF middleware that the request is being
-	// served over plaintext HTTP and not over TLS as it presumes by default.
-	return withSetPlaintext(csrfProtect(h))
 }
 
 func (s *Server) modeAPIHandler(mode ServerMode) (http.Handler, string) {
@@ -450,9 +453,8 @@ func (s *Server) authorizeRequest(w http.ResponseWriter, r *http.Request) (ok bo
 
 // serveLoginAPI serves requests for the web login client.
 // It should only be called by Server.ServeHTTP, via Server.apiHandler,
-// which protects the handler using gorilla csrf.
+// which protects the handler using s.withSecFetchSite.
 func (s *Server) serveLoginAPI(w http.ResponseWriter, r *http.Request) {
-	w.Header().Set("X-CSRF-Token", csrf.Token(r))
 	switch {
 	case r.URL.Path == "/api/data" && r.Method == httpm.GET:
 		s.serveGetNodeData(w, r)
@@ -575,7 +577,6 @@ func (s *Server) serveAPI(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	w.Header().Set("X-CSRF-Token", csrf.Token(r))
 	path := strings.TrimPrefix(r.URL.Path, "/api")
 	switch {
 	case path == "/data" && r.Method == httpm.GET:
@@ -834,12 +835,11 @@ type nodeData struct {
 	KeyExpiry  string // time.RFC3339
 	KeyExpired bool
 
-	TUNMode     bool
-	IsSynology  bool
-	DSMVersion  int // 6 or 7, if IsSynology=true
-	IsUnraid    bool
-	UnraidToken string
-	URLPrefix   string // if set, the URL prefix the client is served behind
+	TUNMode    bool
+	IsSynology bool
+	DSMVersion int // 6 or 7, if IsSynology=true
+	IsUnraid   bool
+	URLPrefix  string // if set, the URL prefix the client is served behind
 
 	UsingExitNode               *exitNode
 	AdvertisingExitNode         bool
@@ -899,7 +899,6 @@ func (s *Server) serveGetNodeData(w http.ResponseWriter, r *http.Request) {
 		IsSynology:       distro.Get() == distro.Synology || envknob.Bool("TS_FAKE_SYNOLOGY"),
 		DSMVersion:       distro.DSMVersion(),
 		IsUnraid:         distro.Get() == distro.Unraid,
-		UnraidToken:      os.Getenv("UNRAID_CSRF_TOKEN"),
 		RunningSSHServer: prefs.RunSSH,
 		URLPrefix:        strings.TrimSuffix(s.pathPrefix, "/"),
 		ControlAdminURL:  prefs.AdminPageURL(),
@@ -1276,37 +1275,6 @@ func (s *Server) proxyRequestToLocalAPI(w http.ResponseWriter, r *http.Request)
 	}
 }
 
-// csrfKey returns a key that can be used for CSRF protection.
-// If an error occurs during key creation, the error is logged and the active process terminated.
-// If the server is running in CGI mode, the key is cached to disk and reused between requests.
-// If an error occurs during key storage, the error is logged and the active process terminated.
-func (s *Server) csrfKey() []byte {
-	csrfFile := filepath.Join(os.TempDir(), "tailscale-web-csrf.key")
-
-	// if running in CGI mode, try to read from disk, but ignore errors
-	if s.cgiMode {
-		key, _ := os.ReadFile(csrfFile)
-		if len(key) == 32 {
-			return key
-		}
-	}
-
-	// create a new key
-	key := make([]byte, 32)
-	if _, err := rand.Read(key); err != nil {
-		log.Fatalf("error generating CSRF key: %v", err)
-	}
-
-	// if running in CGI mode, try to write the newly created key to disk, and exit if it fails.
-	if s.cgiMode {
-		if err := os.WriteFile(csrfFile, key, 0600); err != nil {
-			log.Fatalf("unable to store CSRF key: %v", err)
-		}
-	}
-
-	return key
-}
-
 // enforcePrefix returns a HandlerFunc that enforces a given path prefix is used in requests,
 // then strips it before invoking h.
 // Unlike http.StripPrefix, it does not return a 404 if the prefix is not present.
diff --git a/client/web/web_test.go b/client/web/web_test.go
index 2a6bc787a..ba31dbdff 100644
--- a/client/web/web_test.go
+++ b/client/web/web_test.go
@@ -11,7 +11,6 @@ import (
 	"fmt"
 	"io"
 	"net/http"
-	"net/http/cookiejar"
 	"net/http/httptest"
 	"net/netip"
 	"net/url"
@@ -21,7 +20,6 @@ import (
 	"time"
 
 	"github.com/google/go-cmp/cmp"
-	"github.com/gorilla/csrf"
 	"tailscale.com/client/local"
 	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/ipn"
@@ -1491,82 +1489,77 @@ func mockWaitAuthURL(_ context.Context, id string, src tailcfg.NodeID) (*tailcfg
 	}
 }
 
-func TestCSRFProtect(t *testing.T) {
-	s := &Server{}
+func TestSecFetchSiteProtect(t *testing.T) {
+	tests := []struct {
+		name           string
+		withSameOrigin bool
+		withSameSite   bool
+		withCrossSite  bool
+		expectError    bool
+	}{
+		{
+			name:           "requests with same-origin pass",
+			withSameOrigin: true,
+		},
+		{
+			name:         "requests with same-site fail",
+			withSameSite: true,
+			expectError:  true,
+		},
+		{
+			name:          "requests with cross-site fail",
+			withCrossSite: true,
+			expectError:   true,
+		},
+		{
+			name:        "requests with no Sec-Fetch-Site fail",
+			expectError: true,
+		},
+	}
 
-	mux := http.NewServeMux()
-	mux.HandleFunc("GET /test/csrf-token", func(w http.ResponseWriter, r *http.Request) {
-		token := csrf.Token(r)
-		_, err := io.WriteString(w, token)
-		if err != nil {
-			t.Fatal(err)
-		}
-	})
-	mux.HandleFunc("POST /test/csrf-protected", func(w http.ResponseWriter, r *http.Request) {
-		_, err := io.WriteString(w, "ok")
-		if err != nil {
-			t.Fatal(err)
-		}
-	})
-	h := s.withCSRF(mux)
-	ser := nettest.NewHTTPServer(nettest.GetNetwork(t), h)
-	defer ser.Close()
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			s := &Server{}
+			mux := http.NewServeMux()
 
-	jar, err := cookiejar.New(nil)
-	if err != nil {
-		t.Fatalf("unable to construct cookie jar: %v", err)
-	}
+			mux.HandleFunc("POST /protected", func(w http.ResponseWriter, r *http.Request) {
+				_, err := io.WriteString(w, "ok")
+				if err != nil {
+					t.Fatal(err)
+				}
+			})
 
-	client := ser.Client()
-	client.Jar = jar
+			h := s.withSecFetchSite(mux)
 
-	// make GET request to populate cookie jar
-	resp, err := client.Get(ser.URL + "/test/csrf-token")
-	if err != nil {
-		t.Fatalf("unable to make request: %v", err)
-	}
-	defer resp.Body.Close()
-	if resp.StatusCode != http.StatusOK {
-		t.Fatalf("unexpected status: %v", resp.Status)
-	}
-	tokenBytes, err := io.ReadAll(resp.Body)
-	if err != nil {
-		t.Fatalf("unable to read body: %v", err)
-	}
+			serv := nettest.NewHTTPServer(nettest.GetNetwork(t), h)
+			defer serv.Close()
 
-	csrfToken := strings.TrimSpace(string(tokenBytes))
-	if csrfToken == "" {
-		t.Fatal("empty csrf token")
-	}
+			client := serv.Client()
 
-	// make a POST request without the CSRF header; ensure it fails
-	resp, err = client.Post(ser.URL+"/test/csrf-protected", "text/plain", nil)
-	if err != nil {
-		t.Fatalf("unable to make request: %v", err)
-	}
-	if resp.StatusCode != http.StatusForbidden {
-		t.Fatalf("unexpected status: %v", resp.Status)
-	}
+			req, err := http.NewRequest("POST", serv.URL+"/protected", nil)
+			if err != nil {
+				t.Fatalf("error building request: %v", err)
+			}
 
-	// make a POST request with the CSRF header; ensure it succeeds
-	req, err := http.NewRequest("POST", ser.URL+"/test/csrf-protected", nil)
-	if err != nil {
-		t.Fatalf("error building request: %v", err)
-	}
-	req.Header.Set("X-CSRF-Token", csrfToken)
-	resp, err = client.Do(req)
-	if err != nil {
-		t.Fatalf("unable to make request: %v", err)
-	}
-	if resp.StatusCode != http.StatusOK {
-		t.Fatalf("unexpected status: %v", resp.Status)
-	}
-	defer resp.Body.Close()
-	out, err := io.ReadAll(resp.Body)
-	if err != nil {
-		t.Fatalf("unable to read body: %v", err)
-	}
-	if string(out) != "ok" {
-		t.Fatalf("unexpected body: %q", out)
+			if tt.withSameOrigin {
+				req.Header.Set("Sec-Fetch-Site", "same-origin")
+			} else if tt.withSameSite {
+				req.Header.Set("Sec-Fetch-Site", "same-site")
+			} else if tt.withCrossSite {
+				req.Header.Set("Sec-Fetch-Site", "cross-site")
+			}
+
+			resp, err := client.Do(req)
+			if err != nil {
+				t.Fatalf("unable to make request: %v", err)
+			}
+			defer resp.Body.Close()
+
+			if tt.expectError && resp.StatusCode == http.StatusOK {
+				t.Fatalf("expected error but got ok")
+			} else if !tt.expectError && resp.StatusCode != http.StatusOK {
+				t.Fatalf("expected ok got %q", resp.Status)
+			}
+		})
 	}
 }
diff --git a/cmd/k8s-operator/depaware.txt b/cmd/k8s-operator/depaware.txt
index 4cc4a8d46..481d3321e 100644
--- a/cmd/k8s-operator/depaware.txt
+++ b/cmd/k8s-operator/depaware.txt
@@ -144,8 +144,6 @@ tailscale.com/cmd/k8s-operator dependencies: (generated by github.com/tailscale/
    L    github.com/google/nftables/internal/parseexprfunc            from github.com/google/nftables+
    L    github.com/google/nftables/xt                                from github.com/google/nftables/expr+
         github.com/google/uuid                                       from github.com/prometheus-community/pro-bing+
-        github.com/gorilla/csrf                                      from tailscale.com/client/web
-        github.com/gorilla/securecookie                              from github.com/gorilla/csrf
         github.com/hdevalence/ed25519consensus                       from tailscale.com/clientupdate/distsign+
    L 💣 github.com/illarion/gonotify/v3                              from tailscale.com/net/dns
    L    github.com/illarion/gonotify/v3/syscallf                     from github.com/illarion/gonotify/v3
@@ -1131,13 +1129,12 @@ tailscale.com/cmd/k8s-operator dependencies: (generated by github.com/tailscale/
    W    debug/dwarf                                                  from debug/pe
    W    debug/pe                                                     from github.com/dblohm7/wingoes/pe
         embed                                                        from github.com/tailscale/web-client-prebuilt+
-        encoding                                                     from encoding/gob+
+        encoding                                                     from encoding/json+
         encoding/asn1                                                from crypto/x509+
         encoding/base32                                              from github.com/fxamacker/cbor/v2+
         encoding/base64                                              from encoding/json+
         encoding/binary                                              from compress/gzip+
         encoding/csv                                                 from github.com/spf13/pflag
-        encoding/gob                                                 from github.com/gorilla/securecookie
         encoding/hex                                                 from crypto/x509+
         encoding/json                                                from expvar+
         encoding/pem                                                 from crypto/tls+
@@ -1159,7 +1156,7 @@ tailscale.com/cmd/k8s-operator dependencies: (generated by github.com/tailscale/
         hash/fnv                                                     from google.golang.org/protobuf/internal/detrand
         hash/maphash                                                 from go4.org/mem
         html                                                         from html/template+
-        html/template                                                from github.com/gorilla/csrf+
+        html/template                                                from tailscale.com/util/eventbus
         internal/abi                                                 from crypto/x509/internal/macos+
         internal/asan                                                from internal/runtime/maps+
         internal/bisect                                              from internal/godebug
@@ -1191,7 +1188,7 @@ tailscale.com/cmd/k8s-operator dependencies: (generated by github.com/tailscale/
         internal/runtime/math                                        from internal/runtime/maps+
         internal/runtime/sys                                         from crypto/subtle+
    L    internal/runtime/syscall                                     from runtime+
-        internal/saferio                                             from debug/pe+
+   W    internal/saferio                                             from debug/pe
         internal/singleflight                                        from net
         internal/stringslite                                         from embed+
         internal/sync                                                from sync+
diff --git a/cmd/tailscale/depaware.txt b/cmd/tailscale/depaware.txt
index 7f66e7700..473b23fe0 100644
--- a/cmd/tailscale/depaware.txt
+++ b/cmd/tailscale/depaware.txt
@@ -27,8 +27,6 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
    L    github.com/google/nftables/internal/parseexprfunc            from github.com/google/nftables+
    L    github.com/google/nftables/xt                                from github.com/google/nftables/expr+
   DW    github.com/google/uuid                                       from tailscale.com/clientupdate+
-        github.com/gorilla/csrf                                      from tailscale.com/client/web
-        github.com/gorilla/securecookie                              from github.com/gorilla/csrf
         github.com/hdevalence/ed25519consensus                       from tailscale.com/clientupdate/distsign+
    L 💣 github.com/jsimonetti/rtnetlink                              from tailscale.com/net/netmon
    L    github.com/jsimonetti/rtnetlink/internal/unix                from github.com/jsimonetti/rtnetlink
@@ -318,12 +316,11 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
    W    debug/dwarf                                                  from debug/pe
    W    debug/pe                                                     from github.com/dblohm7/wingoes/pe
         embed                                                        from github.com/peterbourgon/ff/v3+
-        encoding                                                     from encoding/gob+
+        encoding                                                     from encoding/json+
         encoding/asn1                                                from crypto/x509+
         encoding/base32                                              from github.com/fxamacker/cbor/v2+
         encoding/base64                                              from encoding/json+
         encoding/binary                                              from compress/gzip+
-        encoding/gob                                                 from github.com/gorilla/securecookie
         encoding/hex                                                 from crypto/x509+
         encoding/json                                                from expvar+
         encoding/pem                                                 from crypto/tls+
@@ -337,7 +334,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         hash/crc32                                                   from compress/gzip+
         hash/maphash                                                 from go4.org/mem
         html                                                         from html/template+
-        html/template                                                from github.com/gorilla/csrf+
+        html/template                                                from tailscale.com/util/eventbus
         image                                                        from github.com/skip2/go-qrcode+
         image/color                                                  from github.com/skip2/go-qrcode+
         image/png                                                    from github.com/skip2/go-qrcode
@@ -371,7 +368,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         internal/runtime/math                                        from internal/runtime/maps+
         internal/runtime/sys                                         from crypto/subtle+
    L    internal/runtime/syscall                                     from runtime+
-        internal/saferio                                             from debug/pe+
+   W    internal/saferio                                             from debug/pe
         internal/singleflight                                        from net
         internal/stringslite                                         from embed+
         internal/sync                                                from sync+
diff --git a/cmd/tailscaled/depaware.txt b/cmd/tailscaled/depaware.txt
index 329c00e93..af816d5f7 100644
--- a/cmd/tailscaled/depaware.txt
+++ b/cmd/tailscaled/depaware.txt
@@ -116,8 +116,6 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
    L    github.com/google/nftables/internal/parseexprfunc            from github.com/google/nftables+
    L    github.com/google/nftables/xt                                from github.com/google/nftables/expr+
   DW    github.com/google/uuid                                       from tailscale.com/clientupdate+
-        github.com/gorilla/csrf                                      from tailscale.com/client/web
-        github.com/gorilla/securecookie                              from github.com/gorilla/csrf
         github.com/hdevalence/ed25519consensus                       from tailscale.com/clientupdate/distsign+
    L 💣 github.com/illarion/gonotify/v3                              from tailscale.com/net/dns
    L    github.com/illarion/gonotify/v3/syscallf                     from github.com/illarion/gonotify/v3
@@ -580,12 +578,11 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
    W    debug/dwarf                                                  from debug/pe
    W    debug/pe                                                     from github.com/dblohm7/wingoes/pe
         embed                                                        from github.com/tailscale/web-client-prebuilt+
-        encoding                                                     from encoding/gob+
+        encoding                                                     from encoding/json+
         encoding/asn1                                                from crypto/x509+
         encoding/base32                                              from github.com/fxamacker/cbor/v2+
         encoding/base64                                              from encoding/json+
         encoding/binary                                              from compress/gzip+
-        encoding/gob                                                 from github.com/gorilla/securecookie
         encoding/hex                                                 from crypto/x509+
         encoding/json                                                from expvar+
         encoding/pem                                                 from crypto/tls+
@@ -599,7 +596,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         hash/crc32                                                   from compress/gzip+
         hash/maphash                                                 from go4.org/mem
         html                                                         from html/template+
-        html/template                                                from github.com/gorilla/csrf+
+        html/template                                                from tailscale.com/util/eventbus
         internal/abi                                                 from crypto/x509/internal/macos+
         internal/asan                                                from internal/runtime/maps+
         internal/bisect                                              from internal/godebug
@@ -630,7 +627,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         internal/runtime/math                                        from internal/runtime/maps+
         internal/runtime/sys                                         from crypto/subtle+
    L    internal/runtime/syscall                                     from runtime+
-        internal/saferio                                             from debug/pe+
+   W    internal/saferio                                             from debug/pe
         internal/singleflight                                        from net
         internal/stringslite                                         from embed+
         internal/sync                                                from sync+