cmd/derper: fix mesh auth for DERP servers (#16061)

To authenticate mesh keys, the DERP servers used a simple == comparison,
which is susceptible to a side channel timing attack.

By extracting the mesh key for a DERP server, an attacker could DoS it
by forcing disconnects using derp.Client.ClosePeer. They could also
enumerate the public Wireguard keys, IP addresses and ports for nodes
connected to that DERP server.

DERP servers configured without mesh keys deny all such requests.

This patch also extracts the mesh key logic into key.DERPMesh, to
prevent this from happening again.

Security bulletin: https://tailscale.com/security-bulletins#ts-2025-003

Fixes tailscale/corp#28720

Signed-off-by: Simon Law <sfllaw@tailscale.com>

cmd/derper/derper.go

@@ -96,9 +96,6 @@ var (
 var (
 	tlsRequestVersion = &metrics.LabelMap{Label: "version"}
 	tlsActiveVersion  = &metrics.LabelMap{Label: "version"}
-
-	// Exactly 64 hexadecimal lowercase digits.
-	validMeshKey = regexp.MustCompile(`^[0-9a-f]{64}$`)
 )
 
 const setecMeshKeyName = "meshkey"
@@ -159,14 +156,6 @@ func writeNewConfig() config {
 	return cfg
 }
 
-func checkMeshKey(key string) (string, error) {
-	key = strings.TrimSpace(key)
-	if !validMeshKey.MatchString(key) {
-		return "", errors.New("key must contain exactly 64 hex digits")
-	}
-	return key, nil
-}
-
 func main() {
 	flag.Parse()
 	if *versionFlag {
@@ -246,10 +235,9 @@ func main() {
 		log.Printf("No mesh key configured for --dev mode")
 	} else if meshKey == "" {
 		log.Printf("No mesh key configured")
-	} else if key, err := checkMeshKey(meshKey); err != nil {
+	} else if err := s.SetMeshKey(meshKey); err != nil {
 		log.Fatalf("invalid mesh key: %v", err)
 	} else {
-		s.SetMeshKey(key)
 		log.Println("DERP mesh key configured")
 	}
 

cmd/derper/derper_test.go

@@ -138,46 +138,3 @@ func TestTemplate(t *testing.T) {
 		t.Error("Output is missing debug info")
 	}
 }
-
-func TestCheckMeshKey(t *testing.T) {
-	testCases := []struct {
-		name    string
-		input   string
-		want    string
-		wantErr bool
-	}{
-		{
-			name:    "KeyOkay",
-			input:   "f1ffafffffffffffffffffffffffffffffffffffffffffffffffff2ffffcfff6",
-			want:    "f1ffafffffffffffffffffffffffffffffffffffffffffffffffff2ffffcfff6",
-			wantErr: false,
-		},
-		{
-			name:    "TrimKeyOkay",
-			input:   "  f1ffafffffffffffffffffffffffffffffffffffffffffffffffff2ffffcfff6  ",
-			want:    "f1ffafffffffffffffffffffffffffffffffffffffffffffffffff2ffffcfff6",
-			wantErr: false,
-		},
-		{
-			name:    "NotAKey",
-			input:   "zzthisisnotakey",
-			want:    "",
-			wantErr: true,
-		},
-	}
-	for _, tt := range testCases {
-		t.Run(tt.name, func(t *testing.T) {
-			t.Parallel()
-
-			k, err := checkMeshKey(tt.input)
-			if err != nil && !tt.wantErr {
-				t.Errorf("unexpected error: %v", err)
-			}
-			if k != tt.want && err == nil {
-				t.Errorf("want: %s doesn't match expected: %s", tt.want, k)
-			}
-
-		})
-	}
-
-}