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

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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>
pull/16068/head
Simon Law 7 months ago committed by GitHub
parent aa8bc23c49
commit 3ee4c60ff0
No known key found for this signature in database
GPG Key ID: B5690EEEBB952194
9 changed files with 338 additions and 71 deletions
Show Stats Download Patch File Download Diff File

14
cmd/derper/derper.go
Unescape Escape View File

@ -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")
}

43
cmd/derper/derper_test.go
Unescape Escape View File

@ -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)
}
})
}
}

8
derp/derp_client.go
Unescape Escape View File

@ -30,7 +30,7 @@ type Client struct {
logf logger.Logf
nc Conn
br *bufio.Reader
meshKey string
meshKey key.DERPMesh
canAckPings bool
isProber bool
@ -56,7 +56,7 @@ func (f clientOptFunc) update(o *clientOpt) { f(o) }
// clientOpt are the options passed to newClient.
type clientOpt struct {
MeshKey string
MeshKey key.DERPMesh
ServerPub key.NodePublic
CanAckPings bool
IsProber bool
@ -66,7 +66,7 @@ type clientOpt struct {
// access to join the mesh.
//
// An empty key means to not use a mesh key.
func MeshKey(key string) ClientOpt { return clientOptFunc(func(o *clientOpt) { o.MeshKey = key }) }
func MeshKey(k key.DERPMesh) ClientOpt { return clientOptFunc(func(o *clientOpt) { o.MeshKey = k }) }
// IsProber returns a ClientOpt to pass to the DERP server during connect to
// declare that this client is a a prober.
@ -182,7 +182,7 @@ type clientInfo struct {
func (c *Client) sendClientKey() error {
msg, err := json.Marshal(clientInfo{
Version: ProtocolVersion,
MeshKey: c.meshKey,
MeshKey: c.meshKey.String(),
CanAckPings: c.canAckPings,
IsProber: c.isProber,
})

28
derp/derp_server.go
Unescape Escape View File

@ -134,7 +134,7 @@ type Server struct {
publicKey key.NodePublic
logf logger.Logf
memSys0 uint64 // runtime.MemStats.Sys at start (or early-ish)
meshKey string
meshKey key.DERPMesh
limitedLogf logger.Logf
metaCert []byte // the encoded x509 cert to send after LetsEncrypt cert+intermediate
dupPolicy dupPolicy
@ -464,8 +464,13 @@ func genDroppedCounters() {
// amongst themselves.
//
// It must be called before serving begins.
func (s *Server) SetMeshKey(v string) {
s.meshKey = v
func (s *Server) SetMeshKey(v string) error {
k, err := key.ParseDERPMesh(v)
if err != nil {
return err
}
s.meshKey = k
return nil
}
// SetVerifyClients sets whether this DERP server verifies clients through tailscaled.
@ -506,10 +511,10 @@ func (s *Server) SetTCPWriteTimeout(d time.Duration) {
}
// HasMeshKey reports whether the server is configured with a mesh key.
func (s *Server) HasMeshKey() bool { return s.meshKey != "" }
func (s *Server) HasMeshKey() bool { return !s.meshKey.IsZero() }
// MeshKey returns the configured mesh key, if any.
func (s *Server) MeshKey() string { return s.meshKey }
func (s *Server) MeshKey() key.DERPMesh { return s.meshKey }
// PrivateKey returns the server's private key.
func (s *Server) PrivateKey() key.NodePrivate { return s.privateKey }
@ -1355,7 +1360,18 @@ func (c *sclient) requestMeshUpdate() {
// isMeshPeer reports whether the client is a trusted mesh peer
// node in the DERP region.
func (s *Server) isMeshPeer(info *clientInfo) bool {
return info != nil && info.MeshKey != "" && info.MeshKey == s.meshKey
// Compare mesh keys in constant time to prevent timing attacks.
// Since mesh keys are a fixed length, we don’t need to be concerned
// about timing attacks on client mesh keys that are the wrong length.
// See https://github.com/tailscale/corp/issues/28720
if info == nil || info.MeshKey == "" {
return false
}
k, err := key.ParseDERPMesh(info.MeshKey)
if err != nil {
return false
}
return s.meshKey.Equal(k)
}
// verifyClient checks whether the client is allowed to connect to the derper,

103
derp/derp_test.go
Unescape Escape View File

@ -511,11 +511,13 @@ func (ts *testServer) close(t *testing.T) error {
return nil
}
const testMeshKey = "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef"
func newTestServer(t *testing.T, ctx context.Context) *testServer {
t.Helper()
logf := logger.WithPrefix(t.Logf, "derp-server: ")
s := NewServer(key.NewNode(), logf)
s.SetMeshKey("mesh-key")
s.SetMeshKey(testMeshKey)
ln, err := net.Listen("tcp", "127.0.0.1:0")
if err != nil {
t.Fatal(err)
@ -591,8 +593,12 @@ func newRegularClient(t *testing.T, ts *testServer, name string) *testClient {
func newTestWatcher(t *testing.T, ts *testServer, name string) *testClient {
return newTestClient(t, ts, name, func(nc net.Conn, priv key.NodePrivate, logf logger.Logf) (*Client, error) {
mk, err := key.ParseDERPMesh(testMeshKey)
if err != nil {
return nil, err
}
brw := bufio.NewReadWriter(bufio.NewReader(nc), bufio.NewWriter(nc))
c, err := NewClient(priv, nc, brw, logf, MeshKey("mesh-key"))
c, err := NewClient(priv, nc, brw, logf, MeshKey(mk))
if err != nil {
return nil, err
}
@ -1627,3 +1633,96 @@ func TestGetPerClientSendQueueDepth(t *testing.T) {
})
}
}
func TestSetMeshKey(t *testing.T) {
for name, tt := range map[string]struct {
key string
want key.DERPMesh
wantErr bool
}{
"clobber": {
key: testMeshKey,
wantErr: false,
},
"invalid": {
key: "badf00d",
wantErr: true,
},
} {
t.Run(name, func(t *testing.T) {
s := &Server{}
err := s.SetMeshKey(tt.key)
if tt.wantErr {
if err == nil {
t.Fatalf("expected err")
}
return
}
if err != nil {
t.Fatalf("unexpected err: %v", err)
}
want, err := key.ParseDERPMesh(tt.key)
if err != nil {
t.Fatal(err)
}
if !s.meshKey.Equal(want) {
t.Fatalf("got %v, want %v", s.meshKey, want)
}
})
}
}
func TestIsMeshPeer(t *testing.T) {
s := &Server{}
err := s.SetMeshKey(testMeshKey)
if err != nil {
t.Fatal(err)
}
for name, tt := range map[string]struct {
info *clientInfo
want bool
wantAllocs float64
}{
"nil": {
info: nil,
want: false,
wantAllocs: 0,
},
"empty": {
info: &clientInfo{MeshKey: ""},
want: false,
wantAllocs: 0,
},
"invalid": {
info: &clientInfo{MeshKey: "invalid"},
want: false,
wantAllocs: 2, // error message
},
"mismatch": {
info: &clientInfo{MeshKey: "0badf00d00000000000000000000000000000000000000000000000000000000"},
want: false,
wantAllocs: 1,
},
"match": {
info: &clientInfo{MeshKey: testMeshKey},
want: true,
wantAllocs: 1,
},
} {
t.Run(name, func(t *testing.T) {
var got bool
allocs := testing.AllocsPerRun(1, func() {
got = s.isMeshPeer(tt.info)
})
if got != tt.want {
t.Fatalf("got %t, want %t: info = %#v", got, tt.want, tt.info)
}
if allocs != tt.wantAllocs && tt.want {
t.Errorf("%f allocations, want %f", allocs, tt.wantAllocs)
}
})
}
}

2
derp/derphttp/derphttp_client.go
Unescape Escape View File

@ -57,7 +57,7 @@ type Client struct {
TLSConfig *tls.Config // optional; nil means default
HealthTracker *health.Tracker // optional; used if non-nil only
DNSCache *dnscache.Resolver // optional; nil means no caching
MeshKey string // optional; for trusted clients
MeshKey key.DERPMesh // optional; for trusted clients
IsProber bool // optional; for probers to optional declare themselves as such
// WatchConnectionChanges is whether the client wishes to subscribe to

10
derp/derphttp/derphttp_test.go
Unescape Escape View File

@ -212,6 +212,8 @@ func TestPing(t *testing.T) {
}
}
const testMeshKey = "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef"
func newTestServer(t *testing.T, k key.NodePrivate) (serverURL string, s *derp.Server) {
s = derp.NewServer(k, t.Logf)
httpsrv := &http.Server{
@ -224,7 +226,7 @@ func newTestServer(t *testing.T, k key.NodePrivate) (serverURL string, s *derp.S
t.Fatal(err)
}
serverURL = "http://" + ln.Addr().String()
s.SetMeshKey("1234")
s.SetMeshKey(testMeshKey)
go func() {
if err := httpsrv.Serve(ln); err != nil {
@ -243,7 +245,11 @@ func newWatcherClient(t *testing.T, watcherPrivateKey key.NodePrivate, serverToW
if err != nil {
t.Fatal(err)
}
c.MeshKey = "1234"
k, err := key.ParseDERPMesh(testMeshKey)
if err != nil {
t.Fatal(err)
}
c.MeshKey = k
return
}

68
types/key/derp.go
Unescape Escape View File

@ -0,0 +1,68 @@
// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package key
import (
"crypto/subtle"
"encoding/hex"
"errors"
"fmt"
"strings"
"go4.org/mem"
"tailscale.com/types/structs"
)
var ErrInvalidMeshKey = errors.New("invalid mesh key")
// DERPMesh is a mesh key, used for inter-DERP-node communication and for
// privileged DERP clients.
type DERPMesh struct {
_ structs.Incomparable // == isn't constant-time
k [32]byte // 64-digit hexadecimal numbers fit in 32 bytes
}
// DERPMeshFromRaw32 parses a 32-byte raw value as a DERP mesh key.
func DERPMeshFromRaw32(raw mem.RO) DERPMesh {
if raw.Len() != 32 {
panic("input has wrong size")
}
var ret DERPMesh
raw.Copy(ret.k[:])
return ret
}
// ParseDERPMesh parses a DERP mesh key from a string.
// This function trims whitespace around the string.
// If the key is not a 64-digit hexadecimal number, ErrInvalidMeshKey is returned.
func ParseDERPMesh(key string) (DERPMesh, error) {
key = strings.TrimSpace(key)
if len(key) != 64 {
return DERPMesh{}, fmt.Errorf("%w: must be 64-digit hexadecimal number", ErrInvalidMeshKey)
}
decoded, err := hex.DecodeString(key)
if err != nil {
return DERPMesh{}, fmt.Errorf("%w: %v", ErrInvalidMeshKey, err)
}
return DERPMeshFromRaw32(mem.B(decoded)), nil
}
// IsZero reports whether k is the zero value.
func (k DERPMesh) IsZero() bool {
return k.Equal(DERPMesh{})
}
// Equal reports whether k and other are the same key.
func (k DERPMesh) Equal(other DERPMesh) bool {
// Compare mesh keys in constant time to prevent timing attacks.
// Since mesh keys are a fixed length, we don’t need to be concerned
// about timing attacks on client mesh keys that are the wrong length.
// See https://github.com/tailscale/corp/issues/28720
return subtle.ConstantTimeCompare(k.k[:], other.k[:]) == 1
}
// String returns k as a hex-encoded 64-digit number.
func (k DERPMesh) String() string {
return hex.EncodeToString(k.k[:])
}

133
types/key/derp_test.go
Unescape Escape View File

@ -0,0 +1,133 @@
// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package key
import (
"errors"
"testing"
"go4.org/mem"
)
func TestDERPMeshIsValid(t *testing.T) {
for name, tt := range map[string]struct {
input string
want string
wantErr error
}{
"good": {
input: "0123456789012345678901234567890123456789012345678901234567890123",
want: "0123456789012345678901234567890123456789012345678901234567890123",
wantErr: nil,
},
"hex": {
input: "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef",
want: "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef",
wantErr: nil,
},
"uppercase": {
input: "0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF",
want: "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef",
wantErr: nil,
},
"whitespace": {
input: " 0123456789012345678901234567890123456789012345678901234567890123 ",
want: "0123456789012345678901234567890123456789012345678901234567890123",
wantErr: nil,
},
"short": {
input: "0123456789abcdef",
wantErr: ErrInvalidMeshKey,
},
"long": {
input: "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0",
wantErr: ErrInvalidMeshKey,
},
} {
t.Run(name, func(t *testing.T) {
k, err := ParseDERPMesh(tt.input)
if !errors.Is(err, tt.wantErr) {
t.Errorf("err %v, want %v", err, tt.wantErr)
}
got := k.String()
if got != tt.want && tt.wantErr == nil {
t.Errorf("got %q, want %q", got, tt.want)
}
})
}
}
func TestDERPMesh(t *testing.T) {
t.Parallel()
for name, tt := range map[string]struct {
str string
hex []byte
equal bool // are str and hex equal?
}{
"zero": {
str: "0000000000000000000000000000000000000000000000000000000000000000",
hex: []byte{
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
},
equal: true,
},
"equal": {
str: "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef",
hex: []byte{
0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef,
0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef,
0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef,
0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef,
},
equal: true,
},
"unequal": {
str: "0badc0de00000000000000000000000000000000000000000000000000000000",
hex: []byte{
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
},
equal: false,
},
} {
t.Run(name, func(t *testing.T) {
t.Parallel()
k, err := ParseDERPMesh(tt.str)
if err != nil {
t.Fatal(err)
}
// string representation should round-trip
s := k.String()
if s != tt.str {
t.Fatalf("string %s, want %s", s, tt.str)
}
// if tt.equal, then tt.hex is intended to be equal
if k.k != [32]byte(tt.hex) && tt.equal {
t.Fatalf("decoded %x, want %x", k.k, tt.hex)
}
h := DERPMeshFromRaw32(mem.B(tt.hex))
if k.Equal(h) != tt.equal {
if tt.equal {
t.Fatalf("%v != %v", k, h)
} else {
t.Fatalf("%v == %v", k, h)
}
}
})
}
}
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