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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>
This commit is contained in:
Simon Law
2025-05-22 12:14:16 -07:00
committed by GitHub
parent aa8bc23c49
commit 3ee4c60ff0
9 changed files with 338 additions and 71 deletions
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2
derp/derphttp/derphttp_client.go
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
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
}