fix!: remove TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA cipher by default (#13837)

This cipher is included by default in Go as a fallback, but is marked as
an insecure cipher. This removes the 3des cipher by default.

Before:
```
$ nmap --script ssl-enum-ciphers -p 443 xxxxxxx
Starting Nmap 7.94 ( https://nmap.org ) at 2024-07-08 14:16 CDT
Nmap scan report for xxxxx (xxx.xxx.xxx.xxx)
Host is up (0.038s latency).
rDNS record for xxx.xxx.xxx.xxx: xxx.xxx.xxx.xxx.bc.googleusercontent.com

PORT    STATE SERVICE
443/tcp open  https
| ssl-enum-ciphers:
|   TLSv1.2:
|     ciphers:
|       TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256 (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA (secp256r1) - C
|     compressors:
|       NULL
|     cipher preference: server
|     warnings:
|       64-bit block cipher 3DES vulnerable to SWEET32 attack
|   TLSv1.3:
|     ciphers:
|       TLS_AKE_WITH_AES_128_GCM_SHA256 (ecdh_x25519) - A
|       TLS_AKE_WITH_AES_256_GCM_SHA384 (ecdh_x25519) - A
|       TLS_AKE_WITH_CHACHA20_POLY1305_SHA256 (ecdh_x25519) - A
|     cipher preference: server
|_  least strength: C
```

After:
```
$ nmap --script ssl-enum-ciphers -p 443 xxxxxxx
Starting Nmap 7.94 ( https://nmap.org ) at 2024-07-08 15:04 CDT
Nmap scan report for xxxxx (xxx.xxx.xxx.xxx)
Host is up (0.039s latency).
rDNS record for xxx.xxx.xxx.xxx: xxx.xxx.xxx.xxx.bc.googleusercontent.com

PORT    STATE SERVICE
443/tcp open  https
| ssl-enum-ciphers:
|   TLSv1.2:
|     ciphers:
|       TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256 (secp256r1) - A
|     compressors:
|       NULL
|     cipher preference: client
|   TLSv1.3:
|     ciphers:
|       TLS_AKE_WITH_AES_128_GCM_SHA256 (ecdh_x25519) - A
|       TLS_AKE_WITH_AES_256_GCM_SHA384 (ecdh_x25519) - A
|       TLS_AKE_WITH_CHACHA20_POLY1305_SHA256 (ecdh_x25519) - A
|     cipher preference: server
|_  least strength: A
```

* fixup! fix!(cli): remove `TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA` cipher by default

* fixup! fix!(cli): remove `TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA` cipher by default

cli/server.go

@@ -1569,6 +1569,19 @@ func generateSelfSignedCertificate() (*tls.Certificate, error) {
 	return &cert, nil
 }
 
+// defaultCipherSuites is a list of safe cipher suites that we default to. This
+// is different from Golang's list of defaults, which unfortunately includes
+// `TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA`.
+var defaultCipherSuites = func() []uint16 {
+	ret := []uint16{}
+
+	for _, suite := range tls.CipherSuites() {
+		ret = append(ret, suite.ID)
+	}
+
+	return ret
+}()
+
 // configureServerTLS returns the TLS config used for the Coderd server
 // connections to clients. A logger is passed in to allow printing warning
 // messages that do not block startup.
@@ -1599,6 +1612,8 @@ func configureServerTLS(ctx context.Context, logger slog.Logger, tlsMinVersion,
 			return nil, err
 		}
 		tlsConfig.CipherSuites = cipherIDs
+	} else {
+		tlsConfig.CipherSuites = defaultCipherSuites
 	}
 
 	switch tlsClientAuth {

cli/server_internal_test.go

@@ -20,6 +20,28 @@ import (
 	"github.com/coder/serpent"
 )
 
+func Test_configureServerTLS(t *testing.T) {
+	t.Parallel()
+	t.Run("DefaultNoInsecureCiphers", func(t *testing.T) {
+		t.Parallel()
+		logger := slogtest.Make(t, nil)
+		cfg, err := configureServerTLS(context.Background(), logger, "tls12", "none", nil, nil, "", nil, false)
+		require.NoError(t, err)
+
+		require.NotEmpty(t, cfg)
+
+		insecureCiphers := tls.InsecureCipherSuites()
+		for _, cipher := range cfg.CipherSuites {
+			for _, insecure := range insecureCiphers {
+				if cipher == insecure.ID {
+					t.Logf("Insecure cipher found by default: %s", insecure.Name)
+					t.Fail()
+				}
+			}
+		}
+	})
+}
+
 func Test_configureCipherSuites(t *testing.T) {
 	t.Parallel()