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Allow custom SFTP algorithm selection (#19636)

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Algorithms are comma separated.
Note that valid values does not in all cases represent default values.

`--sftp=pub-key-algos=...` specifies the supported client public key
authentication algorithms. Note that this doesn't include certificate types
since those use the underlying algorithm. This list is sent to the client if
it supports the server-sig-algs extension. Order is irrelevant.

Valid values
```
ssh-ed25519
sk-ssh-ed25519@openssh.com
sk-ecdsa-sha2-nistp256@openssh.com
ecdsa-sha2-nistp256
ecdsa-sha2-nistp384
ecdsa-sha2-nistp521
rsa-sha2-256
rsa-sha2-512
ssh-rsa
ssh-dss
```

`--sftp=kex-algos=...` specifies the supported key-exchange algorithms in preference order.

Valid values:

```
curve25519-sha256
curve25519-sha256@libssh.org
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
diffie-hellman-group14-sha256
diffie-hellman-group16-sha512
diffie-hellman-group14-sha1
diffie-hellman-group1-sha1
```

`--sftp=cipher-algos=...` specifies the allowed cipher algorithms.
If unspecified then a sensible default is used.

Valid values:
```
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
chacha20-poly1305@openssh.com
arcfour256
arcfour128
arcfour
aes128-cbc
3des-cbc
```

`--sftp=mac-algos=...` specifies a default set of MAC algorithms in preference order.
This is based on RFC 4253, section 6.4, but with hmac-md5 variants removed because they have
reached the end of their useful life.

Valid values:

```
hmac-sha2-256-etm@openssh.com
hmac-sha2-512-etm@openssh.com
hmac-sha2-256
hmac-sha2-512
hmac-sha1
hmac-sha1-96
```
This commit is contained in:
Klaus Post 2024-04-30 08:15:45 -07:00 committed by GitHub
parent 6579304d8c
commit f64dea2aac
No known key found for this signature in database
GPG Key ID: B5690EEEBB952194
2 changed files with 197 additions and 1 deletions
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123
cmd/sftp-server.go
View File

@ -40,6 +40,85 @@ func (s *sftpLogger) Info(tag xsftp.LogType, msg string) {
logger.Info(msg)
}
const (
kexAlgoDH1SHA1 = "diffie-hellman-group1-sha1"
kexAlgoDH14SHA1 = "diffie-hellman-group14-sha1"
kexAlgoDH14SHA256 = "diffie-hellman-group14-sha256"
kexAlgoDH16SHA512 = "diffie-hellman-group16-sha512"
kexAlgoECDH256 = "ecdh-sha2-nistp256"
kexAlgoECDH384 = "ecdh-sha2-nistp384"
kexAlgoECDH521 = "ecdh-sha2-nistp521"
kexAlgoCurve25519SHA256LibSSH = "curve25519-sha256@libssh.org"
kexAlgoCurve25519SHA256 = "curve25519-sha256"
chacha20Poly1305ID = "chacha20-poly1305@openssh.com"
gcm256CipherID = "aes256-gcm@openssh.com"
aes128cbcID = "aes128-cbc"
tripledescbcID = "3des-cbc"
)
// https://cs.opensource.google/go/x/crypto/+/refs/tags/v0.22.0:ssh/common.go;l=46
// preferredKexAlgos specifies the default preference for key-exchange
// algorithms in preference order. The diffie-hellman-group16-sha512 algorithm
// is disabled by default because it is a bit slower than the others.
var preferredKexAlgos = []string{
kexAlgoCurve25519SHA256, kexAlgoCurve25519SHA256LibSSH,
kexAlgoECDH256, kexAlgoECDH384, kexAlgoECDH521,
kexAlgoDH14SHA256, kexAlgoDH14SHA1,
}
// supportedKexAlgos specifies the supported key-exchange algorithms in
// preference order.
// https://cs.opensource.google/go/x/crypto/+/refs/tags/v0.22.0:ssh/common.go;l=44
var supportedKexAlgos = []string{
kexAlgoCurve25519SHA256, kexAlgoCurve25519SHA256LibSSH,
// P384 and P521 are not constant-time yet, but since we don't
// reuse ephemeral keys, using them for ECDH should be OK.
kexAlgoECDH256, kexAlgoECDH384, kexAlgoECDH521,
kexAlgoDH14SHA256, kexAlgoDH16SHA512, kexAlgoDH14SHA1,
kexAlgoDH1SHA1,
}
// supportedPubKeyAuthAlgos specifies the supported client public key
// authentication algorithms. Note that this doesn't include certificate types
// since those use the underlying algorithm. This list is sent to the client if
// it supports the server-sig-algs extension. Order is irrelevant.
// https://cs.opensource.google/go/x/crypto/+/refs/tags/v0.22.0:ssh/common.go;l=142
var supportedPubKeyAuthAlgos = []string{
ssh.KeyAlgoED25519,
ssh.KeyAlgoSKED25519, ssh.KeyAlgoSKECDSA256,
ssh.KeyAlgoECDSA256, ssh.KeyAlgoECDSA384, ssh.KeyAlgoECDSA521,
ssh.KeyAlgoRSASHA256, ssh.KeyAlgoRSASHA512, ssh.KeyAlgoRSA,
ssh.KeyAlgoDSA,
}
// supportedCiphers lists ciphers we support but might not recommend.
// https://cs.opensource.google/go/x/crypto/+/refs/tags/v0.22.0:ssh/common.go;l=28
var supportedCiphers = []string{
"aes128-ctr", "aes192-ctr", "aes256-ctr",
"aes128-gcm@openssh.com", gcm256CipherID,
chacha20Poly1305ID,
"arcfour256", "arcfour128", "arcfour",
aes128cbcID,
tripledescbcID,
}
// preferredCiphers specifies the default preference for ciphers.
// https://cs.opensource.google/go/x/crypto/+/refs/tags/v0.22.0:ssh/common.go;l=37
var preferredCiphers = []string{
"aes128-gcm@openssh.com", gcm256CipherID,
chacha20Poly1305ID,
"aes128-ctr", "aes192-ctr", "aes256-ctr",
}
// supportedMACs specifies a default set of MAC algorithms in preference order.
// This is based on RFC 4253, section 6.4, but with hmac-md5 variants removed
// because they have reached the end of their useful life.
// https://cs.opensource.google/go/x/crypto/+/refs/tags/v0.22.0:ssh/common.go;l=85
var supportedMACs = []string{
"hmac-sha2-256-etm@openssh.com", "hmac-sha2-512-etm@openssh.com", "hmac-sha2-256", "hmac-sha2-512", "hmac-sha1", "hmac-sha1-96",
}
func (s *sftpLogger) Error(tag xsftp.LogType, err error) {
switch tag {
case xsftp.AcceptNetworkError:
@ -53,13 +132,41 @@ func (s *sftpLogger) Error(tag xsftp.LogType, err error) {
}
}
func filterAlgos(arg string, want []string, allowed []string) []string {
var filteredAlgos []string
found := false
for _, algo := range want {
if len(algo) == 0 {
continue
}
for _, allowedAlgo := range allowed {
algo := strings.ToLower(strings.TrimSpace(algo))
if algo == allowedAlgo {
filteredAlgos = append(filteredAlgos, algo)
found = true
break
}
}
if !found {
logger.Fatal(fmt.Errorf("unknown algorithm %q passed to --sftp=%s\nValid algorithms: %v", algo, arg, strings.Join(allowed, ", ")), "unable to start SFTP server")
}
}
if len(filteredAlgos) == 0 {
logger.Fatal(fmt.Errorf("no valid algorithms passed to --sftp=%s\nValid algorithms: %v", arg, strings.Join(allowed, ", ")), "unable to start SFTP server")
}
return filteredAlgos
}
func startSFTPServer(args []string) {
var (
port int
publicIP string
sshPrivateKey string
)
allowPubKeys := supportedPubKeyAuthAlgos
allowKexAlgos := preferredKexAlgos
allowCiphers := preferredCiphers
allowMACs := supportedMACs
var err error
for _, arg := range args {
tokens := strings.SplitN(arg, "=", 2)
@ -82,6 +189,14 @@ func startSFTPServer(args []string) {
publicIP = host
case "ssh-private-key":
sshPrivateKey = tokens[1]
case "pub-key-algos":
allowPubKeys = filterAlgos(arg, strings.Split(tokens[1], ","), supportedPubKeyAuthAlgos)
case "kex-algos":
allowKexAlgos = filterAlgos(arg, strings.Split(tokens[1], ","), supportedKexAlgos)
case "cipher-algos":
allowCiphers = filterAlgos(arg, strings.Split(tokens[1], ","), supportedCiphers)
case "mac-algos":
allowCiphers = filterAlgos(arg, strings.Split(tokens[1], ","), supportedMACs)
}
}
@ -106,6 +221,12 @@ func startSFTPServer(args []string) {
// An SSH server is represented by a ServerConfig, which holds
// certificate details and handles authentication of ServerConns.
sshConfig := &ssh.ServerConfig{
Config: ssh.Config{
KeyExchanges: allowKexAlgos,
Ciphers: allowCiphers,
MACs: allowMACs,
},
PublicKeyAuthAlgorithms: allowPubKeys,
PasswordCallback: func(c ssh.ConnMetadata, pass []byte) (*ssh.Permissions, error) {
if globalIAMSys.LDAPConfig.Enabled() {
sa, _, err := globalIAMSys.getServiceAccount(context.Background(), c.User())

75
docs/ftp/README.md
View File

@ -167,3 +167,78 @@ Unlike SFTP server, FTP server is insecure by default. To operate under TLS mode
> certs from the server certificate chain, this is mainly to add simplicity of setup. However if you wish to terminate
> TLS certificates via a different domain for your FTP servers you may choose the above command line options.
### Custom Algorithms (SFTP)
Custom algorithms can be specified via command line parameters.
Algorithms are comma separated.
Note that valid values does not in all cases represent default values.
`--sftp=pub-key-algos=...` specifies the supported client public key
authentication algorithms. Note that this doesn't include certificate types
since those use the underlying algorithm. This list is sent to the client if
it supports the server-sig-algs extension. Order is irrelevant.
Valid values
```
ssh-ed25519
sk-ssh-ed25519@openssh.com
sk-ecdsa-sha2-nistp256@openssh.com
ecdsa-sha2-nistp256
ecdsa-sha2-nistp384
ecdsa-sha2-nistp521
rsa-sha2-256
rsa-sha2-512
ssh-rsa
ssh-dss
```
`--sftp=kex-algos=...` specifies the supported key-exchange algorithms in preference order.
Valid values:
```
curve25519-sha256
curve25519-sha256@libssh.org
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
diffie-hellman-group14-sha256
diffie-hellman-group16-sha512
diffie-hellman-group14-sha1
diffie-hellman-group1-sha1
```
`--sftp=cipher-algos=...` specifies the allowed cipher algorithms.
If unspecified then a sensible default is used.
Valid values:
```
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
chacha20-poly1305@openssh.com
arcfour256
arcfour128
arcfour
aes128-cbc
3des-cbc
```
`--sftp=mac-algos=...` specifies a default set of MAC algorithms in preference order.
This is based on RFC 4253, section 6.4, but with hmac-md5 variants removed because they have
reached the end of their useful life.
Valid values:
```
hmac-sha2-256-etm@openssh.com
hmac-sha2-512-etm@openssh.com
hmac-sha2-256
hmac-sha2-512
hmac-sha1
hmac-sha1-96
```