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sftpd: add support for OpenPubkey SSH

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Signed-off-by: Nicola Murino <nicola.murino@gmail.com>
This commit is contained in:
Nicola Murino
2025-11-19 09:16:56 +01:00
parent 92b30fbd1f
commit 3917a5b252
6 changed files with 201 additions and 30 deletions
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4
internal/config/config.go
View File

@@ -278,6 +278,8 @@ func Init() {
PublicKeyAlgorithms: []string{},
TrustedUserCAKeys: []string{},
RevokedUserCertsFile: "",
OPKSSHPath: "",
OPKSSHChecksum: "",
LoginBannerFile: "",
EnabledSSHCommands: []string{},
KeyboardInteractiveAuthentication: true,
@@ -2095,6 +2097,8 @@ func setViperDefaults() {
viper.SetDefault("sftpd.public_key_algorithms", globalConf.SFTPD.PublicKeyAlgorithms)
viper.SetDefault("sftpd.trusted_user_ca_keys", globalConf.SFTPD.TrustedUserCAKeys)
viper.SetDefault("sftpd.revoked_user_certs_file", globalConf.SFTPD.RevokedUserCertsFile)
viper.SetDefault("sftpd.opkssh_path", globalConf.SFTPD.OPKSSHPath)
viper.SetDefault("sftpd.opkssh_checksum", globalConf.SFTPD.OPKSSHChecksum)
viper.SetDefault("sftpd.login_banner_file", globalConf.SFTPD.LoginBannerFile)
viper.SetDefault("sftpd.enabled_ssh_commands", sftpd.GetDefaultSSHCommands())
viper.SetDefault("sftpd.keyboard_interactive_authentication", globalConf.SFTPD.KeyboardInteractiveAuthentication)

41
internal/sftpd/internal_test.go
View File

@@ -16,6 +16,7 @@ package sftpd
import (
"bytes"
"context"
"errors"
"fmt"
"io"
@@ -1818,6 +1819,7 @@ func TestCanReadSymlink(t *testing.T) {
}
func TestAuthenticationErrors(t *testing.T) {
sftpAuthError := newAuthenticationError(nil, "", "")
loginMethod := dataprovider.SSHLoginMethodPassword
username := "test user"
err := newAuthenticationError(fmt.Errorf("cannot validate credentials: %w", util.NewRecordNotFoundError("not found")),
@@ -1842,3 +1844,42 @@ func TestAuthenticationErrors(t *testing.T) {
assert.ErrorIs(t, err, sftpAuthError)
assert.NotErrorIs(t, err, util.ErrNotFound)
}
type mockCommandExecutor struct {
Output []byte
Err error
}
func (f mockCommandExecutor) CombinedOutput(ctx context.Context, name string, args ...string) ([]byte, error) {
return f.Output, f.Err
}
func TestVerifyWithOPKSSH(t *testing.T) {
sshCert := []byte(`ssh-rsa-cert-v01@openssh.com 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 host.example.com`)
key, _, _, _, err := ssh.ParseAuthorizedKey(sshCert) //nolint:dogsled
require.NoError(t, err)
cert, ok := key.(*ssh.Certificate)
require.True(t, ok)
c := Configuration{}
c.executor = mockCommandExecutor{
Err: errors.New("test error"),
}
err = c.verifyWithOPKSSH("user", cert)
assert.Error(t, err)
c.executor = mockCommandExecutor{}
err = c.verifyWithOPKSSH("", cert)
assert.Error(t, err)
c.executor = mockCommandExecutor{
Output: ssh.MarshalAuthorizedKey(cert),
}
err = c.verifyWithOPKSSH("", cert)
assert.Error(t, err)
c.executor = mockCommandExecutor{
Output: ssh.MarshalAuthorizedKey(cert.SignatureKey),
}
err = c.verifyWithOPKSSH("", cert)
assert.NoError(t, err)
}

134
internal/sftpd/server.go
View File

@@ -16,6 +16,8 @@ package sftpd
import (
"bytes"
"context"
"crypto/sha256"
"encoding/hex"
"encoding/json"
"errors"
@@ -25,6 +27,7 @@ import (
"maps"
"net"
"os"
"os/exec"
"path/filepath"
"runtime/debug"
"slices"
@@ -82,10 +85,20 @@ var (
revokedCertManager = revokedCertificates{
certs: map[string]bool{},
}
sftpAuthError = newAuthenticationError(nil, "", "")
)
type commandExecutor interface {
CombinedOutput(ctx context.Context, name string, args ...string) ([]byte, error)
}
type defaultExecutor struct{}
func (d defaultExecutor) CombinedOutput(ctx context.Context, name string, args ...string) ([]byte, error) {
cmd := exec.CommandContext(ctx, name, args...)
cmd.Env = []string{}
return cmd.CombinedOutput()
}
// Binding defines the configuration for a network listener
type Binding struct {
// The address to listen on. A blank value means listen on all available network interfaces.
@@ -150,6 +163,10 @@ type Configuration struct {
// Example content:
// ["SHA256:bsBRHC/xgiqBJdSuvSTNpJNLTISP/G356jNMCRYC5Es","SHA256:119+8cL/HH+NLMawRsJx6CzPF1I3xC+jpM60bQHXGE8"]
RevokedUserCertsFile string `json:"revoked_user_certs_file" mapstructure:"revoked_user_certs_file"`
// Absolute path to the opkssh binary used for OpenPubkey SSH integration
OPKSSHPath string `json:"opkssh_path" mapstructure:"opkssh_path"`
// Expected SHA256 checksum of the opkssh binary. It is verified at application startup
OPKSSHChecksum string `json:"opkssh_checksum" mapstructure:"opkssh_checksum"`
// LoginBannerFile the contents of the specified file, if any, are sent to
// the remote user before authentication is allowed.
LoginBannerFile string `json:"login_banner_file" mapstructure:"login_banner_file"`
@@ -185,6 +202,7 @@ type Configuration struct {
PasswordAuthentication bool `json:"password_authentication" mapstructure:"password_authentication"`
certChecker *ssh.CertChecker
parsedUserCAKeys []ssh.PublicKey
executor commandExecutor
}
type authenticationError struct {
@@ -342,6 +360,7 @@ func (c *Configuration) loadFromProvider() error {
// Initialize the SFTP server and add a persistent listener to handle inbound SFTP connections.
func (c *Configuration) Initialize(configDir string) error {
c.executor = defaultExecutor{}
if err := c.loadFromProvider(); err != nil {
return fmt.Errorf("unable to load configs from provider: %w", err)
}
@@ -365,6 +384,9 @@ func (c *Configuration) Initialize(configDir string) error {
if err := c.initializeCertChecker(configDir); err != nil {
return err
}
if err := c.initializeOPKSSH(); err != nil {
return err
}
c.configureKeyboardInteractiveAuth(serverConfig)
c.configureLoginBanner(serverConfig, configDir)
c.checkSSHCommands()
@@ -1069,6 +1091,49 @@ func (c *Configuration) loadHostCertificates(configDir string) ([]hostCertificat
return certs, nil
}
func (c *Configuration) initializeOPKSSH() error {
if c.OPKSSHPath != "" {
if len(c.parsedUserCAKeys) > 0 {
return errors.New("opkssh and certificate authorities are mutually exclusive")
}
if !util.IsFileInputValid(c.OPKSSHPath) || !filepath.IsAbs(c.OPKSSHPath) {
return fmt.Errorf("opkssh path %q is not valid, it must be an absolute path", c.OPKSSHPath)
}
if c.OPKSSHChecksum == "" {
if _, err := os.Stat(c.OPKSSHPath); err != nil {
return fmt.Errorf("error validating opkssh path %q: %w", c.OPKSSHPath, err)
}
} else {
if err := util.VerifyFileChecksum(c.OPKSSHPath, sha256.New(), c.OPKSSHChecksum, 100*1024*1024); err != nil {
return fmt.Errorf("error validating opkssh checksum: %w", err)
}
}
}
return nil
}
func (c *Configuration) verifyWithOPKSSH(username string, cert *ssh.Certificate) error {
ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
defer cancel()
args := []string{"verify", username, util.BytesToString(ssh.MarshalAuthorizedKey(cert)), cert.Type()}
out, err := c.executor.CombinedOutput(ctx, c.OPKSSHPath, args...)
if err != nil {
logger.Debug(logSender, "", "unable to execute opk verifier: %s", string(out))
return fmt.Errorf("unable to execute opk verifier: %w", err)
}
pubKey, _, _, _, err := ssh.ParseAuthorizedKey(out) //nolint:dogsled
if err != nil {
logger.Debug(logSender, "", "unable to validate the opk verifier output: %s", string(out))
return fmt.Errorf("unable to validate the opk verifier output: %w", err)
}
if !bytes.Equal(pubKey.Marshal(), cert.SignatureKey.Marshal()) {
return errors.New("unable to validate opk result")
}
return nil
}
func (c *Configuration) initializeCertChecker(configDir string) error {
for _, keyPath := range c.TrustedUserCAKeys {
keyPath = strings.TrimSpace(keyPath)
@@ -1144,34 +1209,43 @@ func (c *Configuration) validatePublicKeyCredentials(conn ssh.ConnMetadata, pubK
var certFingerprint string
if ok {
certFingerprint = ssh.FingerprintSHA256(cert.Key)
if cert.CertType != ssh.UserCert {
err := fmt.Errorf("ssh: cert has type %d", cert.CertType)
user.Username = conn.User()
updateLoginMetrics(&user, ipAddr, method, err)
return nil, err
}
if !c.certChecker.IsUserAuthority(cert.SignatureKey) {
err := errors.New("ssh: certificate signed by unrecognized authority")
user.Username = conn.User()
updateLoginMetrics(&user, ipAddr, method, err)
return nil, err
}
if len(cert.ValidPrincipals) == 0 {
err := fmt.Errorf("ssh: certificate %s has no valid principals, user: \"%s\"", certFingerprint, conn.User())
user.Username = conn.User()
updateLoginMetrics(&user, ipAddr, method, err)
return nil, err
}
if revokedCertManager.isRevoked(certFingerprint) {
err := fmt.Errorf("ssh: certificate %s is revoked", certFingerprint)
user.Username = conn.User()
updateLoginMetrics(&user, ipAddr, method, err)
return nil, err
}
if err := c.certChecker.CheckCert(conn.User(), cert); err != nil {
user.Username = conn.User()
updateLoginMetrics(&user, ipAddr, method, err)
return nil, err
if c.OPKSSHPath != "" {
if err := c.verifyWithOPKSSH(conn.User(), cert); err != nil {
err := fmt.Errorf("ssh: verification with OPK failed: %v", err)
user.Username = conn.User()
updateLoginMetrics(&user, ipAddr, method, err)
return nil, err
}
} else {
if cert.CertType != ssh.UserCert {
err := fmt.Errorf("ssh: cert has type %d", cert.CertType)
user.Username = conn.User()
updateLoginMetrics(&user, ipAddr, method, err)
return nil, err
}
if !c.certChecker.IsUserAuthority(cert.SignatureKey) {
err := errors.New("ssh: certificate signed by unrecognized authority")
user.Username = conn.User()
updateLoginMetrics(&user, ipAddr, method, err)
return nil, err
}
if len(cert.ValidPrincipals) == 0 {
err := fmt.Errorf("ssh: certificate %s has no valid principals, user: \"%s\"", certFingerprint, conn.User())
user.Username = conn.User()
updateLoginMetrics(&user, ipAddr, method, err)
return nil, err
}
if revokedCertManager.isRevoked(certFingerprint) {
err := fmt.Errorf("ssh: certificate %s is revoked", certFingerprint)
user.Username = conn.User()
updateLoginMetrics(&user, ipAddr, method, err)
return nil, err
}
if err := c.certChecker.CheckCert(conn.User(), cert); err != nil {
user.Username = conn.User()
updateLoginMetrics(&user, ipAddr, method, err)
return nil, err
}
}
certPerm = &cert.Permissions
}

11
internal/sftpd/sftpd_test.go
View File

@@ -488,6 +488,17 @@ func TestInitialization(t *testing.T) {
assert.NoError(t, err)
sftpdConf.HostKeys = nil
sftpdConf.HostCertificates = nil
sftpdConf.OPKSSHPath = "relative path"
err = sftpdConf.Initialize(configDir)
assert.Error(t, err)
sftpdConf.OPKSSHPath = filepath.Join(os.TempDir(), "missing path")
err = sftpdConf.Initialize(configDir)
assert.Error(t, err)
sftpdConf.OPKSSHChecksum = "invalid checksum"
err = sftpdConf.Initialize(configDir)
assert.Error(t, err)
sftpdConf.OPKSSHPath = ""
sftpdConf.OPKSSHChecksum = ""
sftpdConf.RevokedUserCertsFile = "."
err = sftpdConf.Initialize(configDir)
assert.Error(t, err)

39
internal/util/util.go
View File

@@ -23,6 +23,7 @@ import (
"crypto/rand"
"crypto/rsa"
"crypto/sha256"
"crypto/subtle"
"crypto/tls"
"crypto/x509"
"encoding/hex"
@@ -30,6 +31,7 @@ import (
"encoding/pem"
"errors"
"fmt"
"hash"
"io"
"io/fs"
"math"
@@ -930,3 +932,40 @@ func SlicesEqual(s1, s2 []string) bool {
return true
}
// VerifyFileChecksum computes the hash of the given file using the provided
// hash algorithm and compares it against the expected checksum (in hex format).
// It returns an error if the checksum does not match or if the operation fails.
func VerifyFileChecksum(filePath string, h hash.Hash, expectedHex string, maxSize int64) error {
expected, err := hex.DecodeString(expectedHex)
if err != nil {
return fmt.Errorf("invalid checksum %q: %w", expectedHex, err)
}
f, err := os.Open(filePath)
if err != nil {
return err
}
defer f.Close()
if maxSize > 0 {
fi, err := f.Stat()
if err != nil {
return err
}
if fi.Size() > maxSize {
return fmt.Errorf("file too large: %s", ByteCountIEC(fi.Size()))
}
}
if _, err := io.Copy(h, f); err != nil {
return err
}
actual := h.Sum(nil)
if subtle.ConstantTimeCompare(actual, expected) != 1 {
return errors.New("checksum mismatch")
}
return nil
}

2
sftpgo.json
View File

@@ -100,6 +100,8 @@
"public_key_algorithms": [],
"trusted_user_ca_keys": [],
"revoked_user_certs_file": "",
"opkssh_path": "",
"opkssh_checksum": "",
"login_banner_file": "",
"enabled_ssh_commands": [
"md5sum",