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ssh: add server side multi-step authentication
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Add support for sending back partial success to the client while
handling authentication in the server. This is implemented by a special
error that can be returned by any of the authentication methods, which
contains the authentication methods to offer next.

This patch is based on CL 399075 with some minor changes and the
addition of test cases.

Fixes golang/go#17889
Fixes golang/go#61447
Fixes golang/go#64974

Co-authored-by: Peter Verraedt <[email protected]>
Change-Id: I05c8f913bb407d22c2e41c4cbe965e36ab4739b0
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/516355
Reviewed-by: Andrew Lytvynov <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>
Auto-Submit: Filippo Valsorda <[email protected]>
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@drakkan @peterverraedt @gopherbot
2 people authored and gopherbot committed Apr 3, 2024
1 parent 8d0d405 commit 6f79b5a
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Showing 2 changed files with 530 additions and 50 deletions.
168 changes: 118 additions & 50 deletions ssh/server.go
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Original file line number Diff line number Diff line change
Expand Up @@ -426,6 +426,35 @@ func (l ServerAuthError) Error() string {
return "[" + strings.Join(errs, ", ") + "]"
}

// ServerAuthCallbacks defines server-side authentication callbacks.
type ServerAuthCallbacks struct {
// PasswordCallback behaves like [ServerConfig.PasswordCallback].
PasswordCallback func(conn ConnMetadata, password []byte) (*Permissions, error)

// PublicKeyCallback behaves like [ServerConfig.PublicKeyCallback].
PublicKeyCallback func(conn ConnMetadata, key PublicKey) (*Permissions, error)

// KeyboardInteractiveCallback behaves like [ServerConfig.KeyboardInteractiveCallback].
KeyboardInteractiveCallback func(conn ConnMetadata, client KeyboardInteractiveChallenge) (*Permissions, error)

// GSSAPIWithMICConfig behaves like [ServerConfig.GSSAPIWithMICConfig].
GSSAPIWithMICConfig *GSSAPIWithMICConfig
}

// PartialSuccessError can be returned by any of the [ServerConfig]
// authentication callbacks to indicate to the client that authentication has
// partially succeeded, but further steps are required.
type PartialSuccessError struct {
// Next defines the authentication callbacks to apply to further steps. The
// available methods communicated to the client are based on the non-nil
// ServerAuthCallbacks fields.
Next ServerAuthCallbacks
}

func (p *PartialSuccessError) Error() string {
return "ssh: authenticated with partial success"
}

// ErrNoAuth is the error value returned if no
// authentication method has been passed yet. This happens as a normal
// part of the authentication loop, since the client first tries
Expand All @@ -441,6 +470,15 @@ func (s *connection) serverAuthenticate(config *ServerConfig) (*Permissions, err
authFailures := 0
var authErrs []error
var displayedBanner bool
partialSuccessReturned := false
// Set the initial authentication callbacks from the config. They can be
// changed if a PartialSuccessError is returned.
authConfig := ServerAuthCallbacks{
PasswordCallback: config.PasswordCallback,
PublicKeyCallback: config.PublicKeyCallback,
KeyboardInteractiveCallback: config.KeyboardInteractiveCallback,
GSSAPIWithMICConfig: config.GSSAPIWithMICConfig,
}

userAuthLoop:
for {
Expand Down Expand Up @@ -471,6 +509,11 @@ userAuthLoop:
return nil, errors.New("ssh: client attempted to negotiate for unknown service: " + userAuthReq.Service)
}

if s.user != userAuthReq.User && partialSuccessReturned {
return nil, fmt.Errorf("ssh: client changed the user after a partial success authentication, previous user %q, current user %q",
s.user, userAuthReq.User)
}

s.user = userAuthReq.User

if !displayedBanner && config.BannerCallback != nil {
Expand All @@ -491,20 +534,17 @@ userAuthLoop:

switch userAuthReq.Method {
case "none":
if config.NoClientAuth {
// We don't allow none authentication after a partial success
// response.
if config.NoClientAuth && !partialSuccessReturned {
if config.NoClientAuthCallback != nil {
perms, authErr = config.NoClientAuthCallback(s)
} else {
authErr = nil
}
}

// allow initial attempt of 'none' without penalty
if authFailures == 0 {
authFailures--
}
case "password":
if config.PasswordCallback == nil {
if authConfig.PasswordCallback == nil {
authErr = errors.New("ssh: password auth not configured")
break
}
Expand All @@ -518,17 +558,17 @@ userAuthLoop:
return nil, parseError(msgUserAuthRequest)
}

perms, authErr = config.PasswordCallback(s, password)
perms, authErr = authConfig.PasswordCallback(s, password)
case "keyboard-interactive":
if config.KeyboardInteractiveCallback == nil {
if authConfig.KeyboardInteractiveCallback == nil {
authErr = errors.New("ssh: keyboard-interactive auth not configured")
break
}

prompter := &sshClientKeyboardInteractive{s}
perms, authErr = config.KeyboardInteractiveCallback(s, prompter.Challenge)
perms, authErr = authConfig.KeyboardInteractiveCallback(s, prompter.Challenge)
case "publickey":
if config.PublicKeyCallback == nil {
if authConfig.PublicKeyCallback == nil {
authErr = errors.New("ssh: publickey auth not configured")
break
}
Expand Down Expand Up @@ -562,11 +602,18 @@ userAuthLoop:
if !ok {
candidate.user = s.user
candidate.pubKeyData = pubKeyData
candidate.perms, candidate.result = config.PublicKeyCallback(s, pubKey)
if candidate.result == nil && candidate.perms != nil && candidate.perms.CriticalOptions != nil && candidate.perms.CriticalOptions[sourceAddressCriticalOption] != "" {
candidate.result = checkSourceAddress(
candidate.perms, candidate.result = authConfig.PublicKeyCallback(s, pubKey)
_, isPartialSuccessError := candidate.result.(*PartialSuccessError)

if (candidate.result == nil || isPartialSuccessError) &&
candidate.perms != nil &&
candidate.perms.CriticalOptions != nil &&
candidate.perms.CriticalOptions[sourceAddressCriticalOption] != "" {
if err := checkSourceAddress(
s.RemoteAddr(),
candidate.perms.CriticalOptions[sourceAddressCriticalOption])
candidate.perms.CriticalOptions[sourceAddressCriticalOption]); err != nil {
candidate.result = err
}
}
cache.add(candidate)
}
Expand All @@ -578,8 +625,8 @@ userAuthLoop:
if len(payload) > 0 {
return nil, parseError(msgUserAuthRequest)
}

if candidate.result == nil {
_, isPartialSuccessError := candidate.result.(*PartialSuccessError)
if candidate.result == nil || isPartialSuccessError {
okMsg := userAuthPubKeyOkMsg{
Algo: algo,
PubKey: pubKeyData,
Expand Down Expand Up @@ -629,11 +676,11 @@ userAuthLoop:
perms = candidate.perms
}
case "gssapi-with-mic":
if config.GSSAPIWithMICConfig == nil {
if authConfig.GSSAPIWithMICConfig == nil {
authErr = errors.New("ssh: gssapi-with-mic auth not configured")
break
}
gssapiConfig := config.GSSAPIWithMICConfig
gssapiConfig := authConfig.GSSAPIWithMICConfig
userAuthRequestGSSAPI, err := parseGSSAPIPayload(userAuthReq.Payload)
if err != nil {
return nil, parseError(msgUserAuthRequest)
Expand Down Expand Up @@ -689,49 +736,70 @@ userAuthLoop:
break userAuthLoop
}

authFailures++
if config.MaxAuthTries > 0 && authFailures >= config.MaxAuthTries {
// If we have hit the max attempts, don't bother sending the
// final SSH_MSG_USERAUTH_FAILURE message, since there are
// no more authentication methods which can be attempted,
// and this message may cause the client to re-attempt
// authentication while we send the disconnect message.
// Continue, and trigger the disconnect at the start of
// the loop.
//
// The SSH specification is somewhat confusing about this,
// RFC 4252 Section 5.1 requires each authentication failure
// be responded to with a respective SSH_MSG_USERAUTH_FAILURE
// message, but Section 4 says the server should disconnect
// after some number of attempts, but it isn't explicit which
// message should take precedence (i.e. should there be a failure
// message than a disconnect message, or if we are going to
// disconnect, should we only send that message.)
//
// Either way, OpenSSH disconnects immediately after the last
// failed authnetication attempt, and given they are typically
// considered the golden implementation it seems reasonable
// to match that behavior.
continue
var failureMsg userAuthFailureMsg

if partialSuccess, ok := authErr.(*PartialSuccessError); ok {
// After a partial success error we don't allow changing the user
// name and execute the NoClientAuthCallback.
partialSuccessReturned = true

// In case a partial success is returned, the server may send
// a new set of authentication methods.
authConfig = partialSuccess.Next

// Reset pubkey cache, as the new PublicKeyCallback might
// accept a different set of public keys.
cache = pubKeyCache{}

// Send back a partial success message to the user.
failureMsg.PartialSuccess = true
} else {
// Allow initial attempt of 'none' without penalty.
if authFailures > 0 || userAuthReq.Method != "none" {
authFailures++
}
if config.MaxAuthTries > 0 && authFailures >= config.MaxAuthTries {
// If we have hit the max attempts, don't bother sending the
// final SSH_MSG_USERAUTH_FAILURE message, since there are
// no more authentication methods which can be attempted,
// and this message may cause the client to re-attempt
// authentication while we send the disconnect message.
// Continue, and trigger the disconnect at the start of
// the loop.
//
// The SSH specification is somewhat confusing about this,
// RFC 4252 Section 5.1 requires each authentication failure
// be responded to with a respective SSH_MSG_USERAUTH_FAILURE
// message, but Section 4 says the server should disconnect
// after some number of attempts, but it isn't explicit which
// message should take precedence (i.e. should there be a failure
// message than a disconnect message, or if we are going to
// disconnect, should we only send that message.)
//
// Either way, OpenSSH disconnects immediately after the last
// failed authnetication attempt, and given they are typically
// considered the golden implementation it seems reasonable
// to match that behavior.
continue
}
}

var failureMsg userAuthFailureMsg
if config.PasswordCallback != nil {
if authConfig.PasswordCallback != nil {
failureMsg.Methods = append(failureMsg.Methods, "password")
}
if config.PublicKeyCallback != nil {
if authConfig.PublicKeyCallback != nil {
failureMsg.Methods = append(failureMsg.Methods, "publickey")
}
if config.KeyboardInteractiveCallback != nil {
if authConfig.KeyboardInteractiveCallback != nil {
failureMsg.Methods = append(failureMsg.Methods, "keyboard-interactive")
}
if config.GSSAPIWithMICConfig != nil && config.GSSAPIWithMICConfig.Server != nil &&
config.GSSAPIWithMICConfig.AllowLogin != nil {
if authConfig.GSSAPIWithMICConfig != nil && authConfig.GSSAPIWithMICConfig.Server != nil &&
authConfig.GSSAPIWithMICConfig.AllowLogin != nil {
failureMsg.Methods = append(failureMsg.Methods, "gssapi-with-mic")
}

if len(failureMsg.Methods) == 0 {
return nil, errors.New("ssh: no authentication methods configured but NoClientAuth is also false")
return nil, errors.New("ssh: no authentication methods available")
}

if err := s.transport.writePacket(Marshal(&failureMsg)); err != nil {
Expand Down
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