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Authentication.md

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Authentication Authentication

Xpra's authentication modules can be useful for:

  • securing socket connections
  • making the unix domain socket accessible to other users safely
  • using the proxy server

For more information on the different types of connections, see network. For more generic security information, please see security considerations

SSL mode can also be used for authentication using certificates (see #1252)

When using SSH to connect to a server, encryption and authentication can be skipped: by default the unix domain sockets used by ssh do not use authentication.

Server Syntax

Starting with version 4.0, the preferred way of specifying authentication is within the socket option itself.
ie for starting a seamless server with a TCP socket protected by a password stored in a file:

xpra start --start=xterm -d auth
     --bind-tcp=0.0.0.0:10000,auth=file,filename=password.txt

So that multiple sockets can use different authentication modules, and those modules can more easily be chained:

xpra start --start=xterm -d auth \
     --bind-tcp=0.0.0.0:10000,auth=hosts,auth=file,filename=password.txt \
     --bind-tcp=0.0.0.0:10001,auth=sys

Server Authentication Modules

Xpra supports many authentication modules. Some of these modules require extra dependencies.

server authentication modules
Module Result Purpose
allow always allows the user to login, the username used is the one supplied by the client dangerous / only for testing
none always allows the user to login, the username used is the one the server is running as dangerous / only for testing
fail always fails authentication, no password required useful for testing
reject always fails authentication, pretends to ask for a password useful for testing
env matches against an environment variable (XPRA_PASSWORD by default) alternative to file module
password matches against a password given as a module option, ie: auth=password,value=mysecret alternative to file module
multifile matches usernames and passwords against an authentication file proxy: see password-file below
file compares the password against the contents of a password file, see password-file below simple password authentication
pam linux PAM authentication Linux system authentication
win32 win32security authentication MS Windows system authentication
sys system authentication virtual module which will choose win32 or pam authentication automatically
sqlite sqlite database authentication #1488
capability matches values in the capabilities supplied by the client #3575
peercred SO_PEERCRED authentication #1524
tcp hosts TCP Wrapper #1730
exec Delegates to an external command #1690
kerberos-password Uses kerberos to authenticate a username + password #1691
kerberos-token Uses a kerberos ticket to authenticate a client #1691
gss_auth Uses a GSS ticket to authenticate a client #1691
keycloak Uses a keycloak token to authenticate a client #3334
ldap Uses ldap via python-ldap #1791
ldap3 Uses ldap via python-ldap3 #1791
u2f Universal 2nd Factor #1789
more examples
  • XPRA_PASSWORD=mysecret xpra start --bind-tcp=0.0.0.0:10000,auth=env
  • SOME_OTHER_ENV_VAR_NAME=mysecret xpra start --bind-tcp=0.0.0.0:10000,auth=env,name=SOME_OTHER_ENV_VAR_NAME
  • xpra start --bind-tcp=0.0.0.0:10000,auth=password,value=mysecret
  • xpra start --bind-tcp=0.0.0.0:10000,auth=file,filename=/path/to/mypasswordfile.txt
  • xpra start --bind-tcp=0.0.0.0:10000,auth=sqlite,filename=/path/to/userlist.sdb

Beware when mixing environment variables and password files as the latter may contain a trailing newline character whereas the former often do not.

syntax for older versions

The syntax with older versions used a dedicated switch for each socket type:

  • --auth=MODULE for unix domain sockets and named pipes
  • --tcp-auth=MODULE for TCP sockets
  • --vsock-auth=MODULE for vsock (#983) etc

For more information on the different socket types, see network examples

Client Syntax

By default, challenge-handlers=all which means that the python client will try all authentication handlers available until one succeeds. If the server is configured with multiple authentications modules for the same socket, the client will do the same.

Basic examples

Authenticating as username foo with password bar using the URI:

xpra attach tcp://foo:bar@host:port/

For a more secure option, storing the password value in a file, with debugging enabled:

echo -n "foo" > ./password.txt
xpra attach tcp://host:port/ --challenge-handlers=file:filename=./password.txt --debug auth
client challenge handlers
Module Behaviour and options
env name specifies the environment variable containing the password
defaults to XPRA_PASSWORD
file filename specifies the file containing the passowrd
gss use gss-services to specify the name of the security context
kerberos kerberos-services specifies the valid kerberos services to connect to
the wildcard * may be used
prompt GUI clients should see a dialog, console users a text prompt
u2f APP_ID specifies the u2f authentication application ID
uri Uses values parsed from the connection string, ie: tcp://foo:bar@host

Password File

  • with the file module, the password-file contains a single password, the whole file is the password (including any trailing newline characters). To write a password to a file without the trailing newline character, you can use echo -n "thepassword" > password.txt
  • with multifile, the password-file contains a list of authentication values, see proxy server - this module is deprecated in favour of the sqlite module which is much easier to configure

Usernames

The username can be specified:

  • in the connection files you can save from the launcher
  • in the client connection string
tcp example 
xpra attach tcp://username:password@host:port/

When an authentication module is used to secure a single session, many modules will completely ignore the username part, and it can be omitted from the connection string.

example: specifying the password only

for connecting to the TCP socket and specifying the password only:

xpra attach tcp://:password@host:port/

Since the username is ignored, it can also be replaced with any string of your liking, ie using foobar here:

xpra attach tcp://foobar:password@host:port/

Only the following modules will make use of both the username and password to authenticate against their respective backend: kerberos-password, ldap, ldap3, sys (pam and win32), sqlite, multifile and u2f. In this case, using an invalid username will cause the authentication to fail.

The username is usually more relevant when authenticating against a proxy server (see authentication details there).

Development Documentation

Authentication Process

The steps below assume that the client and server have been configured to use authentication:

  • if the server is not configured for authentication, the client connection should be accepted and a warning will be printed
  • if the client is not configured for authentication, a password dialog may show up, and the connection will fail with an authentication error if the correct value is not supplied
  • if multiple authentication modules are specified, the client may bring up multiple authentication dialogs
  • how the client handles the challenges sent by the server can be configured using the challenge-handlers option, by default the client will try the following handlers in the specified order: uri (whatever password may have been specified in the connection string), file (if the password-file option was used), env (if the environment variable is present), kerberos, gss, keycloak, u2f and finally prompt
module and platform specific notes
  • this information applies to all clients except the HTML5 client: regular GUI clients as well as command line clients like xpra info
  • each authentication module specifies the type of password hashing it supports (usually HMAC)
  • some authentication modules (pam, win32, kerberos-password, ldap and ldap3) require the actual password to be sent across to perform the authentication on the server - they therefore use the weak xor hashing, which is insecure
  • you must use encryption to be able to use xor hashing so that the password is protected during the exchange: the system will refuse to send a xor hashed password unencrypted
  • encryption is processed before authentication
  • when used over TCP sockets, password authentication is vulnerable to man-in-the-middle attacks where an attacker could intercept the initial exchange and use the stolen authentication challenge response to access the session, encryption prevents that
  • the client does not verify the authenticity of the server, using encryption effectively does
  • enabling auth debug logging may leak some authentication information
  • if you are concerned about security, use SSH as transport instead

For more information on packets, see network.

Salt handling is important
  • 64-bit entropy is nowhere near enough against a serious attacker: If you want to defend against rainbow tables, salts are inevitable, because you need a full rainbow table per unique salt, which is computationally and storage-wise intense
  • SHA-512 w/ per User Salts is Not Enough: In the event the hash was disclosed or the database was compromised, the attacker will already have one of the two values (i.e. the salt), used to construct the hash
  • about hmac: _Those people should know that HMAC is as easy to precompute as naked SHA1 is; you can "rainbow-table" HMAC_* and we did get it wrong before...