Authentication

Introduction

Version 1.0 also supports SSL (see #1252) which can 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).

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 mode

For more information on the different types of connections, see wiki/Network.

Modules

The authentication modules used are specified using

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

etc

With version 2.3 and later, you may specify more than one module. ie: --tcp-auth=hosts --tcp-auth=sqlite will verify "TCP Wrappers" before checking the sqlite database. (see #1728 for details)

Here are the modules that can be used:

Module	Result	Purpose	Version requirements
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 below
file	compares the password against the contents of a password file, see 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	see ticket:1488#comment:37	>=2.1
peercred	SO_PEERCRED authentication	see r15886	>=2.1
hosts	​TCP Wrapper	see #1730	>=2.3
exec	Delegates to an external command	see ticket:1690#comment:4	>=2.3

Module Options

The same module may be used with different types of sockets (tcp-auth vs auth), each with a different set of options. For more details see ticket:1159#comment:1.

Some examples:

• XPRA_PASSWORD=mysecret xpra start --auth=env
• SOME_OTHER_ENV_VAR_NAME=mysecret xpra start --auth=env:name=SOME_OTHER_ENV_VAR_NAME
• xpra start --auth=password:value=mysecret
• xpra start --auth=file:filename=/path/to/mypasswordfile.txt
• xpra start --auth=multifile:filename=/path/to/userlist.txt

Beware when mixing environment variables and password files as the latter may contain a trailing newline character whereas the former often do not. The most common used option is the file / multifile filename option, see below.

Password File

"file" vs "multifile":

• "file" contains a single password, the whole file is the password
• "multifile" contains a list of authentication values, see proxy server file authentication - this module is deprecated in favour of "sqlite" which is easier to configure.

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 but a warning will be printed
• If the client is not configured for authentication, the connection will fail with an authentication error

Notes

• This information applies to all clients: 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)
• The "sys" authentication modules (pam and win32) require the actual password to be sent across to perform the authentication on the server - they therefore use the weak "xor" hashing
• You must use wiki/Encryption to be able to use "xor" hashing so that the password is protected during the exchange: the system will refuse to send "xor" hashed password unencrypted
• Encryption is processed before authentication

For more information on packets, see wiki/NetworkProtocol.

Negotiation

• All clients first send a hello packet to the server. If the client expects the server to request authentication for the connection, the client packet may omit most of the regular configuration information since a second packet will need to be sent. Until the server accepts the connection with its own hello packet response, the only packets that will be accepted by the clients are challenge and set_deflate (used to control packet compression). The client will exit unless the server responds within the XPRA_SOCKET_TIMEOUT delay + 10 seconds.
• The server sends back a challenge packet containing a random salt and the digest method to use as specified by the authentication module. If the client does not respond within the XPRA_SOCKET_TIMEOUT delay (defaults to 10 seconds), it is disconnected. The only packets that will be accepted by the server until the client has successfully authenticated are hello and disconnect.
• The client generates its own random salt and responds with a new "hello" packet containing the challenge response (the hashed password and the client salt used), the client and server salts are combined before hashing the password. (so there is no way for the server or client to predict the actual salt used)
• When the server receives the hello packet containing the challenge response, it hands it over to the authentication module, which can then:
  • with hmac: verify that it obtains the same hash by combining its own password value with the two salts
  • with xor: xor the password again with the two salts to retrieve the original password value
• these steps may be repeated if there is more than one authentication module configured (though not all authentication modules require a challenge exchange)

Security Considerations

• 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, Encryption does
• Enabling auth wiki/Logging may leak some authentication information
• if you are concerned about security, use SSH as transport instead