Citrix Linux client credential leak

The Citrix Linux client emits its session credentials when starting a Citrix
session. These credentials end up being recorded in the client’s system log.

Citrix do not consider this to be a security vulnerability.

Software affected

• Citrix Workspace App for Linux versions 2212.

Other versions are likely affected.

Context

When connecting to a Citrix session via a web browser such as Firefox on Linux,
typically you access a web application known as Citrix Storefront. This
provides clickable icons for the applications and remote desktop sessions
available to you.

When you click on one of these, your browser is instructed to open a URL of the
form receiver://..... which is handled using /opt/Citrix/ICAClient/util/ctxwebhelper.
ctxwebhelper parses the URL and uses the decoded information to make a HTTP
GET request to the remote server for an ‘ica’ file, which contains the
connection details necessary to launch the Citrix client software,
/opt/Citrix/ICAClient/wfica.

The ICA file contains details such as the server hostname and temporary session
credentials needed to authenticate the session.

The issue

When making the GET request to retrieve the ICA file, ctxwebhelper echos the
full HTTP response (headers & body) to standard output, which ends up feeding
into journald and then into the system log files.

This can be demonstrated by connecting to a Citrix session and running:

grep receiver\\.desktop.*LogonTicket= /var/log/syslog

which will produce output such as

2023-01-12T11:15:46.816466+00:00 myhostname receiver.desktop[9999]: LogonTicket=1234567890ABCDEF1234567890ABCD

Vendor response

Citrix responded to my report on 2023-01-05 to say they do not consider this a product vulnerability:

Thank you for bearing with us. We have concluded the security  
investigation into the reported issue and determined that the contents  
of /var/log/syslog can only be read or written by root user, or a  
syslog user or an adm group but not by an unprivileged user. As a  
result, we do not consider this finding as a vulnerability in the  
product.

We would like to thank you for submitting the finding and helping to  
keep Citrix customers safe.

Best Regards,

Citrix Security Response Team

This is short-sighted in my opinion - logs should not be considered safe places
to store credentials, even temporary ones.

Workaround

Since Citrix do not consider this a vulnerability it seems unlikely this behaviour will change.

You can work around this issue by replacing ctxwebhelper with a wrapper script
that either discards or filters its output.

First, rename ctxwebhelper:

mv /opt/Citrix/ICAClient/util/ctxwebhelper /opt/Citrix/ICAClient/util/ctxwebhelper.real

Next, place a script in its place, which first redirects stdout and stderr to /dev/null before executing the real ctxwebhelper:

#!/bin/bash  
set -eu  
exec &>/dev/null  
"$(dirname "$0")"/ctxwebhelper.real "$@"

Don’t forget to chmod +x /opt/Citrix/ICAClient/util/ctxwebhelper after doing this.

This script is available from this repository - see ctxwebhelper.wrapper.

Note that this will be overwritten if the Citrix client is reinstalled.

Timeline

2022-12-11: Issue disclosed to Citrix via email to [email protected]

2022-12-13: Citrix acknowledges receipt of the report, assigns identifier CASE-8324.

2023-01-05: Citrix reponds to say they do not consider it a vulnerability.

2023-01-07: Reply to Citrix requesting they reconsider their assessment.

2023-01-14: Public disclosure.

Author

Russell Howe. Github Twitter.

ctxwebhelper.wrapper:

#!/bin/bash

Brexit flags

set -eu

Ensure stdout and stderr are discarded

exec &>/dev/null

Execute the real ctxwebhelper

"$(dirname “$0”)"/ctxwebhelper.real “$@”
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