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.
- Citrix Workspace App for Linux versions 2212 and 2302.
Other versions are likely affected.
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.
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
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.
This is an example of CWE-532: Insertion of Sensitive Information into Log File.
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.
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: Submitted to Full Disclosure mailing list.
2023-01-16: Submitted to VulDB, assigned VDB-218413.