# 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 secure@citrix.com 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](https://github.com/rhowe) [Twitter](https://twitter.com/rhowe212). ctxwebhelper.wrapper: #!/bin/bash # Brexit flags set -eu # Ensure stdout and stderr are discarded exec &>/dev/null # Execute the real ctxwebhelper "$(dirname "$0")"/ctxwebhelper.real "$@" Footer