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Xymon Systems and Network Monitor - remote file deletion vulnerability


a security vulnerability has been found in version 4.x of the Xymon Systems & Network Monitor tool (https://sourceforge.net/projects/xymon/).

The error permits a remote attacker to delete files on the server running the Xymon trend-data daemon "xymond_rrd". File deletion is done with the privileges of the user that Xymon is running with, so it is limited to files available to the userid running the Xymon service. This includes all historical data stored by the Xymon monitoring system.

Vulnerable versions
All Xymon 4.x versions prior to 4.3.12 with the xymond_rrd module enabled (this is the default configuration).

Note that Xymon was called "Hobbit" from version 4.0 to 4.2; all of the "Hobbit" versions are also vulnerable.

Mitigating factors
The attack requires access to the xymond network port (default: tcp port 1984).

If access to administrative commands is limited by use of the "--admin-senders" option for the "xymond" daemon, then the attack is restricted to the commands sent from the IP-adresses listed in the --admin-senders access list. However, the default configuration permits these commands to be sent from any IP.

Systems where xymond_rrd is disabled are not vulnerable, but this is not the default configuration.

Xymon stores historical data, trend-data etc. for each monitored host in a set of directories below the Xymon "server/data/" directory. Each monitored host has a set of directories named by the hostname.

When a host is no longer monitored, the data stored for the host can be removed by sending a "drop HOSTNAME" command to the Xymon master daemon. This is forwarded to xymond_rrd and other modules which then handle deleting various parts of the stored data, essentially by performing the equivalent of "rm -rf <xymondatadirectory>/rrd/HOSTNAME". In the vulnerable versions of Xymon, the hostname sent to xymond was used without any checking, so a hostname could include one or more "../" sequences to delete files outside the intended directory.

There are other modules that delete files in response to a "drophost" command, but for various reasons these are not vulnerable to the attack.

Credit and timeline
The bug was discovered by "cleaver" during investigation of a bug originally reported to the Xymon mailing list on July 17 - http://lists.xymon.com/archive/2013-July/037838.html - and I was notified via private e-mail on July 21st when it was realized to be a security related issue.

A bugfix - r7199 - was committed to the Sourceforge SVN code repository on July 23rd, and version 4.3.12 was released on July 24th.

Henrik Størner
Xymon developer