Last week I had some time to play with the monitoring tool Xymon. Xymon is a monitoring and alerting software mostly written in C. Its server component provides you with a web-interface to check the health of your systems. I only quickly investigated this web-interface.

After walking through the various kinds of pages I found that almost all parameters are correctly checked or sanitized. But on one page the code was doing something odd (criticalview.c):

fprintf(output, "<a href="\&quot;%s&amp;NKPRIO=%d&amp;NKTTGROUP=%s&amp;NKTTEXTRA=%s\&quot;">",

hostsvcurl(itm->;hostname, colname, 1),

prio,

htmlgroupstr, htmlextrastr);

This is strange, the htmlgroupstr and htmlextrastr variables do not get used anywhere else on criticalview.c. The link it generates points to svcstatus.c (well its wrapper, svcstatus.sh). On that page the NKTTGROUP and NKTTEXTRA parameters simple get displayed on the page, without any further cleanup. With that we can generate links like this:

http://localhost/xymon-cgi/svcstatus.sh?HOST=foo&SERVICE=cpu&NKPRIO=1&NKTTGROUP=admins&NKTTEXTRA=foo%3Cscript%3Ealert(1);%3C/script%3E

This nicely executes our injected JavaScript. Bug was reported at SourceForge.