UPDATE2: a Metasploit module has been released. Some limited workarounds may be available. Otherwise patch now!

UPDATE: a link to a working exploit has been seen. As of yet no IDS or WAF signatures/rules have been posted. (2017/09/05 20:30h EDT)

Anyone using Struts 2 should immediately upgrade to Struts 2.5.13 due to a  remote code execution vulnerability. It has been assigned CVE-2017-9805 and a detailed technical writeup is available here: https://lgtm.com/blog/apache_struts_CVE-2017-9805_announcement.

A work around would be to disable access to the REST API used by Struts as it does not correctly deserialize objects when invoked. 

Every once in a while along comes a vulnerability that should cause you to consider actually updating the platform your application runs on! Now that the patch is available it will not be long before a working exploit is out in the wild. 

Cheers,
Adrien de Beaupré, SANS Instructor and Co-author of #SEC642
Intru-shun.ca Inc.