Apache Struts, Again? What’s Going On?

Yesterday’s Apache Struts vulnerability announcement describes an XML Deserialization issue in the popular Java framework for web applications. Deserialization of untrusted user input, also known as CWE-502, is a somewhat well-known vulnerability pattern, and I would expect crimeware kits to incorporate this vulnerability well before most enterprises have committed to a patch, given the complications that this patch introduces.

What’s The Catch?

The problem with deserialization vulnerabilities is that oftentimes, application code relies precisely on the unsafe deserialization routines being exploited—therefore, anyone who is affected by this vulnerability needs to go beyond merely applying a patch and restarting the service, since the patch can make changes to how the underlying application will treat incoming data. Apache mentions this in the "Backward Compatibility" section of S2-052. Updates that mention, "it is possible that some REST actions stop working" is enough to cause cold sweats for IT operations folks who need to both secure their infrastructure and ensure that applications continue to function normally.

What Can I Do?

Organizations that rely on Apache Struts to power their websites need to start that application-level testing now so as to avoid becoming the next victims in a wave of automated attacks that leverage this vulnerability. Remote code execution means everything from defacements to ransoms and everything in between.

In the meantime, Rapid7’s product engineering teams are working up coverage for organizations to detect, verify, and remediate this critical issue. A Metasploit module is in progress, and will be released shortly to help validate any patching or other mitigations.

InsightVM customers with content at “Wednesday 6th September 2017” or later (check Administration --> General to confirm content version) can determine whether they have a vulnerable version of Apache Struts present on Unix hosts in their environment by performing an authenticated scan. The vulnerability id is struts-cve-2017-9805 should you wish to set up a scan template with just this check enabled. It has also been tagged with 'Rapid7 Critical.' An unauthenticated check for CVE-2017-9805 is available for InsightVM and Nexpose under the same id, struts-cve-2017-9805. This check does not remotely execute code; instead, it detects the presence of the vulnerable component against the root and default showcase URIs of Apache Struts instances.

In addition to these specific updates, we’ve also produced a quick guide with step-by-step instructions on how InsightVM and Nexpose can be used to discover, assess, and track remediation of critical vulnerablities, including this Apache Struts vuln.

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Should I Panic?

Yes, you should panic. For about two minutes. Go ahead and get it out of your system. Once that’s done, though, the work of evaluating the Apache Struts patch and how it’ll impact your business needs to get started. We can’t stress enough the impact here—Java deserialization nearly always leads to point-and-click remote code execution in the context of the web service, and patching against new deserialization bugs carries some risk of altering the intended logic for your specific web application. It’s not a great situation to be in, but it’s surmountable.

If you have any questions about this issue, feel free to comment below, or get in touch with your regular Rapid7 support contacts.