A critical-level remote code execution vulnerability (CVE-2018-11776) has been identified in Apache Struts. Struts is an open source framework used in the development of Java-based web apps. The vulnerability has been classified as critical because the flaw could allow remote attackers to execute their code on a server that is running the Struts-developed app.

The Apache Software Foundation announced that it has released a new version of Struts that patches the issue. Affected versions include Struts 2.3 through Struts 2.3.34, as well as Struts 2.5 through Struts 2.5.16. ASF strongly recommends that users upgrade to Struts 2.3.35 or Struts 2.5.17 immediately, even though a workaround that they characterize as "weak" is possible.

Adding urgency, Tenable has blogged, "A working proof of concept (PoC) has been discovered and verified on Github by Tenable's research team. In addition, there are indications that attackers are already probing for vulnerable Apache Struts instances."

Man Yue Mo from the Semmle Security Research team has been credited with identifying the problem.

A previous vulnerability in Struts (CVE-2017-5638) that was also discovered by Semmle has been blamed as the cause of the massive data breach that occurred at Equifax. In that instance, the financial records of more than 140 million users were exposed.

There are specific configurations of Struts that enable the vulnerability.

They are:

1. The alwaysSelectFullNamespace flag is set to true in the Struts configuration. This automatically occurs if the application uses the Struts Convention plugin.
2. The application's Struts configuration file contains an tag that does not specify the optional namespace attribute or specifies a wildcard namespace ("/*").

Dayne Topkin via Unsplash

Semmle notes, "If your application's configuration does not meet these two conditions, you are likely not vulnerable to the two attack vectors described. However, new attack vectors that apply to different configurations may be discovered in the near future."

The researchers found that vulnerable applications may be attacked by threat actors injecting their own namespace as a parameter in an HTTP request. It seems that the value of that parameter is insufficiently validated by the Struts framework, and can be any OGNL string.

OGNL ("Object-Graph Navigation Language") is a domain-specific language that is used to customize Apache Struts' behavior.

Mo describes the process that led to the exposure of the vulnerability in a blog. He looks at the underlying data flow of Struts and how libraries that are used in determining results will affect it.

He also wonders how this vulnerability was missed by everyone. He thinks they were just not looking widely enough.

He notes in the blog that, "In my opinion, it's because the similarities between [previous vulnerabilities] are, in a sense, local, in that they all occur in a similar location in the source code (all in the Rest plugin). They are linked by the tainted source, rather than the location of the source code, so any software or tool that would successfully find variants like this would need to be able to perform […] semantic analysis across the whole code base."

What should worry the security professional is that further vulnerabilities may be exposed by this approach to Struts. Attention in the upcoming days will be needed.

Correction (8/24/2018): This article was updated to correct the mention of the Equifax breach that had identified the company as Experian.

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— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.