Emergency update for Apache Struts fixes incomplete patch for critical flaw

The previous patch did not block an attack vector that could lead to remote code execution in certain environments

The Apache Software Foundation rushed last week to update the popular Apache Struts framework after a previous security patch for a high-risk vulnerability proved to be incomplete.

The newly released Struts strengthens the protection against a ClassLoader manipulation issue through the ParametersInterceptor feature that was thought to have been resolved in March in Struts

The Struts developers warned users Thursday that the previous patch was insufficient and provided manual mitigation instructions until a new version of the framework for developing Java Web applications was ready.

Struts was released later that day and extends the fix to CookieInterceptor, another functionality that according to a new advisory is vulnerable to the same type of ClassLoader manipulation attack when configured to accept all cookies.

The new advisory does not clarify if this "zero-day exploit," as Apache called it, could lead to remote code execution, but in a discussion on the Full Disclosure mailing list over the weekend, Struts developer Rene Gielen clarified that remote code execution was possible in certain environments.

"Our initial impact statement was 'classloader manipulation' which precisely names the problem," he said in an email sent to the list. "By the time we released security bulletin S2-020 [original advisory in March], we had no knowledge about how this could lead to RCE [remote code execution]."

"Strange enough, there are some smart and creative people out there," Gielen said. "Even stranger, some of them find how to exploit the said vulnerability to allow for RCE in a certain environment, a few weeks after the announcement. Luckily, the white-to-grey-hatted of these folks tend to reach a security response team then. This is what happened last week. From then on we knew we had to deal with an RCE."

Attackers have exploited remote code execution vulnerabilities in Apache Struts in the past to compromise servers that hosted applications developed with the framework. Security researchers from Trend Micro reported in August that Chinese hackers had created an automated tool for exploiting known Struts vulnerabilities.

The fact that attackers are interested in Struts increases the risk of attacks for users, so upgrading to the newly released Struts as soon as possible is highly recommended.

There are also third-party products that include Struts and might need to be patched because of this flaw. For example, in the past, Cisco Systems updated several of its products in order to integrate a Struts patch for a critical vulnerability.

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