Chinese hackers are using an automated tool to exploit known vulnerabilities in Apache Struts, in order to install backdoors on servers hosting applications developed with the framework.
Apache Struts is a popular open-source framework for developing Java-based Web applications that's maintained by the Apache Software Foundation.
Several security updates were released for Struts this year, including last month, to address highly critical vulnerabilities that could enable remote attackers to execute arbitrary commands on Web servers running applications built with the framework.
Hackers have since taken notice and are now actively exploiting those flaws, according to researchers from security firm Trend Micro, who found a tool on Chinese underground forums that automates attacks against vulnerable Struts versions.
The tool exploits the following Struts vulnerabilities to compromise servers: S2-016 (CVE-2013-2251), which was patched in Struts 184.108.40.206 on July 16; S2-013 (CVE-2013-1966), patched in Struts 220.127.116.11 on May 22; S2-009 (CVE-2011-3923), patched in Struts 18.104.22.168 on Jan. 22, 2012; and S2-005 (CVE-2010-1870), patched in Struts 2.2.1 on Aug. 16, 2010.
The existence of the attack tool was confirmed on July 19, three days after the most recent vulnerability was disclosed to the public, Noriaki Hayashi, a senior threat researcher at Trend Micro, said Wednesday in a blog post.
"We have observed attacks against Asian targets using this specific hacking tool, which indicates these Struts flaws are being actively exploited by potential threat actors in the wild," he said.
Once hackers break into a Linux-based or Windows-based server using the Struts attack tool, they can execute pre-configured commands in order to extract information about the server's operating system, directory structure, active users and network configuration.
The tool also allows attackers to plant a so-called Web shell that acts as a backdoor, giving them persistent access to the servers to execute other commands and use them as they see fit, Hayashi said.
The Web shell installed by the tool is called JspWebShell and is coded using JavaServer Pages (JSP).
Web shells with more powerful capabilities are easily available on hacker forums and they allow attackers to search for and steal information from compromised servers, the researcher said.
Struts 22.214.171.124, which is currently the most secure version of the framework, removed several vulnerable features like the "redirect:" and "redirectAction:" prefixes of the DefaultActionMapper class. The Struts developers warned that upgrading to this version might break some applications that rely on those features and recommended replacing the retired prefixes with fixed navigation rules.
Upgrading to the latest version is strongly recommended, Hayashi said. "The potential risks from a successful attack outweigh the inconvenience of modifying any deployed apps."