Attackers Rush to Exploit Bash Flaw Before Systems Are Patched

Attackers use the bug in Bash to scan for vulnerable servers, augment backdoor Trojans and create botnets, aiming to strike before the systems are patched.

Shellshock Bash Flaw

As companies and software developers rush to patch vulnerabilities in the Bourne Again Shell, or Bash, attackers have already incorporated exploit code into a variety of tools, from network scanners to malware, attempting to urgently exploit the vulnerabilities before the lion's share of systems are patched.

Web security firm CloudFlare, for example, has seen 1.5 million attacks each day against the bash vulnerability, which is popularly known as Shellshock. Cloud security rival Incapsula estimates that attacks have targeted approximately 4 percent of its customer base, according to data on the probe attempts released by the firm on Sept. 29. While many of the "attacks" could be site owners testing their servers for the vulnerability, three major spikes in network events represent widespread scanning efforts, Marc Gaffan, Incapsula co-founder and chief business officer, told eWEEK.

"If you extrapolate, you are going to get a very, very large number of Websites being targeted," he said.

Less than a week after the widespread vulnerability first became known, companies are rushing to patch the flaw in how Bash handles certain types of parameters, known as environmental variables. Many types of software, such as the Common Gateway Interface (CGI) used to add dynamic content to Websites, execute shell commands and so have an existing link to Bash.

It took five days for the original flaw to be fully patched. On September 30, Apple issued its patch for Mac OS X.

Yet attackers have not waited idly. A variety of Web traffic incorporating an exploit for Shellshock had already been noticed by security firm FireEye, which stated on Sept. 27 that the attack had been incorporated into malware droppers, backdoors and distributed denial-of-service tools. The exploit is easy to code and simple to use, according to an analysis penned by three FireEye researchers.

"The salient points to keep in mind about the CGI vector is that it can be delivered by any HTTP request parameter, the server doesn't have to directly call Bash scripts for it to be vulnerable, and it is OS-agnostic," they wrote. "Its only dependencies are the Web server, the CGI content it hosts, and the use of Bash as the shell."

FireEye noted that some of the traffic originated from Russia. In its own analysis, Incapsula found that 19 percent of the traffic came from the United States and 10 percent from China. Sources from Brazil, France, Germany and the Netherlands each accounted for about 3 percent of the overall attacks, according to Incapsula's data.

Security firm Kaspersky detected attacks within a day of the disclosure of the original flaw. Attackers attempted to use bash to have the targeted server connect back to a specific Internet address, a technique known as a reverse shell. In addition, the attack was used to install a backdoor onto vulnerable Linux systems, according to researcher Stefan Ortloff.

"The binary contains two hard-coded IP addresses," he wrote in the analysis. "The first one is only used to notify the criminals about a new successful infection. The second IP address is used as a command-and-control server (C&C) to communicate directly with the malware."

Robert Lemos

Robert Lemos

Robert Lemos is an award-winning freelance journalist who has covered information security, cybercrime and technology's impact on society for almost two decades. A former research engineer, he's...