Pierluigi Paganini October 29, 2016

Security researchers have found vulnerabilities in the source code of the Mirai botnet and devised a method to hack back it.

The Mirai botnet, this name is familiar to security experts due to the massive DDoS attack that it powered against the Dyn DNS service a few days ago.

The Mirai malware was first spotted by the researcher MalwareMustDie that confirmed it was designed to hack into poorly configured and vulnerable IoT devices. Its source code was leaked on the popular criminal hacker forum Hackforum by a user with moniker “Anna-senpai” giving the opportunity to anyone to compile and customize its own version of the threat.

Experts who reviewed the code have discovered a weakness that could be exploited to shut down the botnet stopping it from flooding the targets with HTTP requests, this means that it is possible to hack back the threat. The experts from Invincea discovered three vulnerabilities in the Mirai code, one of them, a stack buffer overflow, could be exploited to halt the DDoS attack powered by the botnet. The buffer overflow vulnerability affects the way Mirai parses responses from HTTP packets.

“Perhaps the most significant finding is a stack buffer overflow vulnerability in the HTTP flood attack code. When exploited it will cause a segmentation fault (i.e. SIGSEV) to occur, crash the process, and therefore terminate the attack from that bot. The vulnerable code has to do with how Mirai processes the HTTP location header that may be part of the HTTP response sent from an HTTP flood request.” reported the analysis published by the security firm Invincea.

The researchers highlighted that their attack would not have helped in the DNS-based DDoS attack against provider Dyn, but it would halt the Layer 7 attack capabilities of the Mirai botnet implemented in the coded leaked online.

The researchers at Invincea successfully tested a proof-of-concept exploit in a virtual environment setting up a debug instance of the Mirai bot, a command and control server and a target machine.

“This simple “exploit” is an example of active defense against an IoT botnet that could be used by any DDoS mitigation service to defend against a Mirai-based HTTP flood attack in real-time. While it can’t be used to remove the bot from the IoT device, it can be used to halt the attack originating from that particular device. Unfortunately, it’s specific to the HTTP flood attack, so it would not help mitigate the recent DNS-based DDoS attack that rendered many websites inaccessible.” explained the Scott Tenaglia, Research Director in the cyber capabilities team at Invincea Labs.

Tenaglia remarked that the method proposed by the company doesn’t clean the compromised devices, instead it could be effective against HTTP flooding powered by the Mirai Botnet.

The method proposed by the researchers is a form of active defense that has important legal implications because anyway, who is defending its system from the attack power a response attack against the attacker’s infrastructure.

Hacking back is illegal under the Computer Fraud and Abuse Act.

Hacking a bot means to make an unauthorized access to a computer system and such kind of operations have to be authorized by a court order.

Invincea has made an excellent work and isn’t suggesting the hacking back, but limited its analysis on the technical aspect of the Mirai botnet and its vulnerabilities.

“It’s in the gray space of active defense,” Tenaglia told to ThreatPost. “In the defense world, this is a hotly contested issue. Say if your IoT is already compromised and bad code is already running, if I do something to the bad guy’s code, am I breaking law?”

“I would never comment on the legality of this,” Tenaglia said. “I think this gives us another point to discuss with regard to active defense. Is this something we think is ok? I don’t think it would hurt the system; it might help it. If a bot is degrading performance of the Internet connection because of the packets it’s sending out, and if this attack kills the process and the connection gets better, have we helped you? That’s why this is a gray area.”

Pierluigi Paganini

(Security Affairs – Linux Mirai malware, IoT)

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