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Tuesday, 07 August 2018 11:30

TCP vulnerability found in Linux versions 4.9 and above

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A vulnerability in the transport control protocol in version 4.9 of the Linux kernel and above leaves it open to a denial of service by a remote attacker, according to an advisory from CERT.

The vulnerability, reported by Juha-Matti Till at Aalto University, Department of Communications and Networking/Nokia Bell Labs, means an attacker can bring about a Dos by sending specially modified packets during an ongoing TCP session.

However, since maintaining the DoS calls for continuous two-way TCP sessions to a reachable open port, attacks cannot be carried out using spoofed IP addresses.

The Linux kernel project has released patches to address the vulnerability.

British security researcher Kevin Beaumont provided some useful information about the vulnerability on his blog.

"It’s well worth noting that most enterprise grade Linux distributions do not yet use Linux kernels 4.9 or above, so (they) aren’t impacted," Beaumont wrote. "By the time they do, patches will be built in."

He said the following points should be noted:

  • The vulnerability does not allow remote code execution.
  • If the server is fully firewalled from an attacker, it is not exploitable.
  • To exploit the vulnerability, you need inbound TCP access to the server.
  • You cannot spoof packets during exploitation, as it requires an established, two-way TCP stream.
  • You need the server to echo certain packets back to you over TCP to exploit this — the packets need to be sent by the Linux server *outbound*. So only certain protocols would allow this.
  • There is no proof of concept for the exploit available at the moment.

Beaumont also listed the kernel versions in some widely-used Linux distrubutions so users could see if they were vulnerable.

"Don't panic," he said. "For enterprise grade Linux distributions it is unlikely you are actually impacted due to kernel versions. You should, of course, check your systems with a Vulnerability Management approach — and if so, patch your system with apt-get upgrade, yum update etc.

"It is obviously more complex with embedded devices such as NAS devices  –  although I would recommend not exposing these directly the Internet regardless."

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Sam Varghese

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Sam Varghese has been writing for iTWire since 2006, a year after the site came into existence. For nearly a decade thereafter, he wrote mostly about free and open source software, based on his own use of this genre of software. Since May 2016, he has been writing across many areas of technology. He has been a journalist for nearly 40 years in India (Indian Express and Deccan Herald), the UAE (Khaleej Times) and Australia (Daily Commercial News (now defunct) and The Age). His personal blog is titled Irregular Expression.

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