Brute-Force SSH Server Attacks Surge

The number of brute-force SSH attacks is rising, the SANS Internet Storm Center warned on Monday.

There "has been a significant amount of brute force scanning reported by some of our readers and on other mailing lists," said Internet Storm Center handler Scott Fendley in a blog post. "... From the most recent reports I have seen, the attackers have been using either 'low and slow' style attacks to avoid locking out accounts and/or being detected by IDS/IPS systems. Some attackers seem to be using botnets to do a distributed-style attack, which also is not likely to exceed thresholds common on the network."

Data gathered by DenyHosts.org, a site that tracks SSH hacking attempts, appears to confirm Fendley's claim. A graph of the site's data shows SSH hacking attempts rising sharply over the past weekend.

SSH stands for secure shell. It is a network protocol for creating a secure communications channel between two computers using public key cryptography.

A brute-force SSH attack, a kind of dictionary attack, is simply a repeating, typically automated, attempt to guess SSH client user names and/or passwords. If such an attack succeeds, the attacker may be able to view, copy, or delete important files on the accessed server or execute malicious code.

The SANS Institute last year said that brute-force password-guessing attacks against SSH, FTP, and Telnet servers were "the most common form of attack to compromise servers facing the Internet."

A paper published earlier this year by Jim Owens and Jeanna Matthews of Clarkson University, "A Study of Passwords and Methods Used in Brute-Force SSH Attacks," found, based on an analysis of network traffic, that even "strong" passwords may not be enough to foil password-guessing attacks. ("Strong" passwords are typically a combination of letters and numbers, both upper and lower case, that don't form recognizable words.)

The paper focuses on the vulnerability of Linux systems to brute-force SSH attacks. "While it is true that computers running Linux are not subject to the many worms, viruses, and other malware that target Windows platforms, the Linux platform is known to be vulnerable to other forms of exploitation," the paper states. "A 2004 study conducted by the London-based security analysis and consulting firm mi2g found that Linux systems accounted for 65% of 'digital breaches' recorded during the 12-month period ending in October 2004."

The paper points to remarks by Dave Cullinane, CISO at eBay, and Alfred Huger, VP at Symantec Security Response, to the effect that Linux machines make up a large portion of the command and control networks of botnets.

It also notes that "Linux systems face a unique threat of compromise from brute-force attacks against SSH servers that may be running without the knowledge of system owners/operators. Many Linux distributions install the SSH service by default, some without the benefit of an effective firewall."

Thus, all it takes to compromise such systems is to guess the password, and attackers have machines trying to do just that at all hours of the day. To make matters worse, attackers are sharing dictionaries of username/password pairs that include a significant number of "strong" passwords.

Fendley recommends that IT administrators consider defenses advocated by Owens and Matthews in their paper. These include: using host-based security tools to block access to servers; disabling direct access to root accounts; avoiding easily guessed usernames, such as a person's first or last name; enforcing the use of strong passwords, public key authentication, or multifactor authentication, depending on the security posture of the organization in question; and limiting publicly accessible network services through iptables or other host-based security measures.