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Brute-force SSH attacks surge

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An incident handler from SANS' Internet Storm Center has warned businesses to ensure their servers are secure as SSH attacks rose five-fold early this week.

The SANS Internet Storm Center has warned businesses of rising numbers of brute-force SSH attacks.

An SSH attack is a type of dictionary attack which aims to guess secure shell client usernames and passwords.

Writing in the ISC's website diary, incident handler Scott Fendley warned security professionals to be aware of what he called a "significant new trend".

He linked to statistics produced by denyhosts.net, a site which offers a script to defend against SSH hacking attempts, which showed a huge spike in SSH attacks on Monday. In total, the site found nearly 10,000 SSH attacks on that one day: the daily norm is roughly 2,000. He added that anecdotal evidence from mailing lists confirmed the trend.

"This does appear to be a significant new trend of which we all should be aware," wrote Fendlay, who is also a security analyst at the University of Arkansas.

He suggested businesses should check which of their servers have SSH open to outsiders on the default port. He also suggested:

- Using host-based tools such as DenyHosts, fail2ban or BlockHosts in conjunction with TCP-Wrappers to block access to servers;
- Disabling access to the root account;
- Making sure usernames were not easily guessable;
- Using multiple factors of authentication or public keys if possible;
- Generally reducing the number of publicly accessible services through iptables or similar host-based security measures, in addition to using network firewalls.

Fendley said that hackers were trying to avoid detection by launching 'low and slow' attacks which avoid locking out accounts and being detected by intrusion detection systems. Some attackers were using botnets to perform distributed attacks to keep under detection thresholds, he said.

Fendley linked to a whitepaper from New York's Clarkson University which investigated methods used in brute-force SSH attacks.

SANS last year noted the widespread nature of brute-force password-guessing attacks. It said such attacks against SSH, FTP and Telnet servers were "the most common form of attack to compromise servers facing the internet."

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