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Sly new tactic sneaks hackers past security dogs

Advanced evasion techniques can bypass network security, warn experts

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Updated A new hacking technique creates a mechanism for hackers to smuggle attacks past security defences, such as firewalls and intrusion prevention systems.

So-called advanced evasion techniques (AET) are capable of bypassing network security defences, according to net appliance security firm Stonesoft, which was the first to document the approach. Researchers at the Finnish firm came across the attack while testing its security appliance against the latest hacker exploits.

Various evasion techniques including splicing and fragmentation have existed for years. Security devices have to normalise traffic using these approaches before they can inspect payloads and block attacks.

AET take this basic approach to the next level. Traffic is disguised and modified using a variety of evasion techniques in several protocol layers. By bundling IP fragmentation and SMB session mixing together at the same time it's more likely that security defences will correctly handle garbled traffic. And if devices don't recognise combined attacks then it more likely that these assault will make their way past security defences.

AETs are already in circulation on the net as part of targeted attacks and offer a mechanism to bypass network security systems before attacking exposed enterprise servers, according to Stonesoft.

Stonesoft reported its find and sent samples of AETs to Finland's national computer security incident response team (CERT-FI) earlier this month. It also sent samples to ICSA Labs, an independent third-party testing and security product certification division of Verizon Business.

CERT-FI plans to issue an updated advisory on the attack technique later on Monday (18 October). Stonesoft reckons that AETs are a particular problem for firms that still rely on hardware-coded inspection engines, which may be difficult if not impossible to upgrade.

Amichai Shulman, CTO of database security firm Imperva, described the evasion technique as the latest round in the constant cat and mouse game between malicious crackers and security defenders.

"A lot of what attackers are doing today is about evasion at various levels, there is substantial vigilance out there," Shulman commented. ®

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