Sebastian Roschke, Feng Cheng, Christoph Meinel: "BALG: Bypassing Application Layer Gateways Using Multi-Staged Encrypted Shellcodes" in Proceedings of the 12th IFIP/IEEE International Symposium on Integrated Network Management (IM 2011), IEEE Press, Dublin, Ireland, pp. 399-406, 5, 2011. ISBN: 978-1-4244-9219-0. BALG: Bypassing Application Layer Gateways Using Multi-Staged Encrypted Shellcodes Sebastian Roschke Feng Cheng Christoph Meinel Hasso Plattner Institute (HPI) Hasso Plattner Institute (HPI) Hasso Plattner Institute (HPI) University of Potsdam University of Potsdam University of Potsdam 14482, Potsdam, Germany 14482, Potsdam, Germany 14482, Potsdam, Germany Email:
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[email protected] Abstract—Modern attacks are using sophisticated and inno- easily penetrated by simple tunneling. IDS needs to handle vative techniques. The utilization of cryptography, self-modified efficient evasion techniques. ALGs provide more restrictions code, and integrated attack frameworks provide more possibili- for network access by combining filtering on the application ties to circumvent most existing perimeter security approaches, such as firewalls and IDS. Even Application Layer Gateways layer and IDS techniques, such as deep packet inspection. (ALG) which enforce the most restrictive network access can be Most of ALG implementations provide filtering due to ap- exploited by using advanced attack techniques. In this paper, plication layer protocol compliance and even allow to block we propose a new attack for circumventing ALGs. By using certain commands within a specific protocol. Although ALGs polymorphic and encrypted shellcode, multiple shellcode stages, enforce a very restrictive access policy, it is still possible to protocol compliant and encrypted shell tunneling, and reverse channel discovery techniques, we are able to effectively bypass circumvent such devices by using modern attack techniques.