Vulnerabilities & attack vectors of VPNs (Pt 1)

This is the first part of an article that will give an overview of known vulnerabilities and potential attack vectors against commonly used Virtual Private Network (VPN) protocols and technologies. This post will cover vulnerabilities and mitigation controls of the Point-to-Point Tunneling Protocol (PPTP) and IPsec. The second post will cover SSL-based VPNs like OpenVPN and the Secure Socket Tunneling Protocol (SSTP). As surveillance of Internet communications has become an important issue, besides the traditional goals of information security, typically referred as  confidentiality, integrity and authenticity, another security goal has become explicitly desirable: Perfect Forward Secrecy (PFS). PFS may be achieved if the initial session-key agreement generates unique keys for each session. This ensures that even if the private key would be compromised, older sessions (that one may have captured) can’t be decrypted. The concept of PFS will be covered in the second post.
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