PPTP, by design had known vulnerabilities. Especially when comes to Windows based systems. Most of the known vulnerabilities were addressed but some of those still exist which will lead to attacks like DoS or information compromization. When considering Windows based systems, even MSChap v2. Microsoft's Point to Point Encryption quite weak strength (a maximum of 128 bits). You basically need to use EAP methods for strong protection.
Even though it provides authentication, encryption, and packet filtering, Other issues exist:
- No protection for IP, GRE, and PPP header information (basically, protection issues for NCP PPP packets). Data is secure but initial connection negotiations can be vulnerable if captured (during authentication).
- Vulnerabilities like low defense against session hijacking, bit-flipping attacks etc.
- If RC4 encryption has some weaknesses.
L2TP with IPSec (or any other with ssl/tls ipsec/ike) is recommended.
Penetration tools can be used to test but it is not an easy task for an end user. Basically tools developed for Linux with the intention of penetration testing (no single tool, combinations) can be used but the process does not suit for regular user.
User has the ability to check the security methods used in the connection. As an example, by opening the dial in properties and going to the Security Tab (Windows). Data Encryption should be "Maximum strength encryption(disconnect if server declines)". Authentication should be EAP-TLS, PEAP or Smart Cards. MSChapv2 is absolutely not recommended (least secure). Certificate based security it recommended. In order to check whether the DNS requests are secure, he/she can use online tools such as GRC. FYI: PPTP is not secure at all. Its outdated and can be cracked using nthash