Learn PPTP - Security Analysis & Vulnerabilities

#Introduction

All of PPTP's components have been dissected one by one. Now it is time to piece them together into a complete picture: how insecure PPTP really is, which attacks have been proven, and how to assess the risk professionally.

Episode 13 gathers the evidence: the MS-CHAPv2 brute-force, RC4 weaknesses in MPPE, downgrade attacks, unencrypted GRE, risk assessment, and a list of CVEs and advisories. This is the most important episode for making decisions about the fate of PPTP servers in your organization.

#The MS-CHAPv2 Brute-Force (2012)

#"Divide and Conquer"

In 2012, Moxie Marlinspike and David Hulton published a technique that turns the MS-CHAPv2 challenge-response into an offline attack against the password. They called it divide and conquer because it splits the problem into smaller, easier-to-crack pieces.

With tools like pptp_bf or online cracking services, a weak password can be cracked within hours. Even more worrying: because the MPPE key is derived from the same password, cracking the password means cracking the encryption of every recorded session.

sudo tcpdump -i any port 1723 -w mschap.pcap

tcpdump -i any port 1723 -w mschap.pcap saves the control channel traffic to a file for analysis. In a lab, this can demonstrate the weakness; in production, it is a reminder that anyone able to sniff the network has the same material for an attack.

#Why This Is an Offline Attack

The key difference between this attack and an online attack is the material required. An online attack must interact with the server repeatedly — slow and noisy. An offline attack only needs to capture a single challenge-response pair from the network, then crack it on its own machine without the server knowing.

The consequences are severe: an attacker does not need to keep a connection alive or worry about being tracked. Once the control channel traffic is recorded, the attack material is complete and can be processed at any time — even months after the connection ended, as long as the user's password has not changed.

#RC4 Weaknesses in MPPE (2013)

#A Cipher Growing Fragile

RC4 has statistical biases that make it vulnerable to various attacks. In 2013-2015, research on RC4 biases intensified, and the conclusion was emphatic: RC4 is no longer considered secure for modern protocols.

MPPE inherits all of RC4's problems. Combined with key derivation from a password (rather than a secure key exchange) and the absence of an integrity check, MPPE encryption provides a level of security far below modern VPN standards.

#A Deadly Combination

What makes MPPE worse than plain RC4 is its combination with the key derivation design. Because the encryption key is the same material cracked by the MS-CHAPv2 attack, a single traffic capture hands over two things at once: the credentials and the data key.

In practice, an attacker only needs to record traffic, crack the password via divide and conquer, then decrypt every session that used that key. There is no independent second line of defense — data confidentiality scales directly with password strength.

#Downgrade Attacks and Unencrypted GRE

#Forcing Clients Down to Weaker Security

A downgrade attack is a scenario in which an attacker in the middle of a connection makes the client and server agree on weaker options than they should. If MPPE can be downgraded or skipped entirely, the whole tunnel becomes plaintext.

#Unencrypted GRE

GRE itself is never encrypted. When MPPE fails to negotiate or is deliberately skipped, all data crosses GRE in its raw form. This combination is what makes PPTP so dangerous on untrusted networks.

#Risk Assessment

#Main Attack Scenarios

• Credential interception: passwords can be extracted from traffic via an offline attack.
• Man-in-the-middle: connections can be hijacked or downgraded in security.
• Passive eavesdropping: recorded traffic can be decrypted once the password is found.

These three scenarios place PPTP in the insecure category. No configuration combination eliminates these risks.

#Assessing with CVSS

For formal purposes, the risk can be mapped to the CVSS scale. Vulnerabilities that enable eavesdropping and credential theft without user interaction typically score high — in the category that demands immediate remediation.

A CVSS score is not an end in itself; it is a common language with security teams and management. Use the score to compare PPTP against other risks in the organization, so migration priorities can be justified objectively.

#CVEs and Advisories

#The RC4 Ecosystem Context

Several well-known CVEs and advisories relate to the era of RC4 and TLS weaknesses and reinforce the conclusions about PPTP:

• BEAST (CVE-2011-3389): an attack on CBC in TLS, showing the fragility of old crypto.
• POODLE (CVE-2014-3566): an SSLv3 downgrade attack.
• Bar Mitzvah and RC4 bias attacks in TLS: confirming RC4 is insecure.

These CVEs attack TLS, not MPPE directly, but they illustrate the era in which equally weak ciphers were removed from modern protocols — while MPPE kept using RC4 until the very end.

#Tools for Auditing PPTP

Several tools can be used in a lab to test the security of a PPTP server:

• pptp_bf: cracks passwords from captured challenge-responses.
• wireshark: analyzes MS-CHAPv2 negotiation and MPPE payloads.
• nmap: checks whether port 1723 is truly open to the public.

sudo nmap -sS -p 1723 203.0.113.10

nmap -sS -p 1723 203.0.113.10 checks whether the control channel port is open from an external viewpoint. A well-managed PPTP server should only accept connections from allowed subnets, not from the entire internet.

#Closing

Episode 13 summarized the evidence of PPTP's insecurity: the MS-CHAPv2 brute-force since 2012, fragile RC4 in MPPE, downgrade attacks, unencrypted GRE, and the CVE context from the era of old crypto.

Key takeaways:

• MS-CHAPv2 challenge-responses can be cracked offline since 2012.
• Cracking the password means cracking the MPPE encryption of every session.
• RC4 in MPPE inherits biases that make it considered insecure.
• Downgrade attacks can make the tunnel run without encryption.
• GRE is never encrypted; security depends entirely on MPPE.
• PPTP is insecure against passive eavesdropping, MITM, and credential theft.

In the next episode, episode 14, we will discuss why PPTP is deprecated — the decisions by Microsoft, NIST, and the NSA, comparisons with OpenVPN, WireGuard, IKEv2, and L2TP/IPsec, and a migration strategy framework.