Source: cve@mitre.org
The design of the Internet Key Exchange (IKE) protocol, when using Aggressive Mode for shared secret authentication, does not encrypt initiator or responder identities during negotiation, which may allow remote attackers to determine valid usernames by (1) monitoring responses before the password is supplied or (2) sniffing, as originally reported for FireWall-1 SecuRemote.
IKE Aggressive Mode, used in VPN implementations, transmits unencrypted user identities during the initial negotiation phase. This allows attackers to sniff network traffic and harvest valid usernames, enabling them to launch brute-force attacks or other credential-based exploits. Successful exploitation can lead to unauthorized access to sensitive network resources and data exfiltration.
Step 1: Traffic Sniffing: An attacker monitors network traffic, specifically looking for IKE Aggressive Mode negotiations. This can be done passively using tools like Wireshark or tcpdump.
Step 2: Identity Extraction: The attacker captures the IKE packets and extracts the initiator and responder identities (usernames) from the unencrypted payloads.
Step 3: Credential Harvesting: The attacker compiles a list of valid usernames obtained from the sniffed traffic.
Step 4: Brute-Force/Credential Stuffing: The attacker uses the harvested usernames to attempt to authenticate to the VPN server using brute-force or credential stuffing attacks, leveraging the knowledge of valid usernames to increase the likelihood of success.
Step 5: Unauthorized Access: If successful, the attacker gains unauthorized access to the VPN and potentially the internal network.
The vulnerability stems from a design flaw in the Internet Key Exchange (IKE) protocol, specifically when using Aggressive Mode for authentication. In Aggressive Mode, the initiator and responder identities (e.g., usernames) are transmitted in the clear during the first two messages of the IKE exchange. This lack of encryption allows attackers to passively capture these identities. The root cause is the protocol's design choice to prioritize speed and simplicity over security in the initial exchange, failing to encrypt sensitive information before authentication is fully established. This is not a specific code flaw like a buffer overflow or race condition, but a fundamental protocol weakness.
This vulnerability is a foundational building block for attacks. While not directly linked to specific APTs, it's a technique that any attacker, including state-sponsored actors, could use. It lowers the barrier to entry for attacks. The vulnerability itself is not listed in the CISA KEV catalog, but the resulting compromised credentials could be used in attacks that are.
Network traffic analysis using tools like Wireshark or tcpdump to identify IKE Aggressive Mode negotiations (identified by specific IKE packet types and payloads).
Reviewing VPN server logs for failed login attempts from multiple IP addresses, especially after periods of network traffic that could have been used for sniffing.
Monitoring for unusual network traffic patterns, such as a sudden increase in failed login attempts or brute-force attacks against VPN endpoints.
Using Intrusion Detection Systems (IDS) or Intrusion Prevention Systems (IPS) configured to detect IKE Aggressive Mode traffic and potential username extraction attempts.
Analyzing VPN server configuration to determine if Aggressive Mode is enabled.
Disable IKE Aggressive Mode if possible. This is the most effective mitigation.
Configure VPN clients and servers to use IKE Main Mode, which encrypts identities during negotiation. This is the preferred and more secure configuration.
Implement strong password policies, including password complexity requirements and regular password changes.
Enforce multi-factor authentication (MFA) to add an extra layer of security and mitigate the risk of compromised credentials.
Monitor VPN server logs for suspicious activity, such as failed login attempts, and implement alerting to notify security teams of potential attacks.
Regularly update VPN software and firmware to patch any known vulnerabilities and security flaws.
Segment the network to limit the impact of a successful VPN compromise.