CVE-2015-4171 in strongSwan
Summary
by MITRE
strongSwan 4.3.0 through 5.x before 5.3.2 and strongSwan VPN Client before 1.4.6, when using EAP or pre-shared keys for authenticating an IKEv2 connection, does not enforce server authentication restrictions until the entire authentication process is complete, which allows remote servers to obtain credentials by using a valid certificate and then reading the responses.
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Analysis
by VulDB Data Team • 05/20/2022
The vulnerability identified as CVE-2015-4171 affects strongSwan implementations version 4.3.0 through 5.x prior to 5.3.2 and the corresponding VPN client versions before 1.4.6. This issue specifically impacts IKEv2 connections that utilize EAP or pre-shared key authentication methods, creating a critical security gap in the authentication process. The flaw stems from the improper timing of server authentication enforcement within the IKEv2 protocol implementation, allowing malicious actors to exploit the authentication flow before proper validation occurs.
The technical root cause of this vulnerability lies in the timing discrepancy between when authentication restrictions should be enforced and when they actually are applied during the IKEv2 handshake process. When using EAP or pre-shared key authentication, the system accepts initial connection attempts and begins processing authentication responses without first validating that the server meets the required authentication criteria. This creates an attack surface where remote servers can present valid certificates and then observe the authentication response messages to extract sensitive credentials. The vulnerability essentially allows for credential harvesting through a man-in-the-middle attack vector that exploits the protocol's authentication timing weakness.
From an operational impact perspective, this vulnerability enables attackers to perform credential theft attacks against VPN connections that rely on EAP or pre-shared key authentication methods. The attack process allows adversaries to capture authentication responses and potentially extract usernames, passwords, or other sensitive authentication data. This represents a significant risk for organizations using strongSwan VPN implementations, particularly those with remote access requirements or enterprise network connections that depend on IKEv2 with EAP or pre-shared key authentication. The vulnerability can lead to unauthorized access to protected networks and potential data breaches.
The mitigation strategy for this vulnerability involves upgrading to strongSwan versions 5.3.2 or later for the server implementation and version 1.4.6 or later for the client software. Organizations should also consider implementing additional network security controls such as certificate pinning, enhanced monitoring of authentication flows, and network segmentation to limit the potential impact of credential theft. Security teams should review their current IKEv2 configurations and ensure that authentication processes are properly enforced before accepting any authentication responses. This vulnerability aligns with CWE-284, which addresses improper access control in authentication systems, and represents a specific implementation weakness in the IKEv2 protocol that could be exploited through techniques categorized under the ATT&CK framework's credential access tactics.
The vulnerability demonstrates a classic example of improper authentication flow control where the security boundary is not properly enforced until after the authentication process completes. This timing issue creates a window of opportunity for attackers to observe and potentially exploit authentication responses, representing a significant weakness in the security architecture of the affected strongSwan implementations. Organizations should prioritize patching this vulnerability as it directly impacts the integrity of their VPN authentication processes and could lead to unauthorized access to sensitive network resources.