CVE-2015-8023 in strongSwan
Summary
by MITRE
The server implementation of the EAP-MSCHAPv2 protocol in the eap-mschapv2 plugin in strongSwan 4.2.12 through 5.x before 5.3.4 does not properly validate local state, which allows remote attackers to bypass authentication via an empty Success message in response to an initial Challenge message.
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Analysis
by VulDB Data Team • 11/12/2024
The vulnerability identified as CVE-2015-8023 represents a critical authentication bypass flaw within the strongSwan virtual private network implementation. This issue affects the EAP-MSCHAPv2 protocol server component, specifically within the eap-mschapv2 plugin that handles Microsoft Challenge Handshake Authentication Protocol version 2 authentication. The vulnerability exists in strongSwan versions ranging from 4.2.12 through 5.x prior to the release of version 5.3.4, creating a significant security gap that could allow malicious actors to compromise network access controls. The flaw manifests in the server's failure to properly validate local authentication state during the EAP exchange process, specifically when handling Challenge-Response authentication sequences.
The technical exploitation of this vulnerability occurs through a carefully crafted empty Success message that an attacker can send in response to an initial Challenge message. During normal EAP-MSCHAPv2 authentication, the server sends a Challenge message containing a random challenge value that the client must respond to with a properly computed response. The server implementation in affected strongSwan versions fails to validate the integrity of the authentication state machine, allowing an attacker to bypass the authentication process entirely by sending an empty Success message that appears to satisfy the authentication requirements. This improper state validation creates a condition where the server accepts the empty response as valid authentication, effectively granting access without proper credential verification. The vulnerability directly relates to CWE-284, which addresses improper access control in authentication mechanisms, and represents a failure in implementing proper protocol state validation.
The operational impact of this vulnerability extends beyond simple authentication bypass, as it can lead to complete network compromise when exploited in environments relying on strongSwan for IPsec VPN connections. Attackers can leverage this weakness to establish unauthorized VPN sessions, potentially gaining access to internal network resources, sensitive data, and privileged systems. The vulnerability is particularly dangerous in enterprise environments where strongSwan is commonly deployed for secure remote access, as it allows attackers to impersonate legitimate users without knowledge of valid credentials. This authentication bypass can facilitate lateral movement within networks, data exfiltration, and persistent access to organizational infrastructure. The attack vector is relatively simple, requiring only network access to send the specially crafted empty Success message, making it a high-risk vulnerability that can be exploited by both internal and external threat actors.
Organizations should prioritize immediate remediation by upgrading to strongSwan version 5.3.4 or later, which includes the necessary patches to address the authentication state validation flaw. System administrators should also implement network monitoring to detect anomalous authentication patterns that might indicate exploitation attempts. Additional mitigations include implementing strict network segmentation, deploying intrusion detection systems that can identify suspicious EAP message sequences, and conducting thorough vulnerability assessments of all VPN infrastructure components. The vulnerability demonstrates the critical importance of proper state machine validation in cryptographic protocol implementations and aligns with ATT&CK technique T1566, which covers credential harvesting through network protocols. Organizations should also consider implementing additional authentication layers such as two-factor authentication to reduce the impact of potential authentication bypasses, as the vulnerability specifically affects the EAP-MSCHAPv2 authentication mechanism rather than the underlying IPsec encryption.