CVE-2025-62291: strongSwan Buffer Overflow Vulnerability

CVE-2025-62291 Overview

A critical vulnerability has been discovered in the EAP-MSCHAPv2 plugin (client-side) in strongSwan before version 6.0.3. This vulnerability allows a malicious EAP-MSCHAPv2 server to send a crafted message of size 6 through 8, causing an integer underflow that potentially results in a heap-based buffer overflow.

Critical Impact

Remote attackers operating malicious authentication servers can exploit this integer underflow to potentially achieve arbitrary code execution on vulnerable strongSwan VPN clients, compromising confidentiality, integrity, and availability of the affected system.

Affected Products

• strongSwan versions prior to 6.0.3
• Systems using the eap-mschapv2 plugin for client-side EAP authentication
• VPN clients configured to authenticate via EAP-MSCHAPv2

Discovery Timeline

• 2026-01-16 - CVE CVE-2025-62291 published to NVD
• 2026-01-16 - Last updated in NVD database

Technical Details for CVE-2025-62291

Vulnerability Analysis

This vulnerability (CWE-191: Integer Underflow) exists in the client-side implementation of the eap-mschapv2 plugin within strongSwan. When processing EAP-MSCHAPv2 authentication messages from a server, the plugin fails to properly validate message length boundaries before performing arithmetic operations on the received size values.

A malicious EAP-MSCHAPv2 server can craft and send authentication messages with a size value between 6 and 8 bytes. When the plugin processes these messages, the improper handling of these small size values triggers an integer underflow during length calculation. This underflow causes a subsequent memory operation to reference an incorrect buffer size, leading to a heap-based buffer overflow condition.

The network-based attack vector means any strongSwan client configured to use EAP-MSCHAPv2 authentication is potentially vulnerable when connecting to untrusted or compromised authentication servers.

Root Cause

The root cause lies in insufficient bounds checking on the message length field received from the EAP-MSCHAPv2 server. When the plugin calculates buffer sizes for processing authentication response data, it subtracts a fixed header size from the received length value. With specially crafted messages of size 6-8 bytes, this subtraction results in an integer underflow, wrapping the value to a very large positive number and causing subsequent buffer operations to overflow allocated heap memory.

Attack Vector

The attack requires a man-in-the-middle position or control over a malicious authentication server. The attacker must be able to intercept or initiate an EAP-MSCHAPv2 authentication session with the vulnerable strongSwan client. The exploitation flow involves:

1. The attacker establishes or intercepts a VPN connection where the client uses EAP-MSCHAPv2 authentication
2. The malicious server sends a crafted EAP-MSCHAPv2 response message with a length field between 6 and 8 bytes
3. The strongSwan client processes this message, triggering the integer underflow in length calculation
4. The resulting heap buffer overflow can potentially be leveraged to corrupt memory and execute arbitrary code

The vulnerability is exploited through the VPN authentication protocol itself, requiring no user interaction beyond initiating a VPN connection to the malicious server.

Detection Methods for CVE-2025-62291

Indicators of Compromise

• Unexpected strongSwan process crashes or core dumps during VPN connection attempts
• Anomalous network traffic containing EAP-MSCHAPv2 messages with unusually small payload sizes (6-8 bytes)
• Memory corruption errors or segmentation faults in strongSwan daemon logs
• Suspicious outbound connections to unknown VPN servers

Detection Strategies

• Monitor strongSwan daemon logs for authentication failures followed by process termination
• Deploy network intrusion detection rules to identify malformed EAP-MSCHAPv2 packets with suspicious size values
• Implement endpoint detection to alert on strongSwan process crashes or unexpected memory access patterns
• Use SentinelOne's behavioral AI to detect anomalous process behavior following VPN connection events

Monitoring Recommendations

• Enable verbose logging for the strongSwan daemon to capture detailed authentication events
• Configure system monitoring to alert on abnormal memory usage by strongSwan processes
• Implement network flow analysis to detect connections to unauthorized or suspicious VPN endpoints
• Review authentication server certificates and configurations to ensure connections only occur to trusted infrastructure

How to Mitigate CVE-2025-62291

Immediate Actions Required

• Upgrade strongSwan to version 6.0.3 or later immediately
• Audit VPN configurations to ensure EAP-MSCHAPv2 is only used with trusted and verified authentication servers
• Consider temporarily disabling EAP-MSCHAPv2 authentication in favor of alternative methods if patching is not immediately possible
• Restrict VPN client configurations to connect only to known, trusted authentication infrastructure

Patch Information

The strongSwan project has addressed this vulnerability in version 6.0.3. Organizations should update their strongSwan installations immediately. Detailed patch information is available through the strongSwan GitHub Release Notes. The specific code changes addressing this vulnerability can be reviewed in the eap_mschapv2 commit history. Debian users should refer to the Debian LTS Security Announcement for distribution-specific updates.

Workarounds

• Disable the eap-mschapv2 plugin if EAP-MSCHAPv2 authentication is not required for VPN operations
• Configure network-level controls to restrict VPN connections only to pre-approved authentication servers
• Implement certificate pinning or mutual TLS to ensure strongSwan clients only authenticate with trusted servers
• Deploy network segmentation to limit exposure of VPN clients to potentially malicious authentication servers

# Disable eap-mschapv2 plugin in strongSwan configuration
# Edit /etc/strongswan.conf or /etc/strongswan.d/charon.conf
charon {
    # Disable the vulnerable plugin
    load_modular = yes
    plugins {
        eap-mschapv2 {
            load = no
        }
    }
}