CVE-2025-62291: 
strongSwan vulnerability analysis and mitigation

CVE-2025-62291 is a heap-based buffer overflow vulnerability in the client-side eap-mschapv2 plugin of strongSwan, an open-source IPsec-based VPN solution. A malicious EAP-MSCHAPv2 server can send a crafted Failure Request packet of size 6 through 8 bytes, triggering an integer underflow (CWE-191) that can result in a heap-based buffer overflow on the connecting client. The vulnerability affects strongSwan versions before 6.0.3, including Microsoft Azure Linux packages azl3_strongswan_5.9.14-7/8 and cbl2_strongswan_5.9.10-3/4. It was disclosed by the strongSwan project on October 27, 2025, and formally published to NVD on January 16, 2026. The CVSS v3.1 base score is 8.1 (High) (strongSwan Blog, Microsoft MSRC).

The root cause is an integer underflow (CWE-191) in the client-side length validation logic of the eap-mschapv2 plugin (src/libcharon/plugins/eap_mschapv2). When the plugin processes an EAP-MSCHAPv2 Failure Request packet, it performs an arithmetic operation on the packet length field without adequate bounds checking; a crafted packet of 6–8 bytes causes the length calculation to wrap around to a very large value, which is then used to copy data into a heap buffer, resulting in a heap-based buffer overflow. The attack vector is network-based and requires no user interaction or privileges, but does require high complexity — specifically, the attacker must control or impersonate an EAP-MSCHAPv2 authentication server that the victim client connects to. The fix was committed on October 27, 2025, in two commits addressing length checks for Failure Request and successful response packets on the client side (GitHub Commits, strongSwan Blog).

Successful exploitation could allow a malicious EAP-MSCHAPv2 server to achieve arbitrary code execution or denial of service on the strongSwan client, with high impact to confidentiality, integrity, and availability. Because the overflow occurs in the VPN client process during authentication, an attacker who can position themselves as a rogue or compromised VPN server could potentially compromise the connecting endpoint before a secure tunnel is established. This could expose sensitive credentials, private keys, or other data on the client system, and may enable lateral movement within the victim's network (strongSwan Blog, Microsoft MSRC).

As of the time of reporting, there is no public proof-of-concept exploit and no evidence of in-the-wild exploitation (Feedly). The EPSS score is approximately 0.043%, indicating a low current probability of exploitation in the wild. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Exploitation requires the attacker to control or impersonate an EAP-MSCHAPv2 server that the target client authenticates against, raising the practical bar for opportunistic attacks. Detection coverage is available via multiple Nessus and Qualys scanner plugins (Feedly).

1. Setup a rogue EAP-MSCHAPv2 server: Deploy a malicious VPN/RADIUS server configured to respond to EAP-MSCHAPv2 authentication requests, or perform a man-in-the-middle attack to intercept client authentication traffic.
2. Lure or redirect the target client: Position the rogue server so that a vulnerable strongSwan client (version < 6.0.3) initiates an EAP-MSCHAPv2 authentication exchange with it — for example, via DNS spoofing, BGP hijacking, or by advertising a rogue access point.
3. Craft a malicious Failure Request packet: During the EAP-MSCHAPv2 handshake, respond to the client's authentication attempt with a specially crafted Failure Request packet of exactly 6–8 bytes in length, designed to trigger the integer underflow in the client's length validation logic.
4. Trigger integer underflow and heap overflow: The client's eap-mschapv2 plugin processes the malformed packet; the undersized length causes an arithmetic underflow, resulting in a very large computed buffer size and a subsequent heap-based buffer overflow.
5. Achieve code execution or crash: Depending on heap layout and memory protections, the overflow may allow overwriting heap metadata or function pointers to redirect execution flow, potentially achieving remote code execution on the client, or at minimum causing a denial of service (strongSwan Blog, GitHub Commits).

• Network: Unexpected or unauthorized EAP-MSCHAPv2 authentication attempts to unknown or untrusted VPN/RADIUS servers; anomalous short EAP packets (6–8 bytes) in network captures during IKE/EAP exchanges.
• Logs: strongSwan daemon (charon) crash logs or core dumps following an EAP-MSCHAPv2 authentication attempt; error messages related to EAP-MSCHAPv2 Failure Request processing in /var/log/syslog or journalctl.
• Process: Unexpected termination or restart of the charon or charon-systemd process; unusual child processes spawned from the strongSwan daemon after a VPN connection attempt.
• File System: Presence of core dump files (e.g., core, core.charon) in the working directory of the strongSwan daemon following a failed authentication event.

The primary remediation is to upgrade strongSwan to version 6.0.3 or later, which contains fixes for the integer underflow in the eap-mschapv2 plugin's Failure Request packet length validation (strongSwan Releases, strongSwan Blog). Debian LTS users should apply the update announced via the Debian LTS security list (Debian LTS). If immediate patching is not possible, restrict network access so that strongSwan clients only connect to trusted, verified EAP-MSCHAPv2 servers, and consider disabling the eap-mschapv2 plugin entirely if it is not required for operations.

The strongSwan project published a detailed security advisory on their official blog on October 27, 2025, the same day the fix was committed, demonstrating proactive disclosure (strongSwan Blog). Multiple Linux distributions — including Debian, Ubuntu, SUSE, Mageia, and Fedora — issued security advisories and updated packages in the weeks following disclosure. Microsoft also published an advisory through the MSRC for affected Azure Linux packages (Microsoft MSRC). Security scanner vendors Tenable (Nessus) and Qualys rapidly released detection plugins, indicating broad industry awareness of the vulnerability.