CVE-2026-77206: Libreswan IKEv2 DOS Vulnerability

CVE-2026-77206 Overview

CVE-2026-77206 is a null pointer dereference vulnerability in the Libreswan IKEv2 state machine. The flaw resides in the Extensible Authentication Protocol (EAP) handling code used during authentication. Due to inconsistency within the IKEv2 state machine, a fall-through path incorrectly dereferences ike->sa.st_eap, which is NULL. Any IKEv2 connection can trigger the condition, even when EAP is not configured. A single malicious IKE_AUTH packet is sufficient, and the peer does not need to be authenticated. The result is a crash and restart of the pluto daemon. Repeated exploitation produces a sustained denial of service against affected VPN gateways.

Critical Impact
An unauthenticated remote attacker can crash the Libreswan pluto daemon with a single crafted IKE_AUTH packet, enabling denial of service against IPsec VPN endpoints.

Affected Products

Libreswan IPsec implementation (IKEv2 state machine, EAP authentication handler)
Deployments using the pluto daemon for IKEv2 negotiation
Systems exposing IKEv2 (UDP/500, UDP/4500) to untrusted networks

Discovery Timeline

2026-10-13 - CVE-2026-77206 published to NVD
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-77206

Vulnerability Analysis

The vulnerability is a null pointer dereference [CWE-476] in Libreswan's IKEv2 state machine. Libreswan is an open-source IPsec implementation that negotiates security associations through the pluto daemon. The IKEv2 protocol defines distinct exchanges for authentication, including optional EAP-based flows.

The bug lives in the EAP handling code reached during IKE_AUTH processing. A state-machine fall-through dispatches packets into the EAP path even when the responder has not negotiated or configured EAP. That path dereferences ike->sa.st_eap, which is NULL for non-EAP sessions. The dereference faults the process and terminates the daemon.

Because the fault occurs before peer authentication completes, no credentials or prior session state are required. The impact is limited to availability. The advisory confirms no remote code execution is possible, but repeated packets force continuous pluto restarts and disrupt VPN tunnels.

Root Cause

The root cause is missing state validation in the IKEv2 EAP dispatch logic. Code intended for EAP-authenticated sessions executes against SA structures whose st_eap field was never initialized. The handler assumes EAP state exists whenever the fall-through branch is taken, violating the invariant enforced by the IKEv2 negotiation flow.

Attack Vector

An attacker sends a crafted IKE_AUTH message to a reachable Libreswan responder over UDP/500 or UDP/4500. The responder parses the packet, enters the vulnerable state transition, and dereferences the null st_eap pointer. The pluto process crashes. Service managers such as systemd will typically restart the daemon, but an attacker can loop the exploit to maintain denial of service. See the Libreswan CVE-2026-77206 Advisory for the vendor description.

No authenticated code example is published. The vulnerability mechanism is documented in prose by the vendor; refer to the Libreswan CVE-2026-77206 Details technical note for the exact state-machine path.

Detection Methods for CVE-2026-77206

Indicators of Compromise

Repeated pluto daemon crashes and restarts recorded in systemd journal or /var/log/pluto.log
IKEv2 IKE_AUTH packets from unexpected source addresses immediately preceding daemon termination
Spikes in SIGSEGV or abnormal exit codes attributed to the pluto process
Loss of established IPsec security associations without a corresponding administrative action

Detection Strategies

Monitor process supervision logs for frequent pluto restarts within short time windows
Correlate IKEv2 traffic captures with daemon crash timestamps to isolate triggering peers
Alert on crash loops exceeding a baseline restart rate on IPsec gateways

Monitoring Recommendations

Enable verbose Libreswan logging and forward pluto events to a central SIEM
Track UDP/500 and UDP/4500 source address diversity for anomalous scanners
Capture tunnel state transitions and alert on repeated authentication-phase failures

How to Mitigate CVE-2026-77206

Immediate Actions Required

Upgrade Libreswan to the patched release listed in the Libreswan CVE-2026-77206 Advisory as soon as it is available in your distribution
Restrict IKEv2 exposure using firewall rules that permit UDP/500 and UDP/4500 only from known peer addresses
Enable automatic restart policies for the pluto service to reduce outage duration during attempted exploitation

Patch Information

Libreswan has published a security advisory for CVE-2026-77206 with corrected IKEv2 state machine logic. Apply the fixed version provided by your Linux distribution or build from the upstream release referenced in the Libreswan CVE-2026-77206 Details note. Verify the installed version with ipsec --version after upgrade.

Workarounds

Apply network ACLs that limit IKEv2 traffic to trusted VPN peers until patching is complete
Place IPsec responders behind a filtering device that enforces source-address allowlists for UDP/500 and UDP/4500
Monitor pluto with a watchdog and configure throttled restart to prevent fast crash loops from exhausting resources

bash

# Example iptables rules restricting IKEv2 to known peer addresses
iptables -A INPUT -p udp --dport 500  -s <peer_ip>/32 -j ACCEPT
iptables -A INPUT -p udp --dport 4500 -s <peer_ip>/32 -j ACCEPT
iptables -A INPUT -p udp --dport 500  -j DROP
iptables -A INPUT -p udp --dport 4500 -j DROP