CVE-2026-78126: strongSwan Use-After-Free Vulnerability

CVE-2026-78126 Overview

CVE-2026-78126 is a NULL pointer dereference vulnerability [CWE-476] in the strongSwan IPsec implementation. The flaw resides in the eap-aka plugin, which handles Extensible Authentication Protocol (EAP) Authentication and Key Agreement exchanges. Affected releases span strongSwan 4.1.10 through 6.0.7. A remote unauthenticated attacker can trigger the dereference during EAP-AKA negotiation, causing the charon daemon to crash and interrupting IPsec tunnel services. The vendor addressed the issue in strongSwan 6.1.0.

Critical Impact
Remote attackers can crash the strongSwan IKE daemon over the network without authentication, disrupting availability of VPN and IPsec services.

Affected Products

strongSwan 4.1.10 through 6.0.7
Systems using the eap-aka plugin for IPsec authentication
IPsec gateways and clients built on strongSwan libraries

Discovery Timeline

2026-09-07 - strongSwan project publishes vulnerability advisory
2026-09-11 - CVE-2026-78126 published to NVD
2026-09-14 - Last updated in NVD database

Technical Details for CVE-2026-78126

Vulnerability Analysis

The vulnerability is a NULL pointer dereference in strongSwan's eap-aka plugin. The plugin implements EAP-AKA, an authentication method originally designed for mobile networks that uses UMTS SIM credentials. When the plugin processes a malformed or unexpected EAP-AKA message, it accesses a pointer that has not been initialized or has been left NULL, causing the charon IKE daemon to terminate.

Because EAP negotiation occurs before user authentication completes, the attacker does not need valid credentials. Successful exploitation degrades availability only; there is no impact on confidentiality or integrity. Attack complexity is elevated because the attacker must reach a strongSwan endpoint that has the eap-aka plugin loaded and initiate an EAP-AKA exchange.

Root Cause

The defect is a missing NULL check in the EAP-AKA message handling path within the eap-aka plugin. The code assumes a data structure returned during protocol processing is always populated, but attacker-controlled input can leave that structure NULL. Dereferencing it causes a segmentation fault in the charon process.

Attack Vector

Exploitation is network-based. An attacker sends a crafted EAP-AKA message to a strongSwan responder or initiator that supports the plugin. Because IPsec gateways typically expose IKE on UDP 500 and 4500, the attack surface can extend to internet-facing VPN concentrators. Repeated exploitation prevents legitimate IPsec sessions from establishing, producing a sustained denial-of-service condition.

No public proof-of-concept exploit is available. Technical specifics are documented in the strongSwan vulnerability advisory.

Detection Methods for CVE-2026-78126

Indicators of Compromise

Unexpected charon daemon crashes with segmentation fault signatures in system logs
Repeated IKE_SA_INIT or IKE_AUTH failures involving EAP-AKA method identifiers
systemd or init supervisor entries showing strongswan or charon service restarts
Core dumps referencing the eap-aka plugin call stack

Detection Strategies

Parse charon.log and syslog for SIGSEGV, plugin 'eap-aka', and abnormal daemon termination messages
Monitor IKE traffic on UDP 500 and 4500 for anomalous EAP-AKA message patterns from unknown peers
Correlate daemon restarts with concurrent inbound IKE sessions to identify probing behavior
Track strongSwan package versions across the fleet to identify hosts running vulnerable builds

Monitoring Recommendations

Enable verbose IKE and EAP logging at charon startup to capture failed negotiations
Forward strongSwan logs to a centralized SIEM for correlation and alerting on repeated crashes
Alert when the charon process restart count exceeds a baseline threshold within a short window

How to Mitigate CVE-2026-78126

Immediate Actions Required

Upgrade strongSwan to release 6.1.0 or later on all affected gateways and clients
Inventory systems running strongSwan 4.1.10 through 6.0.7 and prioritize internet-facing IPsec endpoints
If patching is delayed, disable the eap-aka plugin on hosts that do not require it
Restrict IKE traffic to known peer addresses using firewall access control lists where feasible

Patch Information

The fix is included in strongSwan 6.1.0, published by the project on the GitHub strongSwan 6.1.0 release page. Additional context is available in the strongSwan vulnerability blog post.

Workarounds

Disable the eap-aka plugin in the strongSwan configuration if EAP-AKA authentication is not required
Limit IKE responder exposure by allowing only trusted source addresses on UDP 500 and 4500
Deploy IKE rate limiting or connection throttling to slow repeated crash attempts against unpatched hosts

bash

# Disable the eap-aka plugin at build time or via strongswan.conf
# In strongswan.conf:
charon {
    load_modular = yes
    plugins {
        eap-aka {
            load = no
        }
    }
}

# Restart the service after applying the configuration
systemctl restart strongswan