CVE-2026-77206 in Libreswan
Summary
by MITRE • 10/14/2026
Libreswan's IKEv2 state machine has a null pointer dereference bug in the EAP handling code for authentication. As a result of inconsistency within the IKEv2 state machine code, this bug can be triggered by any IKEv2 connection, even if EAP is not configured at all. The error is caused by a fall-through where it incorrectly dereferences ike->sa.st_eap which is NULL. A malicious IKE_AUTH packet can trigger this bug, meaning the peer does not need to be authenticated. The bug results in a crash and restart of the pluto daemon. No remote code execution is possible. Continued exploitation can cause a denial of service.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified within Libreswan's Internet Key Exchange version 2 implementation represents a critical flaw in the state machine logic governing authentication procedures, specifically affecting Extensible Authentication Protocol handling. This issue stems from an inconsistency in how the IKEv2 state machine processes incoming packets during the initial phases of connection establishment. The core technical defect is a null pointer dereference that occurs when the system attempts to access the st_eap member of the security association structure without first verifying its validity. In normal operational scenarios, this field would be populated if EAP authentication was explicitly configured and initiated; however, due to a code fall-through error in the state transition logic, the application proceeds to dereference this pointer even when no EAP context has been established or is required for the specific connection type.
This architectural flaw allows any peer attempting an IKEv2 connection to trigger the vulnerability without needing prior authentication credentials. A malicious actor can craft a specifically designed IKE_AUTH packet that exploits this state machine inconsistency, forcing the Libreswan daemon, known as pluto, to access memory at address zero or another invalid location associated with the null pointer. Because the check for EAP configuration is bypassed by the flawed control flow, the vulnerability is reachable from an unauthenticated remote position over the network. The immediate consequence of this exploitation vector is a segmentation fault that causes the abrupt termination and subsequent restart of the pluto daemon process on the affected system.
From an operational impact perspective, while the flaw does not permit arbitrary code execution or privilege escalation due to the nature of memory access violations in this context, it poses a severe risk to service availability. The crash of the primary IKEv2 daemon effectively breaks all active and new IPsec tunnels managed by that instance, leading to a complete denial of service for VPN connectivity provided by the affected Libreswan installation. An attacker capable of sending continuous malformed packets can sustain this condition through repeated crashes, ensuring persistent disruption of network services. This aligns with Common Weakness Enumeration category CWE-476, which defines null pointer dereference as a software error where code attempts to use a pointer that references no valid memory address, often leading to application crashes or undefined behavior.
Furthermore, the exploitation technique maps directly to MITRE ATT&CK tactic T1499, Endpoint Denial of Service, specifically under sub-techniques involving resource exhaustion through service disruption. The vulnerability highlights the importance of rigorous input validation and state verification in cryptographic protocol implementations, particularly where multiple authentication methods are supported but not all are enabled for every connection. Mitigation strategies primarily involve applying vendor-provided patches that correct the state machine logic to ensure proper null checks before dereferencing pointers related to EAP contexts. Until such updates are deployed, administrators may consider restricting IKEv2 access through network-level firewall rules or disabling specific vulnerable endpoints if feasible, though patching remains the definitive remediation path for this class of vulnerability in Libreswan deployments.