CVE-2026-119425 in OPNsense
Summary
by MITRE • 10/30/2026
OPNsense core 26.1.6 through 26.7.7 contains an authentication bypass vulnerability in the captive portal roaming feature that lets unauthenticated attackers hijack authorized client sessions by spoofing unauthenticated MAC-to-IP observations. Attackers on the same Layer-2 segment can send forged gratuitous ARP replies binding their IP to a victim's MAC, gaining portal access with traffic attributed to the victim.
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Analysis
by VulDB Data Team • 10/30/2026
The vulnerability identified in OPNsense core versions 26.1.6 through 26.7.7 represents a critical authentication bypass within the captive portal roaming feature, specifically exploiting weaknesses in how Layer-2 network identity is mapped to user sessions. This flaw allows unauthenticated attackers who are positioned on the same local area network segment as the target victims to hijack authorized client sessions. The core of the issue lies in the system's reliance on Media Access Control addresses and Internet Protocol address mappings for session management without sufficient cryptographic verification or binding integrity checks. By spoofing MAC-to-IP observations, an attacker can effectively impersonate a legitimate user who has already passed through the captive portal authentication process.
From a technical perspective, the exploitation mechanism involves the injection of forged gratuitous Address Resolution Protocol replies into the network traffic stream. In standard Ethernet networking, devices use ARP to resolve IP addresses to MAC addresses for local communication. When an attacker sends a spoofed gratuitous ARP reply that binds their own IP address to the victim's MAC address, they manipulate the switch and router forwarding tables within the LAN segment. Consequently, subsequent traffic intended for the victim is redirected or associated with the attacker’s connection context. Because OPNsense tracks sessions based on these observed MAC-to-IP associations rather than requiring continuous re-authentication or binding tokens tied to specific hardware identifiers securely, the system incorrectly attributes the new session activity to the previously authenticated user.
The operational impact of this vulnerability is severe for organizations relying on captive portals for network access control and policy enforcement. An attacker can gain unauthorized access to internal resources that are restricted to authenticated users by simply spoofing the MAC address of a legitimate client who has already logged in. This bypasses not only authentication but also any role-based access controls or bandwidth limiting policies associated with that user’s profile. Furthermore, because the traffic is attributed to the victim, it can lead to false positive security alerts and complicate forensic investigations, as logs will show activity originating from a trusted account rather than an external threat actor. This undermines the fundamental trust model of network segmentation and access control systems deployed in enterprise environments.
This vulnerability aligns with Common Weakness Enumeration category CWE-287, which describes Improper Authentication, specifically where identity verification is bypassed through spoofing or manipulation of credentials or session identifiers. In terms of offensive security frameworks, this behavior corresponds to MITRE ATT&CK technique T1094.003, known as Spoof Gateway Address under the Adversary-in-the-Middle tactic. The attacker acts as a man-in-the-middle at Layer 2, intercepting and potentially modifying traffic while maintaining the guise of an authorized user. This highlights the risks associated with relying solely on MAC addresses for session continuity in dynamic network environments where ARP spoofing is feasible.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. The primary defense is to apply the vendor-provided patch that updates OPNsense core to a version beyond 26.7.7, which likely implements stricter binding mechanisms or cryptographic validation for session continuity. For environments where updating may not be immediately feasible, network administrators should consider implementing Dynamic ARP Inspection on managed switches to prevent hosts from sending unsolicited or invalid ARP replies. Additionally, enabling port security features that limit the number of MAC addresses per switch port can restrict an attacker’s ability to spoof multiple identities. From a policy standpoint, reducing session timeout durations and requiring periodic re-authentication for sensitive operations can minimize the window of opportunity for such hijacking attacks. Ultimately, relying on Layer-2 identifiers alone for authentication continuity is inherently risky in untrusted or shared network segments, and organizations should evaluate whether their captive portal implementation requires stronger identity binding mechanisms that are resistant to ARP spoofing.