CVE-2026-101303 in FreeBSD
Summary
by MITRE • 10/15/2026
The IPv6 UDP send path for unconnected sockets did not apply the jail policy of rewriting a loopback destination address to the jail's primary IPv6 address.
A process in a classic (non-VNET) jail can send UDP datagrams to services listening on the host's IPv6 loopback address, bypassing jail network isolation.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability described involves a critical flaw in the IPv6 User Datagram Protocol implementation within FreeBSD jails, specifically affecting unconnected sockets that do not have an explicit destination address bound at creation time. In this scenario, when a process inside a classic non-VNET jail attempts to send UDP datagrams without first establishing a connection or binding a specific local endpoint, the kernel's network stack fails to enforce the expected security policy regarding loopback addresses. The core technical flaw lies in the handling of destination address resolution for unconnected sockets during the IPv6 transmission path. Normally, when traffic is directed toward the host's IPv6 loopback interface (::1), the jailing mechanism should intercept this request and rewrite the destination address to match the primary IPv6 address assigned specifically to that jail instance. This rewriting process is essential for maintaining network isolation boundaries between the guest environment and the underlying host system. However, due to the oversight in the code path handling unconnected sockets, this translation step was skipped entirely.
This omission results in a significant breach of containerization security principles by allowing processes within an isolated jail to communicate directly with services running on the host's loopback interface. In standard network configurations, the loopback address is often used for inter-process communication or local service access that should remain internal to the system. By bypassing this restriction, attackers who have gained code execution privileges inside a compromised jail can reach sensitive administrative interfaces, debugging tools, or other services bound exclusively to localhost on the host machine. This effectively nullifies one of the primary layers of defense provided by jails, which is network isolation from the parent environment and potentially other sibling jails if they share the same loopback space.
The operational impact of this vulnerability extends beyond simple access control bypasses. It enables potential privilege escalation scenarios where an attacker leverages local services on the host to escalate their privileges or pivot further into the infrastructure. For instance, many administrative tools rely on localhost connections for authentication and data exchange without requiring additional network-level security checks such as firewalls or TLS encryption, assuming that physical or virtual isolation is sufficient protection. An adversary exploiting this flaw can interact with these trusted local services directly from within a restricted container environment, potentially extracting sensitive configuration data, manipulating system states, or launching attacks against other components of the host system that are not exposed to external networks but were incorrectly deemed safe due to their loopback binding.
From an industry standards perspective, this vulnerability aligns closely with CWE-250, which refers to Execution with Unnecessary Privileges, as it allows a process operating under restricted permissions to perform actions outside its designated scope. It also relates to CWE-693, Protection Mechanism Failure, specifically in the context of network isolation mechanisms failing to enforce intended security policies. In terms of the MITRE ATT&CK framework, this behavior facilitates lateral movement and privilege escalation techniques where an attacker moves from a compromised containerized environment into the host operating system by exploiting misconfigured trust relationships between local services and isolated processes.
To mitigate this risk, immediate patching is required to update the FreeBSD kernel or relevant network stack components to include the missing logic for rewriting loopback addresses in unconnected UDP socket transmissions. System administrators should also consider implementing additional defense-in-depth measures such as strict firewall rules that block inbound traffic from jail IP ranges to host loopback interfaces where possible, although this may not be feasible if jails share the same network namespace entirely. Furthermore, auditing applications running within jails for any reliance on localhost connectivity with the host can help identify potential attack vectors before exploitation occurs. Regular security audits and penetration testing focused on container escape techniques are recommended to ensure that isolation boundaries remain intact against evolving threat landscapes.