CVE-2026-80735 in Linux
Summary
by MITRE • 09/03/2026
In the Linux kernel, the following vulnerability has been resolved:
ovpn: ensure socket is owned by ovpn before deref sk_user_data
Some subsystems, like BPF SOCKMAP, set sk_user_data without actually setting the encap_type.
For this reason, we must make sure that the type is the one ovpn expects before dereferencing sk_user_data.
Failing to do so may lead to out-of-bounds reads.
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Analysis
by VulDB Data Team • 09/03/2026
The Linux kernel vulnerability identified in the OpenVPN (ovpn) subsystem highlights a critical race condition and logic error related to socket data handling, specifically concerning the interaction between network sockets and user-space context pointers. The core issue stems from how the ovpn module accesses private data associated with a socket via the sk_user_data field within the sock structure. In normal operation, this pointer is expected to reference an ovpn-specific control block that contains essential state information for managing virtual tunnel interfaces. However, the vulnerability arises because other kernel subsystems, most notably BPF SOCKMAP and potentially others utilizing generic socket attachment mechanisms, can populate the sk_user_data field without adhering to the specific protocol expectations of the ovpn driver. These subsystems may set sk_user_data to point to arbitrary or unrelated data structures while leaving the encap_type field unset or set to a value that does not indicate an OpenVPN encapsulation context.
This discrepancy creates a dangerous scenario where the ovpn code proceeds to dereference sk_user_data under the false assumption that it points to a valid and correctly structured ovpn control block. Since there is no prior verification of the encap_type, which serves as the definitive indicator that the socket is actually owned by or associated with an OpenVPN instance, the kernel may interpret unrelated memory contents as ovpn-specific data structures. This misinterpretation leads directly to out-of-bounds reads, where the kernel attempts to access fields within a structure that does not exist at the expected offset in memory. Such invalid memory accesses can result in information disclosure by leaking sensitive kernel or user-space data contained in adjacent memory regions, or they may trigger general protection faults leading to local denial of service conditions through kernel panics.
From a technical classification perspective, this flaw aligns with CWE-125 Out-of-bounds Read and CWE-403 Exposure of System Information if the read operation successfully retrieves sensitive data before crashing. The attack vector typically involves an unprivileged user or a compromised process that can manipulate socket attributes via BPF programs or other network stack interfaces to trigger this path in the ovpn driver. This scenario is relevant to MITRE ATT&CK techniques involving T1083 File and Directory Discovery if information disclosure occurs, or T1529 System Shutdown/Reboot if a denial of service is achieved through kernel instability. The vulnerability underscores the importance of strict ownership verification before dereferencing generic pointers in network subsystems, particularly when multiple kernel modules interact with shared socket structures.
To mitigate this risk, developers must enforce rigorous type checking prior to accessing sk_user_data within the ovpn code path. Specifically, the encap_type field should be validated against known OpenVPN encapsulation identifiers to ensure that the socket is genuinely owned by an ovpn instance before any dereference operations occur. This defensive programming practice prevents the kernel from interpreting arbitrary memory as valid control blocks and eliminates the possibility of out-of-bounds access resulting from mismatched data structures. Additionally, maintaining clear separation between generic socket attachment mechanisms like BPF SOCKMAP and protocol-specific drivers helps prevent such cross-subsystem confusion. System administrators should ensure that their Linux kernels are updated to include patches addressing this specific ovpn validation logic error, thereby closing the gap that allows unauthorized or malformed sk_user_data references to compromise system stability and confidentiality.