CVE-2026-64044 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
ovpn: respect peer refcount in CMD_NEW_PEER error path
ovpn_nl_peer_new_doit()'s error path calls ovpn_peer_release() directly rather than ovpn_peer_put(), bypassing the kref. The accompanying comment ("peer was not yet hashed, thus it is not used in any context") holds for UDP but not for TCP.
For UDP, the ovpn_socket union uses the .ovpn arm and never points back at a peer; UDP encap_recv looks up peers via the not-yet-populated hashtables, so the new peer is unreachable until ovpn_peer_add() publishes it.
For TCP, ovpn_socket_new() sets ovpn_sock->peer and ovpn_tcp_socket_attach() publishes ovpn_sock via rcu_assign_sk_user_data(). From that moment until ovpn_socket_release() detaches in the error path, the TCP fd is fully wired: userspace recvmsg / sendmsg / close / poll on the fd, as well as the strparser-driven ovpn_tcp_rcv() path, can reach the peer through sk_user_data -> ovpn_sock->peer and bump its refcount via ovpn_peer_hold().
ovpn_tcp_socket_wait_finish() (called inside ovpn_socket_release()) drains strparser and the tx work, but does not synchronize with userspace syscall callers that already hold a peer reference. If ovpn_nl_peer_modify() or ovpn_peer_add() returns an error while such a caller is in flight - notably an ovpn_tcp_recvmsg() blocked in __skb_recv_datagram() on peer->tcp.user_queue - the direct ovpn_peer_release() destroys the peer while the caller still holds the reference, and the eventual ovpn_peer_put() from that caller operates on freed memory.
Replace the direct destructor call with ovpn_peer_put() so the kref correctly defers destruction until the last reference is dropped. In the common case where no concurrent user is present, behaviour is unchanged: the kref hits zero immediately and ovpn_peer_release_kref() runs the same destructor.
With this conversion ovpn_peer_release() has no callers outside peer.c - ovpn_peer_release_kref() in the same translation unit is the only remaining user - so make it static and drop its declaration from peer.h.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability exists within the OpenVPN implementation in the Linux kernel, specifically affecting how peer references are managed during error conditions in the command processing flow. This flaw stems from an incorrect handling of reference counting mechanisms when a new peer creation operation fails. The core issue manifests in the `ovpn_nl_peer_new_doit()` function where, upon encountering an error condition, the code directly invokes `ovpn_peer_release()` instead of properly decrementing the reference count through `ovpn_peer_put()`. This bypasses the kernel's reference counting infrastructure known as kref, which is designed to ensure that objects are only destroyed when no active references remain.
The technical complexity of this vulnerability becomes apparent when examining the different transport protocols supported by OpenVPN. For UDP connections, the socket structure uses a union where the `.ovpn` arm is employed and does not maintain back-pointers to peer structures, making the peer unreachable until it's officially added to hash tables through `ovpn_peer_add()`. However, for TCP connections, the situation differs significantly as `ovpn_socket_new()` establishes a direct link between socket and peer structures, while `ovpn_tcp_socket_attach()` publishes this relationship via `rcu_assign_sk_user_data()`. Once established, all userspace operations including recvmsg, sendmsg, close, and poll can access the peer through the socket's user data field and increment its reference count using `ovvpn_peer_hold()`.
This distinction creates a race condition scenario where concurrent access becomes problematic. The `ovpn_tcp_socket_wait_finish()` function handles draining of strparser and transmit work queues but fails to synchronize with active syscall callers that already possess peer references. When an error occurs during operations like `ovpn_nl_peer_modify()` or `ovpn_peer_add()` while a caller is actively processing, such as an `ovpn_tcp_recvmsg()` blocked in `__skb_recv_datagram()` on `peer->tcp.user_queue`, the direct call to `ovpn_peer_release()` destroys the peer object while active references still exist. Subsequent calls to `ovpn_peer_put()` from these callers then operate on freed memory, leading to potential kernel crashes or undefined behavior.
The vulnerability maps directly to CWE-476 which addresses NULL pointer dereferences and improper handling of reference counting mechanisms in kernel code. From an operational perspective, this flaw represents a critical security risk as it allows for potential denial-of-service conditions and memory corruption that could be exploited by malicious actors. The fix implements proper reference counting by replacing the direct destructor call with `ovpn_peer_put()`, ensuring that destruction only occurs when all references are released. This approach maintains backward compatibility in normal operation while correctly handling edge cases involving concurrent access, and also includes a code cleanup by making `ovpn_peer_release()` static since it's no longer called from external modules.
This vulnerability aligns with ATT&CK technique T1059.006 for kernel-level privilege escalation and T1489 for system network configuration modification, as it affects the fundamental reference counting mechanism that governs resource management in the OpenVPN subsystem. The fix ensures proper adherence to kernel security principles by maintaining object lifetime management through established reference counting patterns rather than direct destructor invocation, thus preventing potential exploitation through memory corruption attacks that target kernel data structures.