CVE-2026-80738 in Linux
Summary
by MITRE • 09/03/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: Check sk_state before sk_protocol in bpf_tcp_*_syncookie
bpf_tcp_gen_syncookie and bpf_tcp_check_syncookie accept a socket pointer 'sk' with argument type ARG_PTR_TO_BTF_ID_SOCK_COMMON. However, they access sk->sk_protocol without validating whether 'sk' represents a full socket.
Fix this issue by checking sk->sk_state != TCP_LISTEN before inspecting sk->sk_protocol in both bpf_tcp_gen_syncookie and bpf_tcp_check_syncookie. Since mini-sockets are never in the TCP_LISTEN state, the condition short-circuits and prevents dereferencing fullsock-specific fields.
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Analysis
by VulDB Data Team • 09/03/2026
The Linux kernel's Berkeley Packet Filter subsystem includes specific helper functions designed to facilitate TCP SYN cookie generation and verification within eBPF programs. These helpers, specifically bpf_tcp_gen_syncookie and bpf_tcp_check_syncookie, are intended for use by network security tools and load balancers that need to interact with the TCP handshake process at a low level. The vulnerability arises from an insufficient validation of the socket object passed as an argument to these functions. While the function signatures require an argument of type ARG_PTR_TO_BTF_ID_SOCK_COMMON, which theoretically covers both full sockets and mini-sockets used in listening states, the implementation fails to distinguish between them before accessing fields that are exclusive to fully established or non-listening sockets.
The core technical flaw lies in the order of operations within these helper functions. The code attempts to access sk->sk_protocol without first verifying whether the socket is currently in a state where this field is valid and populated for mini-sockets. In Linux kernel networking, when a socket is in the TCP_LISTEN state, it often operates as a mini-socket or uses a different internal structure that does not maintain standard protocol fields like sk->sk_protocol in the same manner as full sockets. Accessing these fields on an object that does not support them constitutes an out-of-bounds read or use-after-free scenario depending on memory layout and kernel version specifics, leading to potential information disclosure or system instability.
This vulnerability is classified under CWE-125, which describes Out-of-Bounds Read, as the code attempts to dereference a pointer into a region of memory that may not contain valid data for the specific socket type being processed. From an offensive security perspective, this aligns with ATT&CK technique T1083, File and Directory Discovery, if the leak allows reading kernel memory contents, or more broadly as part of privilege escalation vectors where kernel memory corruption can lead to arbitrary code execution. The lack of state validation creates a path for attackers who can influence socket creation states to trigger undefined behavior within the kernel space.
The operational impact of this flaw is significant because it affects the stability and security of systems running vulnerable Linux kernels that utilize eBPF programs for TCP SYN cookie handling. An attacker with the ability to craft specific network packets or manipulate socket states could potentially crash the system, causing a denial of service through a kernel panic. In more severe scenarios involving memory corruption, this vulnerability could be exploited to escalate privileges from an unprivileged user space process to root level within the kernel, compromising the entire host's security posture.
The resolution implemented by the Linux community addresses this issue by introducing a state check before accessing protocol-specific fields. Specifically, the fix adds a validation step that checks if sk->sk_state is not equal to TCP_LISTEN. This ensures that the code only proceeds to inspect sk->sk_protocol when it is certain that the socket object represents a full socket with valid protocol data structures. Since mini-sockets are never in the TCP_LISTEN state during normal operation, this condition effectively short-circuits access for those cases while allowing legitimate operations on full sockets to proceed correctly.
To mitigate the risks associated with this vulnerability, system administrators should ensure that all Linux kernels are updated to versions where this patch is applied. For environments running custom or older kernel builds, it is critical to audit eBPF programs that utilize bpf_tcp_gen_syncookie and bpf_tcp_check_syncookie helpers to verify they do not rely on unpatched helper implementations. Additionally, deploying runtime security monitoring tools can help detect anomalous behavior in network packet processing paths that might indicate exploitation attempts against this specific kernel flaw.