CVE-2026-89698 in Linux
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
nfsd: widen nfsd_genl_rqstp address fields to sockaddr_storage
struct nfsd_genl_rqstp declares rq_daddr and rq_saddr as plain "struct sockaddr" (16 bytes). When an IPv6 NFS client is connected, nfsd_genl_rpc_status_compose_msg() casts these fields to "struct sockaddr_in6 *" (28 bytes) and reads sin6_addr at offset 8..24, which extends 8 bytes past the end of the 16-byte sockaddr field into the adjacent rq_flags member. The 16-byte nla_put_in6_addr then ships 8 bytes of truncated IPv6 address followed by 8 bytes of rq_flags to userspace via the NFSD_A_RPC_STATUS_SADDR6/DADDR6 netlink attributes.
This is reachable by any unprivileged process in the network namespace because NFSD_CMD_RPC_STATUS_GET uses GENL_CMD_CAP_DUMP without GENL_ADMIN_PERM.
Fix by widening rq_daddr and rq_saddr to struct sockaddr_storage so the IPv6 casts operate within bounds, copying sizeof(struct sockaddr_storage) bytes in the memcpy calls so the full address is captured, and zero-initializing the genl_rqstp stack variable to prevent leaking uninitialized tail bytes through netlink.
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Analysis
by VulDB Data Team • 09/11/2026
The Linux kernel vulnerability identified involves a buffer over-read condition within the NFS server daemon component, specifically affecting how network addresses are handled in generic Netlink communication structures. The core technical flaw resides in the definition of struct nfsd_genl_rqstp, where the destination and source address fields, rq_daddr and rq_saddr, are declared as plain struct sockaddr objects with a fixed size of sixteen bytes. This structure is designed to hold IPv4 addresses or truncated representations but lacks the capacity for full IPv6 addressing which requires twenty-eight bytes. When an unprivileged process in a network namespace issues a request via NFSD_CMD_RPC_STATUS_GET, the kernel attempts to compose a message using nfsd_genl_rpc_status_compose_msg(). During this operation, the code incorrectly casts these sixteen-byte fields to struct sockaddr_in6 pointers and proceeds to read data from offset eight through twenty-four. This access pattern extends eight bytes beyond the allocated boundary of the address field, intruding into the adjacent rq_flags member which stores internal state flags for the request structure.
The operational impact of this flaw is significant because it allows an attacker with local unprivileged network namespace access to extract sensitive kernel memory contents via Netlink attributes NFSD_A_RPC_STATUS_SADDR6 and NFSD_A_RPC_STATUS_DADDR6. The nla_put_in6_addr function transmits sixteen bytes to userspace, comprising eight bytes of potentially valid but truncated IPv6 address data followed by eight bytes derived from the rq_flags field. This constitutes an out-of-bounds read that can lead to information disclosure, exposing internal kernel state variables such as request flags which may reveal details about ongoing NFS operations or system configuration. The vulnerability is particularly concerning due to its low privilege requirement; since the command capability dump does not require administrative permissions, any user with network namespace access can trigger this memory leak without authentication barriers.
From a classification perspective, this issue aligns with CWE-125, Out-of-bounds Read, as it involves accessing memory beyond the intended buffer boundary. It also relates to CWE-200, Exposure of Sensitive Information to an Unauthorized Actor, due to the leakage of kernel internal state through network interfaces. In terms of MITRE ATT&CK mapping, this vulnerability facilitates reconnaissance and potentially information gathering techniques by allowing local users to probe kernel memory layout and operational states via standard networking APIs without elevated privileges. The root cause is a fundamental mismatch between data structure definitions and their usage in variable-length address handling scenarios common in modern IPv6-enabled environments.
The remediation strategy implemented addresses the structural deficiency by widening both rq_daddr and rq_saddr fields to struct sockaddr_storage, which provides sufficient space for both IPv4 and IPv6 addresses without truncation or overflow risks. The fix ensures that memcpy operations copy the full sizeof(struct sockaddr_storage) bytes, guaranteeing complete address data integrity when communicating with userspace. Additionally, the solution includes zero-initialization of the genl_rqstp stack variable to prevent information leakage through uninitialized tail bytes that might otherwise be copied into Netlink messages if not explicitly cleared. This comprehensive approach eliminates both the out-of-bounds read and potential secondary leaks from unallocated memory regions. System administrators should apply kernel updates containing this patch immediately, particularly in environments where NFS services are exposed or network namespaces are utilized for containerization purposes to mitigate unauthorized information disclosure risks.