CVE-2026-53191: Linux Kernel io_uring Buffer Flag Loss Vulnerability

Technical summary

CVE-2026-53191 affects io_uring's bundled receive functionality in the Linux kernel. When io_recv_finish() retries a bundle recv operation, it merges completion flags using the expression 'cflags = req->cqe.flags | (cflags & CQE_F_MASK)'. The CQE_F_MASK bitmask was incomplete and did not include IORING_CQE_F_BUF_MORE. When provided buffer rings operate in incremental mode (IOU_PBUF_RING_INC), the io_kbuf_inc_commit() function can leave a ring entry partially consumed, setting IORING_CQE_F_BUF_MORE in cflags to signal userspace that a buffer ID will be reused. However, because this flag was excluded from CQE_F_MASK, it was silently dropped during the merge logic, causing userspace to lose critical state information about buffer reuse. This leads to incorrect ring head advancement and potential use-after-free conditions on kernel buffers.

Business impact

Systems running affected Linux kernel versions with io_uring-based networking applications face elevated risk of denial-of-service and privilege escalation. The vulnerability permits local attackers with sufficient privileges to trigger memory corruption, crash services relying on io_uring, or potentially escalate privileges. This particularly affects database engines, high-performance networking tools, and container runtimes that leverage io_uring for asynchronous I/O. Business continuity of services depends on rapid patching of production systems.

Affected systems

The Linux kernel is affected, specifically versions that support io_uring with bundle recv operations and provided buffer rings in incremental mode. Organizations using container platforms, cloud infrastructure, or custom applications built on io_uring-based networking (particularly databases and message brokers) should assess their kernel versions. Verify your specific kernel release against available security advisories from your Linux distribution vendor.

Exploitability

The vulnerability requires local access and elevated privileges (PR:L in the CVSS vector), making opportunistic remote exploitation unlikely. However, local privilege escalation attacks and malicious containerized workloads can trigger this condition. The straightforward nature of the flag-handling error means that once discovered, exploitation proof-of-concepts may circulate quickly among security researchers and potentially attackers.

Remediation

Apply the kernel security update provided by your Linux distribution vendor that adds IORING_CQE_F_BUF_MORE to the CQE_F_MASK definition. This ensures the flag is properly inherited during retry iterations and stripped from saved cflags between iterations. The fix is minimal and focused; no workarounds or configuration changes are required once the patch is deployed.

Patch guidance

Contact your Linux distribution's security advisory channel (Red Hat, Canonical, SUSE, etc.) for the specific kernel version addressing CVE-2026-53191. Updates typically ship as security errata. If you maintain custom kernels, backport the CQE_F_MASK change to your release branches. Test patches in a non-production environment before rolling out to ensure no regressions in io_uring-dependent workloads. Kernel updates often require a reboot; plan downtime accordingly.

Detection guidance

Monitor system logs for unexpected kernel oops, memory corruption warnings, or io_uring-related errors. Tools like auditd can track io_uring syscalls (io_uring_enter, io_uring_setup) to identify affected workloads. Vulnerability scanners that audit kernel versions against CVE databases can identify at-risk systems. Network-based detection of io_uring exploitation is limited; focus on host-based kernel panic logs and application-level anomalies in services using io_uring.

Why prioritize this

With a CVSS score of 7.8 (HIGH severity), this vulnerability combines significant impact (complete integrity and availability compromise) with moderate exploitability constraints (local, privileged access required). It directly affects modern, high-performance I/O stacks used in production databases, messaging brokers, and containerized environments. Although not currently in the CISA KEV catalog, the straightforward nature of the bug and its presence in a widely-used kernel subsystem justify rapid patching. Organizations running io_uring-dependent services should prioritize this alongside other kernel security patches.

Risk score, explained

The CVSS 3.1 score of 7.8 reflects a HIGH-severity vulnerability affecting confidentiality, integrity, and availability equally (C:H, I:H, A:H). The local attack vector (AV:L) and requirement for low privileges (PR:L) prevent a critical rating, yet the complete compromise of data and system stability justifies immediate attention. Kernel memory corruption has historically led to both denial-of-service and privilege escalation chains, elevating business risk even if the direct attack surface is limited to local users.

Frequently asked questions