CVE-2026-53250: Linux Kernel Race Condition Vulnerability

CVE-2026-53250 Overview

CVE-2026-53250 is a Time-of-Check Time-of-Use (TOCTOU) race condition in the Linux kernel's AF_XDP socket (XSK) transmit path. The vulnerability resides in the xsk_skb_metadata() function, where csum_start and csum_offset fields are read from a userspace-writable UMEM buffer twice: once for bounds validation and again for socket buffer assignment. A local attacker controlling the UMEM mapping can race between these reads to bypass the bounds check, triggering out-of-bounds memory access during checksum computation in the kernel's transmit path.

Critical Impact
Local userspace processes with AF_XDP socket access can corrupt kernel memory through a race window, potentially leading to kernel memory disclosure or denial of service.

Affected Products

Linux kernel versions implementing AF_XDP TX metadata support
Distributions shipping vulnerable kernel branches prior to the upstream stable backports
Systems exposing AF_XDP sockets to unprivileged or sandboxed workloads

Discovery Timeline

2026-06-25 - CVE-2026-53250 published to NVD
2026-06-25 - Last updated in NVD database

Technical Details for CVE-2026-53250

Vulnerability Analysis

The vulnerability is a classic TOCTOU race condition in the kernel's XDP socket transmit metadata handling. The TX metadata area lives in the UMEM buffer, a memory-mapped region shared between kernel and userspace. Because userspace retains write access to this region while the kernel processes transmit descriptors, any field read more than once from this memory may observe inconsistent values.

In the vulnerable xsk_skb_metadata() implementation, the kernel reads csum_start and csum_offset from the shared UMEM to validate they fall within buffer bounds. It then re-reads the same fields to populate the corresponding sk_buff checksum offload fields. A concurrent userspace thread can mutate these values between the validation and the assignment, allowing values that failed the original bounds check to be used during checksum computation, [CWE-367].

Root Cause

The root cause is the kernel trusting that values in a userspace-writable shared mapping remain stable across two separate reads. No local caching or atomic snapshot was used, so the validated value was never guaranteed to equal the value subsequently consumed. The fix reads csum_start and csum_offset once into local stack variables, then uses those local copies for both validation and assignment.

Attack Vector

Exploitation requires local access and the ability to create an AF_XDP socket with a registered UMEM. The attacker maps the UMEM, submits TX descriptors referencing metadata, and uses a sibling thread to flip csum_start or csum_offset to an out-of-bounds value immediately after the kernel performs its bounds check. When the kernel proceeds to apply the checksum offload, the hardware or software checksum logic operates on memory outside the validated region.

The race is benign for other metadata fields such as flags and launch_time because no security-critical invariant depends on them remaining stable. Only the checksum offset pair gates a bounds-sensitive memory operation, which is why the fix is scoped to those two fields. Patch commits are available at the Linux Kernel Commit bfdfd2706d5f, Linux Kernel Commit 22ba97ea9cc1, and Linux Kernel Commit 0dfe05b93843.

Detection Methods for CVE-2026-53250

Indicators of Compromise

Unexpected kernel oops or BUG: messages referencing xsk_skb_metadata, skb_checksum_help, or __skb_checksum in dmesg
KASAN reports flagging out-of-bounds reads inside the XDP socket transmit path
Unprivileged processes creating AF_XDP sockets and registering UMEM regions on hosts that do not normally use XDP networking

Detection Strategies

Audit kernel versions across the fleet and compare against the patched stable branches referenced in the upstream commits
Enable KASAN on test and pre-production kernels to surface the out-of-bounds access if exploitation is attempted
Hunt for processes invoking socket(AF_XDP, SOCK_RAW, 0) outside of approved networking, observability, or DPDK-style workloads

Monitoring Recommendations

Forward dmesg and /var/log/kern.log to a centralized log platform and alert on kernel oops, KASAN, or UBSAN messages tied to XSK symbols
Monitor auditd for socket() syscalls with the AF_XDP family from unexpected UIDs or containers
Track kernel crash and panic rates per host to identify clusters that may indicate exploitation attempts

How to Mitigate CVE-2026-53250

Immediate Actions Required

Apply the upstream stable kernel update containing commits bfdfd2706d5f, 22ba97ea9cc1, and 0dfe05b93843, or the equivalent backport from your distribution vendor
Reboot affected hosts after upgrading the kernel package to ensure the patched code is active
Inventory workloads that legitimately require AF_XDP and restrict the capability elsewhere

Patch Information

The fix caches csum_start and csum_offset into local variables inside xsk_skb_metadata() so that the validated values are the exact values used for sk_buff assignment. The patch is available in the mainline tree and backported to active stable branches via the three referenced commits. Distribution kernels should be updated to the first packaged release that incorporates these commits.

Workarounds

Restrict AF_XDP socket creation by dropping CAP_NET_RAW from untrusted users and containers, or by applying a seccomp filter that blocks the AF_XDP family
Disable XDP zero-copy and TX metadata support on interfaces where it is not required
Use Linux Security Modules such as SELinux or AppArmor to constrain which binaries can open XDP sockets and register UMEM regions

bash

# Example seccomp-style policy fragment to deny AF_XDP socket creation
# AF_XDP == 44 on Linux
# Pseudocode for a seccomp-bpf filter:
#   if (syscall == socket && arg0 == 44) return SECCOMP_RET_ERRNO(EPERM);

# Verify a host has the patched kernel installed
uname -r
grep -E 'xsk_skb_metadata' /proc/kallsyms

# Confirm the distribution package includes the fix
dpkg -l | grep linux-image    # Debian/Ubuntu
rpm -qa | grep kernel         # RHEL/Fedora/SUSE