CVE-2026-63926: Linux Kernel BPF Sockmap Vulnerability

CVE-2026-63926 Overview

CVE-2026-63926 is a Linux kernel vulnerability in the BPF sockmap subsystem. The flaw resides in bpf_msg_push_data(), which incorrectly advances the tail fragment offset when inserting data into the middle of a scatterlist entry. The function uses the message-global insertion point start instead of the fragment-local delta, over-advancing the offset and producing an inconsistent split layout for non-first scatter-gather (SG) entries. Local attackers with the ability to load or attach BPF programs can trigger memory corruption in kernel context. The upstream Linux kernel maintainers have merged patches across multiple stable branches to resolve the flaw.

Critical Impact
Local exploitation can corrupt kernel scatterlist state through the BPF sockmap message-push helper, threatening confidentiality, integrity, and availability of affected Linux systems.

Affected Products

Linux kernel builds shipping the bpf_msg_push_data() sockmap helper prior to the fixing commits
Linux distributions using vulnerable stable kernels that have not backported the patch set
Systems that permit unprivileged or privileged BPF program loading against sockmap-attached sockets

Discovery Timeline

2026-07-19 - CVE-2026-63926 published to NVD
2026-07-20 - Last updated in NVD database

Technical Details for CVE-2026-63926

Vulnerability Analysis

The vulnerability lives in the Berkeley Packet Filter (BPF) sockmap helper bpf_msg_push_data(). This helper allows BPF programs attached to sockets to insert bytes into a message buffer represented as a scatterlist. When the insertion point falls inside an existing scatterlist entry, the helper splits that entry into a left fragment (retained head) and a right fragment (retained tail).

The defect is arithmetic. The right fragment's offset field is page-local, describing the byte offset into its backing page. The buggy code advanced this offset by start, the message-global insertion point that includes the cumulative length of every preceding scatterlist entry. For any insertion into a non-first SG entry, start exceeds the fragment-local delta, so the right fragment ends up pointing beyond the intended page region.

Root Cause

The root cause is a boundary condition error [CWE-193 class] combining incorrect index arithmetic with an inconsistent state after list splitting. The corrective patch advances the right fragment offset by start - offset, the fragment-local delta that matches the number of bytes removed from the front of the original entry. Without this fix, the split scatterlist becomes desynchronized from the underlying pages, corrupting kernel memory bookkeeping.

Attack Vector

Exploitation requires local access and the ability to load or attach an eBPF program that invokes bpf_msg_push_data() on a sockmap-attached socket. An attacker crafts a message layout that forces insertion into a non-first scatterlist entry, then invokes the helper to trigger the miscalculated tail offset. The resulting inconsistent scatterlist can be leveraged to read or write out-of-bounds kernel memory, corrupt socket buffer state, or crash the kernel. The vulnerability yields high impact to confidentiality, integrity, and availability from a local attack vector requiring no user interaction.

No public proof-of-concept code is available. Technical details are captured in the upstream fix commits, including 3075c21d2d76 and f14609d81467.

Detection Methods for CVE-2026-63926

Indicators of Compromise

Unexpected kernel panics, BUG: messages, or KASAN reports referencing bpf_msg_push_data, sk_msg, or scatterlist helpers
Unprivileged or lightly privileged processes loading BPF programs of type BPF_PROG_TYPE_SK_MSG on production systems
Sockmap or sk_msg BPF program attachments on hosts that do not normally use them

Detection Strategies

Audit bpf() syscalls with BPF_PROG_LOAD and BPF_PROG_ATTACH operations, focusing on SK_MSG program types and sockmap targets
Use auditd or eBPF-based process telemetry to record which UIDs invoke BPF loading on Linux hosts
Correlate kernel oops or KASAN traces mentioning sockmap message helpers with the process that most recently loaded a BPF program

Monitoring Recommendations

Ship kernel logs and dmesg output to a central log platform and alert on stack traces involving bpf_msg_push_data or sk_msg_alloc
Track kernel version and patch level inventory across the fleet to identify hosts still exposed
Monitor for privilege changes or capability grants (CAP_BPF, CAP_SYS_ADMIN) that widen the population of users able to reach the vulnerable code path

How to Mitigate CVE-2026-63926

Immediate Actions Required

Apply the vendor kernel update from your Linux distribution that incorporates the upstream sockmap fix
Inventory hosts running vulnerable kernels and prioritize multi-tenant systems, container hosts, and shared workstations
Restrict BPF program loading to trusted administrators by enforcing kernel.unprivileged_bpf_disabled=1 and gating CAP_BPF

Patch Information

The Linux kernel maintainers have merged the fix across stable branches. The change replaces the incorrect advancement offset += start with offset += start - offset, matching the length trimmed from the head of the original scatterlist entry. Consult the following commits and pull the update appropriate for your kernel line: 3075c21d2d76, 5e1902866796, 63f64a510c79, 96b72672ce84, aeb95146848d, d81b323af2dc, f14609d81467, and f72eed9b84fb.

Workarounds

Disable unprivileged BPF by setting sysctl -w kernel.unprivileged_bpf_disabled=1 and persisting it in /etc/sysctl.d/
Remove CAP_BPF and CAP_SYS_ADMIN from non-administrative service accounts and container workloads
Where feasible, avoid deploying SK_MSG BPF programs against sockmap on kernels that have not yet been patched

bash

# Disable unprivileged BPF program loading until the kernel is patched
echo 'kernel.unprivileged_bpf_disabled=1' | sudo tee /etc/sysctl.d/90-bpf-hardening.conf
sudo sysctl --system

# Verify the setting is active
sysctl kernel.unprivileged_bpf_disabled

# Confirm the running kernel version against the stable fix
uname -r