CVE-2026-68186: Linux Kernel Privilege Escalation Flaw

CVE-2026-68186 Overview

CVE-2026-68186 is a NULL pointer dereference vulnerability in the Linux kernel's binfmt_misc subsystem. The flaw resides in load_misc_binary(), which sets bprm->have_execfd when it encounters the 'O' or 'C' flag, before the interpreter file is actually opened. If the interpreter open fails with -ENOEXEC, the flag stays set while bprm->executable remains NULL. A later format handler can then run, and begin_new_exec() dereferences the missing executable via would_dump(bprm, bprm->executable).

Unprivileged users can trigger the condition because binfmt_misc can be mounted inside user namespaces.

Critical Impact
Unprivileged local users can trigger a kernel NULL pointer dereference through user-namespace-mounted binfmt_misc combined with a FUSE-backed interpreter that returns -ENOEXEC.

Affected Products

Linux kernel (upstream) versions containing the binfmt_mischave_execfd handling prior to the referenced fix commits
Distributions shipping vulnerable stable kernels tracked by commits 0f19d54e, 1cd4e9b7, 2dd02989, 5ccc99d5, and bbf5f639
Systems where binfmt_misc can be mounted in user namespaces

Discovery Timeline

2026-08-10 - CVE-2026-68186 published to NVD
2026-08-10 - Last updated in NVD database

Technical Details for CVE-2026-68186

Vulnerability Analysis

The binfmt_misc handler registers user-defined interpreters for arbitrary binary formats. When an entry uses the 'O' (open-binary) or 'C' (credentials) flag, the kernel promises to pass an open file descriptor for the target binary to the interpreter through AT_EXECFD. load_misc_binary() raises bprm->have_execfd as soon as it detects that flag, prior to opening the interpreter.

If the interpreter open fails and returns -ENOEXEC, the kernel's format search falls through to the next handler in the list. However, have_execfd remains set on the bprm structure while bprm->executable is still NULL. When a subsequent handler such as the native ELF loader accepts the binary, begin_new_exec() calls would_dump(bprm, bprm->executable) and dereferences the NULL pointer, causing a kernel oops.

Both would_dump() and the later FD_ADD(0, bprm->executable) call, which rejects NULL with -ENOMEM, are past the point of no return. The exec cannot be unwound safely.

Root Cause

The root cause is a state-tracking error [CWE-476, NULL Pointer Dereference]. have_execfd is set based on a flag inspection rather than on the successful completion of the interpreter open and stage. The flag and bprm->executable are not updated atomically, so an early failure leaves the two fields out of sync.

Attack Vector

An unprivileged local user mounts binfmt_misc inside a user namespace and registers an entry with the 'O' flag pointing to an interpreter path hosted on a FUSE filesystem the user controls. The user then executes a native ELF binary whose magic matches the entry. The FUSE server returns -ENOEXEC from the interpreter open. The kernel falls back to the ELF loader, which proceeds to begin_new_exec() and dereferences the NULL bprm->executable, producing a local denial of service.

The upstream fix moves the assignment of have_execfd next to execfd_creds, after the interpreter has been opened and staged. On open failure, have_execfd now stays clear, and the fallback handler behaves like any native exec. See the kernel patch commit 0f19d54e for the code change.

Detection Methods for CVE-2026-68186

Indicators of Compromise

Kernel oops or panic traces referencing begin_new_exec, would_dump, or load_misc_binary on the call stack
Unexpected process crashes on hosts where binfmt_misc is mounted inside unprivileged user namespaces
FUSE mounts owned by unprivileged users combined with binfmt_misc entries using the O or C flag

Detection Strategies

Audit /proc/sys/fs/binfmt_misc/ and container mount namespaces for user-registered entries whose interpreter paths resolve to FUSE-backed filesystems.
Alert on kernel ring buffer messages containing BUG: unable to handle alongside symbols from fs/exec.c or fs/binfmt_misc.c.
Correlate unshare or clone syscalls creating user namespaces with subsequent mount operations of type binfmt_misc executed by non-root users.

Monitoring Recommendations

Collect dmesg and /var/log/kern.log centrally and parse for NULL pointer dereference signatures in exec paths.
Track process execve events with argv[0] paths that do not match the resolved executable, which indicates a binfmt_misc argv rewrite.
Monitor creation of FUSE mounts by unprivileged UIDs in production workloads that do not require them.

How to Mitigate CVE-2026-68186

Immediate Actions Required

Apply the upstream stable kernel updates referenced by commits 0f19d54e, 1cd4e9b7, 2dd02989, 5ccc99d5, and bbf5f639 as soon as distribution packages become available.
Disable unprivileged user namespaces on hosts that do not require them by setting kernel.unprivileged_userns_clone=0 or user.max_user_namespaces=0.
Restrict who can load FUSE filesystems by removing SUID from fusermount or applying MAC policies to constrain FUSE mounts.

Patch Information

Fixes are distributed across multiple stable kernel trees. The relevant commits are 0f19d54e, 1cd4e9b7, 2dd02989, 5ccc99d5, and bbf5f639. The patch relocates the have_execfd assignment to occur only after the interpreter has been successfully opened and staged, alongside execfd_creds.

Workarounds

Unmount binfmt_misc where it is not required using umount /proc/sys/fs/binfmt_misc.
Remove or restrict binfmt_misc entries that use the O or C flag until patched kernels are deployed.
Prevent unprivileged users from creating user namespaces on affected hosts using sysctl controls or seccomp policies enforced by the container runtime.

bash

# Disable unprivileged user namespaces and audit binfmt_misc entries
sysctl -w kernel.unprivileged_userns_clone=0
sysctl -w user.max_user_namespaces=0
ls -la /proc/sys/fs/binfmt_misc/