[CVE-2026-68186][MODERATE REGULAR] binfmt_misc: set have_execfd only once the interpreter is opened

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-68186
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: binfmt_misc: set have_execfd only once the interpreter is opened
Commit: 0f19d54e2524f0bf183b82f365ae4e49b4a2f788
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0f19d54e2524f0bf183b82f365ae4e49b4a2f788
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68186
Analysis date: Mon, 10 Aug 2026 17:43:22 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended / Actionable Moderate only in hardened or multi tenant namespace setups
Manual review required: YES

Summary:
A local unprivileged user may trigger a binfmt_misc exec state mismatch where have_execfd is set without a staged executable, causing begin_new_exec() to NULL dereference bprm->executable and crash the kernel.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-68186 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68186

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE REGULAR
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-68186	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-703;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL-pointer dereference can occur in the binfmt_misc exec path because bprm have_execfd was set before the interpreter file was actually opened and staged. If opening the interpreter fails with ENOEXEC, binary format search can fall through to a native handler while bprm executable is still NULL but have_execfd incorrectly says an execfd exists. An unprivileged local user can trigger this on systems where binfmt_misc is available in user namespaces, for example by using a FUSE backed interpreter that returns ENOEXEC and then executing a matching native ELF file. For the CVSS the PR:L is used because reliable triggering requires local user controlled setup and execution, but it does not require full host root privileges in the affected configuration. The issue is not network reachable. Impact is local denial of service via kernel oops or crash, with no supported confidentiality or integrity impact because the primitive is a NULL pointer dereference rather than UAF or writable memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended / Actionable Moderate only in hardened or multi tenant namespace setups (with actual score 4)	YES	OOB DISK LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended / Actionable Moderate only in hardened or multi tenant namespace setups**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. The commit states that unprivileged users can reach this when binfmt_misc is available in user namespaces.
* Reliable kernel crash / strong DoS: +1. The failure path reaches begin_new_exec() with bprm->executable NULL and causes a NULL pointer dereference after the point of no return.
* Cross-boundary namespace impact: +1. A user namespace setup can let a reduced privilege user trigger a host kernel failure, affecting other workloads.

Paranoid additional signal:

* Availability impact realistic: +1. If the affected configuration is enabled and FUSE is usable, the trigger is controlled and repeatable enough to be treated as a practical local host DoS.

Not counted:

* No generic memory corruption. The primitive is a NULL pointer dereference, not UAF, OOB write, double free, type confusion, or arbitrary write.
* No privilege escalation plausible signal. The patch and described path do not show attacker controlled reclaim, writable corruption, credential overwrite, or fd theft.
* No confidentiality or integrity impact. The fallback state is inconsistent, but the concrete demonstrated failure is a crash.
* No remote reachability. This is a local exec/binfmt path.

## 3. Reachability analysis

The bug is triggered locally through binfmt_misc handling during exec. The described path needs a binfmt_misc entry with the O or C behavior, an interpreter open failure returning ENOEXEC, and fallback to another binary format such as ELF while have_execfd remains incorrectly set.

Namespaces matter because the commit explicitly says binfmt_misc can be mounted in user namespaces, making this reachable by an unprivileged local user in affected configurations. FUSE is used in the described trigger to make the interpreter open fail in the needed way. This is not a network exposed path and is not triggered by packet input.

The path is not universally enabled by default on all systems, but user namespaces, binfmt_misc, and FUSE are common enough that this should not be treated as a purely theoretical or rare hardware issue.

Call-site confidence: high. The commit identifies the downstream dereference in begin_new_exec() and the relevant state transition in load_misc_binary().

## 4. Severity interpretation

This behaves primarily like a local denial of service issue, not an Important-class memory corruption vulnerability. The realistic impact is a NULL pointer dereference and kernel oops or crash from an unprivileged local trigger under a specific namespace and FUSE capable setup.

Theoretical escalation evidence is weak. There is no UAF, stale object reuse, writable stale pointer, arbitrary write, type confusion, or controlled object replacement shown by the patch. Therefore it should not be scored as LPE or strong memory corruption.

Practical triage result: borderline Moderate with manual review recommended, mainly because an unprivileged namespace user may be able to crash the host kernel.

## 5. One-sentence report phrase

A local unprivileged user may trigger a binfmt_misc exec state mismatch where have_execfd is set without a staged executable, causing begin_new_exec() to NULL dereference bprm->executable and crash the kernel.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the bug is not memory corruption and does not show LPE, but it is explicitly reachable by unprivileged users in user namespace configurations and can cause a host kernel crash.

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: binfmt_misc: set have_execfd only once the interpreter is opened
Commit: 0f19d54e2524f0bf183b82f365ae4e49b4a2f788
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0f19d54e2524f0bf183b82f365ae4e49b4a2f788

Commit description:

load_misc_binary() raises bprm->have_execfd as soon as it sees the 'O'
(or 'C') flag. This happens well before it opens the interpreter. If
that open fails the flag stays set on the bprm. binfmt_misc is at the
head of the format list so an interpreter open failure that returns
-ENOEXEC lets the search fall through to a later format. This means it
runs the matched binary directly having never staged an interpreter. So
bprm->executable is NULL while have_execfd falsely claims a descriptor
is present.

Consequently, begin_new_exec() dereferences the missing executable:

  would_dump(bprm, bprm->executable);

and NULL derefs. Had it not, the hand-off later in the same function
would have failed anyway. FD_ADD(0, bprm->executable) rejects a NULL
file with -ENOMEM. Both sites are past the point of no return so the
exec cannot be unwound either way.

This can be reached by unprivileged users as binfmt_misc can be mounted
in user namespaces. So a user can register an 'O' entry whose
interpreter lives on a FUSE mount, have the FUSE server fail the open
with -ENOEXEC and execute a native ELF file that matches the entry.

have_execfd only means anything alongside the executable it describes
which is not set until the interpreter has been opened and staged.
So lets raise it there, next to execfd_creds, which is already set at
that point. An open failure now leaves it clear, so the fallback format
derives credentials from the binary and emits no AT_EXECFD, as it would
for any native exec. The argv rewrite load_misc_binary() performs before
the open is still not undone. This means the binary sees the interpreter
path in argv[0] and its own path in argv[1] but that predates this
change and only became observable once the exec stopped faulting.

Link: https://patch.msgid.link/20260720-beglichen-kognitiv-organismus-5e1e55326c56@brauner
Fixes: bc2bf33 ("exec: Remove recursion from search_binary_handler")
Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/binfmt_misc.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0f19d54e2524f0bf183b82f365ae4e49b4a2f788

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-68186 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68186

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 3
  Paranoid ActionableScore: 4

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).