[CVE-2026-99042][IMPORTANT] io_uring/net: populate CONNECT filter fields for 24-byte sockaddr_in6

[CVE-2026-99042][IMPORTANT] io_uring/net: populate CONNECT filter fields for 24-byte sockaddr_in6

[AL-KERNEL]

CVE: CVE-2026-99042
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: io_uring/net: populate CONNECT filter fields for 24-byte sockaddr_in6
Commit: 596a955d2d01b7e29d9fed60ebcf7c4ba518b8e5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=596a955d2d01b7e29d9fed60ebcf7c4ba518b8e5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99042
Analysis date: Mon, 26 Oct 2026 21:32:02 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A local process can use a valid 24-byte IPv6 sockaddr to hide the real destination from an io_uring BPF connect filter and connect to an endpoint the policy denies.

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

The original Linux kernel CVE announcement for CVE-2026-99042 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99042

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: IMPORTANT
Manual review required: YES

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

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

CVE-2026-99042	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-99042 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT IMPORTANT AEGIS_AS=4 LEGACY=IMPORTANT RESULT=SAME (final reconciliation agrees; DECISION: both IMPORTANT)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N';CWE-863;*CWE-20;CWE-1284;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '7.1';DESCR 'A logic flaw in io_connect_bpf_populate causes a BPF connect filter to receive zero port and IPv6 address fields when IORING_OP_CONNECT uses an accepted 24 byte sockaddr_in6. The socket layer still uses the real destination, so a local process can bypass a deny rule for an IPv6 port or address by selecting addr_len 24 instead of 28. For the CVSS the PR:L value is used because the attacker must execute a local io_uring connect operation, while no elevated capability is normally required. The direct primitive is a network policy bypass rather than memory corruption, with potentially serious confidentiality and integrity impact when the filter is an isolation boundary for sensitive IPv6 services. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 7.1 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto was applied and KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were B4_PENDING_REVIEW_AUTHORIZATION_PRESERVE and MANUAL_REVIEW_REQUIRED and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 4 with primitive class CLASS_A and manual review REQUIRED. Its independently reconciled impact was IMPORTANT. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	OOB IMPROVEONLY IPV6 IO_URING B4_PENDING_REVIEW_AUTHORIZATION_PRESERVE MANUAL_REVIEW_REQUIRED KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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

ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=YES
ProtectedResourceImpact=CONFIRMED
B4PendingReview=NO
B4PendingConfirmed=YES

1. ActionableScore

* Raw base score: 5.
* Conservative score: 5; no floor or cap applied.
* Raw paranoid score: 5.
* Paranoid score: 5; no additional speculative impact awarded.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Concrete authorization decision bypass: a 24-byte IPv6 sockaddr is accepted by the socket layer, but the BPF filter receives zero port/address fields, changing a configured destination DENY into ALLOW.
* +1 Local unprivileged trigger: issuing `IORING_OP_CONNECT` normally requires neither UID 0 nor an administrative capability.
* +1 Practical exploit chain: the verified sequence is 24-byte sockaddr → zeroed filter metadata → filter permits → socket connects to the actual denied endpoint.
* +1 Integrity impact: the attacker performs a network operation explicitly prohibited by the installed connect policy.
* No memory-corruption, UAF, disclosure, sensitive-service compromise, LPE, or arbitrary access points awarded. The speculative C:H/I:H legacy vector is not treated as proof.

3. Reachability analysis

A privileged policy administrator may need to install the io_uring BPF filter, but the later bypassing connect is performed by the restricted local process. That prior defensive setup does not make the attacker host-admin-equivalent. No namespace capability is needed for the connect itself, and no race, unusual hardware, or retry sequence is involved.

4. Primitive and severity reading

The primitive is a clean authorization-context mismatch: filtering evaluates zero IPv6 destination fields while the socket layer consumes the real fields from the same accepted 24-byte address. The fix changes the population threshold from 28 bytes to `SIN6_LEN_RFC2133`, restoring the intended policy input.

B4-PENDING is confirmed through the concrete path `IORING_OP_CONNECT` → `io_connect_bpf_populate()` → destination filter decision → IPv6 connect. Access to the filter-protected endpoint is demonstrated; whether that endpoint exposes secrets or privileged functionality remains unproven. This is CLASS_A authorization bypass, not memory corruption. ImportantCandidate is retained because a below-host-admin actor demonstrably bypasses an enforced network boundary.

5. One-sentence report phrase

A local process can use a valid 24-byte IPv6 sockaddr to hide the real destination from an io_uring BPF connect filter and connect to an endpoint the policy denies.

6. Manual review advice

MANUAL CHECK REQUIRED

The verified below-host-admin DENY-to-ALLOW policy bypass satisfies B4-PENDING, while downstream service sensitivity and broader impact remain deployment-dependent.

7. Call-site confidence

Call-site confidence: high

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

Patch: io_uring/net: populate CONNECT filter fields for 24-byte sockaddr_in6
Commit: 596a955d2d01b7e29d9fed60ebcf7c4ba518b8e5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=596a955d2d01b7e29d9fed60ebcf7c4ba518b8e5

Commit description:

io_connect_bpf_populate() fills connect.port and connect.v6_addr for
AF_INET6 only when addr_len >= sizeof(struct sockaddr_in6), i.e. 28
bytes.

The IPv6 connect paths accept SIN6_LEN_RFC2133 (24 bytes), which
already holds sin6_port and sin6_addr. io_connect() passes addr_len to
the socket layer unchanged.

At addr_len 24 the filter sees family AF_INET6 with port and address
zero, while the kernel connects to the real destination. A filter that
denies an IPv6 port or address is bypassed. Denying connect.port == P
rejects [::1]:P with -EACCES at addr_len 28 but connects at addr_len 24.

Use SIN6_LEN_RFC2133 as the threshold. sin6_scope_id stays zero when
absent.

The threshold mismatch was found by an AI coding assistant (Claude Code)
comparing this hook against the net/ipv6 length checks on my
instructions; it also drafted the reproducer and this patch. I verified
the bypass and the fix in a VM on 7.3-rc6 and that 7.2 has the same
code.

Fixes: 899bea8 ("io_uring/net: allow filtering on IORING_OP_CONNECT")
Cc: stable@vger.kernel.org # 7.2
Signed-off-by: Natasha Klaus <natalie.klaus@runtimeverification.com>
Link: https://patch.msgid.link/20261009133807.730724-1-natalie.klaus@runtimeverification.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  io_uring/net.c
  net/compat.h
  net/ipv6.h

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=596a955d2d01b7e29d9fed60ebcf7c4ba518b8e5

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

The original Linux kernel CVE announcement for CVE-2026-99042 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99042

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:L/PR:L/UI:N/S:U/C:L/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
  The Best / paranoid CVSS score: 7.1

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: unknown
  Paranoid ActionableScore: 5

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: IMPORTANT
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): IMPORTANT

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).