* [CVE-2026-98751][MODERATE REGULAR] tty: abort break signalling on hangup
@ 2026-10-19 6:33 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-19 6:33 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98751
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: tty: abort break signalling on hangup
Commit: 3bc82999fb85cecc7eb6ec1755ee7d698aa1e783
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3bc82999fb85cecc7eb6ec1755ee7d698aa1e783
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98751
Analysis date: Mon, 19 Oct 2026 02:33:48 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
A race between a timed tty break ioctl and hangup can invoke a driver break callback after device shutdown or reopening, creating a driver-dependent local denial-of-service risk without establishing a generic UAF or stronger corruption primitive.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98751 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98751
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-2026-98751 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98751 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE AEGIS_AS=0 LEGACY=MODERATE RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;*CWE-672;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A race between the TCSBRK or TCSBRKP timed break operation and tty hangup can cause break_ctl() to be called after the underlying device has been removed, powered down, or possibly reopened. The second driver callback occurs after a user controlled timeout, so its effect depends on whether the affected tty driver safely handles stale device state and concurrent shutdown. The directly supported impact is a local denial of service through a driver failure, hang, or kernel crash. A driver that accesses stale driver_data could potentially expose a limited memory safety impact, but the generic tty diff does not establish a reliable UAF or arbitrary write primitive. For the CVSS the PR:L is selected because a local user or process must have access to the tty and issue the break ioctl. AC:H is selected because successful triggering requires the break timeout to overlap with hangup or device shutdown. The issue is not directly network reachable. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 5.8 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 0 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) MAYBE INIT PRINTF-TTY RACE DMAorINTERRUPT HARDWARE LINUS - - checked
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ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::
2. Signal breakdown
Conservative signals:
* +1 Local below-host-admin trigger. A process with access to the tty file can issue TCSBRK or TCSBRKP and control the break duration. No administrative capability check is shown.
* +1 Weak invalid-state callback. The second `break_ctl()` call can occur after hangup when the underlying device is gone, powered down, or potentially reopened.
* Total: 2.
Paranoid signal:
* +1 Plausible driver-dependent kernel failure. A driver that does not safely handle the documented race may dereference stale private state, hang, or fault. This supports review for local DoS but does not establish a generic UAF.
* Total: 3.
Not awarded:
* Real UAF +2 was not awarded. The patch does not establish that `tty`, `tty->ops`, or `driver_data` is freed and then dereferenced.
* No strong or constrained overwrite is shown. NEW52 B5-R does not apply because there is no heap OOB write or retry-accumulated index.
* No active-state trust-boundary, checksum-metadata, or shared-backing write exists.
* No controlled reclaim, arbitrary write, callback-pointer corruption, disclosure, authorization bypass, or privilege escalation is demonstrated.
* No additional availability point was awarded because persistent or host-wide failure is not established.
* No race penalty was applied. The timeout is user-controlled and the evidence does not establish a narrow or irreproducible timing window.
* Remote reachability is not established.
3. Reachability analysis
A local process must hold an accessible tty file and invoke TCSBRK or TCSBRKP. The timeout can be selected through the ioctl argument, while hangup or device shutdown must overlap the wait.
Host UID 0 and host-administrator capabilities are not inherently required after the process has access to the tty. Device-node permissions, a delegated serial-device FD, session ownership, or another local access mechanism may supply that access. This is not a namespace-capability operation, and no prior host configuration beyond exposing the tty to the process is intrinsic to the vulnerable ioctl path.
The path is not directly network-triggerable. Exact repeatability depends on the driver and on who controls hangup, disconnect, shutdown, or reopening. The supplied patch does not establish that an ordinary user can reproduce the race against every driver implementing `break_ctl()`.
4. Primitive and severity interpretation
The established primitive is a race-driven callback into a tty driver after the device has transitioned through hangup. This is invalid lifecycle-state consumption, not a demonstrated post-free object use.
The second `break_ctl()` invocation is an ordinary driver operation rather than a release, put, free, or refcount mutation. Therefore, no concrete repeated-release primitive exists and the UAF floors do not apply.
The demonstrated code does not show that active structured kernel storage is reclaimed or replaced. The practical result may range from an `EIO`-like device failure to a driver-specific hang or kernel fault. A stale `driver_data` UAF remains a driver-specific hypothesis requiring examination of individual `break_ctl()` implementations.
`PrimitiveClass=CLASS_C` is appropriate because the generic patch establishes invalid-state callback behavior without an independently useful corruption primitive. `ImportantCandidate=NO` and `AutoDowngradeAllowed=YES` follow because no unresolved strong corruption, ownership, authorization, or trust-boundary primitive is established.
The likely operational severity is Low to Moderate, with manual review useful for drivers whose `break_ctl()` accesses shutdown-sensitive private state.
5. One-sentence report phrase
A race between a timed tty break ioctl and hangup can invoke a driver break callback after device shutdown or reopening, creating a driver-dependent local denial-of-service risk without establishing a generic UAF or stronger corruption primitive.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Review representative `break_ctl()` implementations for stale `driver_data`, removed-device access, or shutdown-sensitive hardware operations, but the supplied generic tty code does not justify mandatory Important-class preservation.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: tty: abort break signalling on hangup
Commit: 3bc82999fb85cecc7eb6ec1755ee7d698aa1e783
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3bc82999fb85cecc7eb6ec1755ee7d698aa1e783
Commit description:
The break ioctls can race with hangup and end up calling into a tty
driver for a device that is already gone or powered down.
Drivers must handle this race, but TCSBRK and TCSBRKP should still be
aborted to avoid calling back into the driver after a user-controlled
timeout (possibly even after the tty has been reopened).
Note that drivers should disable any break state on shutdown so just
return on hangup (calling break_ctl() again is racy and will most likely
fail for drivers handling the race).
Fixes: f34d7a5 ("tty: The big operations rework")
Cc: stable <[email protected]>
Signed-off-by: Johan Hovold <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
drivers/tty/tty_io.c
include/linux/tty.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=3bc82999fb85cecc7eb6ec1755ee7d698aa1e783
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98751 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98751
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:H/PR:L/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS score: 5.8
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: 2
Paranoid ActionableScore: 3
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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