* [CVE-2026-80847][MODERATE REGULAR] tcp: clamp route advmss to TCP_MIN_MSS
@ 2026-09-04 22:15 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-04 22:15 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-80847
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: tcp: clamp route advmss to TCP_MIN_MSS
Commit: 6b8c20bf61924dfc38fefb145c9f7406d73fae53
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6b8c20bf61924dfc38fefb145c9f7406d73fae53
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80847
Analysis date: Fri, 04 Sep 2026 18:15:36 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-80847 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80847
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-2026-80847 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-80847 CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-369;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.9';DESCR 'A divide by zero can occur in the TCP initial window path because route derived advmss may become smaller than the minimum value assumed by tcp_select_initial_window. The vulnerable state can be reached through an explicitly too small RTAX_ADVMSS value, and the commit message also notes a default advmss path where route MTU and min_adv_mss can drive the effective value down. For the CVSS the PR:N in the paranoid score reflects the case where an attacker only needs network reachability after the vulnerable route or PMTU related state already exists. The base case is local PR:L because reliable setup normally requires a local process with delegated network administration rights such as CAP_NET_ADMIN in a network namespace. Impact is denial of service via kernel crash. There is no concrete memory corruption primitive in the patch context, so confidentiality and integrity are not raised.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) SKIP CVE-2026-80847 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE REMOTE UAF NETWORK LINUS IPV6 LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 2
* Conservative score: 2. No floor or cap applied.
* Raw paranoid score: 3
* Paranoid score: 3. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::
2. Signal breakdown
Conservative positive signals:
* +1 Reliable kernel crash / strong DoS. The patch and commit describe a divide-by-zero reachable when route-derived advmss becomes 0 before tcp_select_initial_window().
* +1 Common networking core path. The affected code is in ordinary TCP IPv4 and IPv6 paths, including SYN receive, SYNACK creation, TCP connect initialization, and advertised MSS handling.
Conservative negative signals:
* No negative privilege penalty applied. Explicit RTAX_ADVMSS manipulation normally requires CAP_NET_ADMIN, but the full trigger is not proven to require host-admin-equivalent privilege in all cases.
* No race penalty applied. This is not a timing race.
* No rare subsystem penalty applied. TCP is common.
Paranoid additional signal:
* +1 Potential network-facing trigger condition needing review. The commit says the divide-by-zero is not limited to explicit RTAX_ADVMSS and may also occur through the default advmss path when route MTU and min_adv_mss drive the effective value down. This does not prove direct remote reachability, but it makes the TCP data-plane exposure worth manual review.
Important non-awarded signals:
* Remote reachable +2 not awarded. The patch does not prove that an external attacker can create the bad advmss state without local routing, MTU, PMTU, sysctl, or namespace conditions.
* Local unprivileged trigger +1 not awarded. Ordinary unprivileged reachability of the complete setup is not established.
* Namespace-scoped capability not set to YES. Route manipulation may be namespace-aware, but the complete trigger chain and product user namespace policy are not proven.
* No memory corruption signal awarded. The established primitive is divide-by-zero, not UAF, OOB, double free, refcount corruption, or arbitrary write.
* C/I impact not awarded. There is no disclosure or unauthorized modification primitive.
* Extra availability not awarded. The same kernel crash is already counted once.
* ImportantCandidate not set. The issue is a crash-class TCP bug without an established corruption or security-boundary primitive.
3. Reachability analysis
The most concrete trigger is local configuration of a too-small RTAX_ADVMSS route metric, which normally requires CAP_NET_ADMIN over the relevant network namespace. That may be a delegated or namespace-scoped privilege in some environments, but the supplied evidence does not prove that an ordinary local user can create the complete vulnerable setup.
The commit also states that the same divide-by-zero can occur through a default advmss path when RTAX_ADVMSS is 0 and effective advmss is reduced by route MTU and min_adv_mss. This creates uncertainty about practical network exposure. A remote peer may be able to exercise the TCP paths once the vulnerable routing or MTU state exists, but the supplied patch does not establish that the peer can create that state unauthenticated.
No special hardware is required. No race is required. Prior local routing or network namespace configuration may be relevant, but it is not fully resolved from the provided input.
4. Primitive and severity interpretation
The exact primitive is an integer validity failure leading to divide-by-zero in TCP receive window calculation. This is Class C crash/DoS behavior.
There is no real UAF, no stale object, no repeated release/refcount mutation, no active structured stale-object consumption, and no controlled reclaim. The demonstrated consequence is kernel crash or panic via divide-by-zero. A stronger consequence such as privilege escalation or information disclosure is speculative and not supported by the patch.
PrimitiveClass=CLASS_C because the bug is an invalid numeric state consumed by TCP code and the demonstrated effect is crash/DoS. ImportantCandidate=NO because no memory corruption, ownership violation, authorization bypass, or other Important-preserving primitive is established. AutoDowngradeAllowed=YES because unresolved exploitation leverage beyond DoS is not established, though manual review is recommended due to possible TCP network-facing exposure.
Likely operational severity is ordinary Moderate to borderline actionable Moderate depending on whether the bad advmss state can be created or influenced by untrusted actors in the evaluated product.
5. One-sentence report phrase
A route-derived TCP advmss value can fall below TCP_MIN_MSS and reach tcp_select_initial_window as zero, causing a divide-by-zero kernel crash when affected TCP connection setup paths consume the invalid MSS value.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
The issue is DoS-only from the patch evidence, but it affects common TCP paths and the default advmss path leaves enough reachability uncertainty to justify checking product-specific routing, PMTU, min_adv_mss, and namespace exposure.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: tcp: clamp route advmss to TCP_MIN_MSS
Commit: 6b8c20bf61924dfc38fefb145c9f7406d73fae53
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6b8c20bf61924dfc38fefb145c9f7406d73fae53
Commit description:
tcp_select_initial_window() assumes that callers never pass an MSS
smaller than 1, but route-derived advmss values can violate that
assumption.
A too-small explicit RTAX_ADVMSS is one way to get there, but it is not
the only one. The same divide-by-zero can also be reached through the
"default advmss" path when RTAX_ADVMSS is left at 0 and the effective
advmss is later driven down by route MTU and min_adv_mss.
Introduce a tcp_dst_advmss() helper that clamps route advmss to
TCP_MIN_MSS before TCP consumes it, and use it in the TCP paths that
derive advmss from dst metrics. This keeps the effective MSS from
dropping to zero before tcp_select_initial_window() rounds the receive
window.
Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: [email protected]
Reported-by: Vega <[email protected]>
Signed-off-by: Yong Wang <[email protected]>
Signed-off-by: Ren Wei <[email protected]>
Link: https://patch.msgid.link/251eaf8277fa7c66364c9815c5da01662d269181.1787074852.git.edragain@163.com
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
include/net/tcp.h
net/ipv4/tcp_ipv4.c
net/ipv4/tcp_minisocks.c
net/ipv4/tcp_output.c
net/ipv6/tcp_ipv6.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=6b8c20bf61924dfc38fefb145c9f7406d73fae53
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-80847 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80847
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:N/I:N/A:H
The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS score: 5.9
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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