* [CVE-2026-99056][IMPORTANT] strparser: make sure __strp_recv isn't running before tearing down the parser
@ 2026-10-27 3:32 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-27 3:32 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99056
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: strparser: make sure __strp_recv isn't running before tearing down the parser
Commit: a18dd388045dc7263d99a07db9de4e0f149ca8f1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a18dd388045dc7263d99a07db9de4e0f149ca8f1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99056
Analysis date: Mon, 26 Oct 2026 23:32:31 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
An ESP-in-TCP teardown race permits in-flight receive processing to use freed strparser state, with upstream reporting a published exploit but not documenting its final primitive or required privilege.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99056 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99056
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-99056 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-99056 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT IMPORTANT AEGIS_AS=6 LEGACY=IMPORTANT RESULT=SAME INDEPENDENT_LEGACY=MODERATE_KPANIC INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_WINS_DISAGREEMENT PRE_MERGE_LEGACY=MODERATE_KPANIC (final reconciliation agrees; DECISION: both IMPORTANT) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-362;*CWE-416;CWE-667;BEST CVSS score: '7';DESCR 'A teardown race in espintcp allows strp_done to proceed while an in flight __strp_recv still uses parser related state, producing an exploitable use after free rather than only a warning or resource leak. The receive path runs under the socket lock, but strp_stop previously blocked only future callbacks and did not wait for callbacks that had already started. A published reproducer reportedly converts the UAF into an exploit, which supports potential local privilege escalation, confidentiality and integrity compromise, and a host kernel crash. For the CVSS the PR:L rating reflects the need for local code execution and access to the espintcp socket lifecycle. AC:H reflects the need to win a teardown versus receive race. No direct remote trigger lacking a local user context is established. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 7 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 KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC because KP specialist did not establish a qualifying panic. final reconciliation stage MODERATE_KPANIC to IMPORTANT because 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 has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP and TEMP_KPANIC_MERGE_IMPORTANT_WINS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 0.0 and ActionableScore 6 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was IMPORTANT. Cross model aggregation changed the production result under an enabled evidence gated merge rule. 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 REMOTE LOCK DANGER RACE UAF DMAorINTERRUPT LINUS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP TEMP_KPANIC_MERGE_IMPORTANT_WINS KPANIC_UNSUPPORTED_REMOVED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 3.
* Conservative score: 4. REAL UAF FLOOR 4 applies because an active parser is used by an in-flight receive operation after teardown can begin, and host-admin-only reachability is not established.
* Raw paranoid score: 5.
* Paranoid score: 5. No cap or additional floor applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2 Real UAF: `strp_done()` can tear down parser resources while an already-running `__strp_recv()` continues using parser state.
* +1 Reliable kernel crash: the commit expressly identifies a crash-capable UAF.
* Paranoid +1 attacker-influenced lifetime sequence: the published reproducer supports driving concurrent receive and socket/parser teardown, although exact privilege requirements are absent.
* Paranoid +1 exploit evidence: the authoritative commit states that the reproducer turns the UAF into an exploit. The supplied material does not reveal its primitive or final effect.
* No race penalty: a race exists, but no evidence establishes narrow, one-shot, or materially uncontrollable timing.
* No controlled reclaim, directed write, callback control, arbitrary read/write, LPE, or C/I impact is shown. The vague exploit statement cannot establish those effects.
* No repeated release/refcount signal exists; this is concurrent post-lifetime structured use, not a shown second `put` or `free`.
* The same possible crash is not counted again as Availability impact.
3. Reachability analysis
The affected deployed user is ESP-in-TCP. A local actor apparently needs access to its socket lifecycle and must race teardown against receive processing. The commit and reproducer establish feasibility but do not show whether ordinary users, delegated network capabilities, namespace-local capabilities, or host administration suffice for the complete setup. Consequently, both host-admin and namespace scope remain unknown. Remote packet traffic alone is not proven sufficient. No special hardware is required.
4. Primitive and severity reading
This is a CLASS_B UAF involving active structured parser state: teardown proceeds without waiting for an in-flight receive-side consumer. It is broader than a shallow cleanup dereference because active packet processing continues concurrently, but the supplied diff does not expose the exact stale field or exploitation mechanics.
A crash and real UAF are supported; controlled post-free reuse and privilege escalation are not. Nevertheless, the upstream statement that a published reproducer converts the race into an exploit makes Important-vs-Moderate review mandatory. Auto-downgrade is unsafe until that exploit and the ESP-in-TCP privilege boundary are examined. This is unrelated to authorization-state/B4-PENDING.
5. One-sentence report phrase
An ESP-in-TCP teardown race permits in-flight receive processing to use freed strparser state, with upstream reporting a published exploit but not documenting its final primitive or required privilege.
6. Manual review advice
MANUAL CHECK REQUIRED
Review the published exploit for reclaim control, resulting kernel operations, final impact, and complete privilege/namespace requirements.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: strparser: make sure __strp_recv isn't running before tearing down the parser
Commit: a18dd388045dc7263d99a07db9de4e0f149ca8f1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a18dd388045dc7263d99a07db9de4e0f149ca8f1
Commit description:
The comment above strp_done() claims that if strp is stopped,
strp_recv will no longer be called. That's only true if the caller has
a mechanism (eg locking) to guarantee that strp_recv() calls that
started before strp_stop() have completed by the time we call
strp_done().
This adds a dummy lock_sock/release_sock pair to guarantee that any
in-flight strp_recv() (which runs under either bh_lock_sock or
lock_sock, depending if it's called from ->sk_data_ready or strp_work)
has completed.
Only espintcp can be affected by this race condition, but this patch
makes sure no future user of strp can have the bug.
This could crash on strp->sk ("general mode" of strp), but this mode
has been here for 9 years and has never been used. It'll be gone soon,
no point worrying about it.
Switch the stopped/paused/etc bits to u8's to avoid races between
strp_stop() and strp_pause/unpause().
A reproducer has been published and turns the possible UAF into an
exploit [1].
Reported-by: Hyunwoo Kim <[email protected]>
Link: https://lore.kernel.org/all/aZLn2Faeg1FB7XOf@v4bel/
Link: https://lore.kernel.org/all/aZgpkyTDU3aXe_V0@v4bel/
Link: https://github.com/m0x41nos/RustyTux [1]
Fixes: e27cca9 ("xfrm: add espintcp (RFC 8229)")
Cc: [email protected]
Signed-off-by: Sabrina Dubroca <[email protected]>
Reviewed-by: Hyunwoo Kim <[email protected]>
Link: https://patch.msgid.link/425d9d926709b542ed5331110c19fee4aee1f28d.1791206940.git.sd@queasysnail.net
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
include/net/strparser.h
net/strparser/strparser.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=a18dd388045dc7263d99a07db9de4e0f149ca8f1
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99056 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99056
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: Not available
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7
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: 4
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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