From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98548][IMPORTANT] sctp: discard the rest of the packet on a stale-cookie error Date: Sun, 18 Oct 2026 22:18:06 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98548 X-AL-KERNEL-Priority: IMPORTANT X-AL-KERNEL-Severity: IMPORTANT X-AL-KERNEL-Base-Severity: IMPORTANT X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 7 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: c0af3fd150fdc43821df3bb77e5ec74faf4dfc7a List-Id: CVE: CVE-2026-98548 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: NO Patch: sctp: discard the rest of the packet on a stale-cookie error Commit: c0af3fd150fdc43821df3bb77e5ec74faf4dfc7a Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0af3fd150fdc43821df3bb77e5ec74faf4dfc7a Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98548 Analysis date: Sun, 18 Oct 2026 22:18:06 -0400 ActionableScore: 7 ActionableScore lower bound: unknown Actionable bucket: Strong Important candidate Manual review required: YES Summary: A malicious SCTP peer can preserve a pointer to an RCU-freed transport through bundled stale-cookie and DATA processing, causing a remote kernel use-after-free with subsequent field access, possible post-free writes, live-list corruption, and denial of service. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98548 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98548 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-2026-98548 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-98548 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=5 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_WINS_DISAGREEMENT PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-672;*CWE-664;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '9.8';DESCR 'A use-after-free in SCTP packet processing occurs when a bundled ERROR Stale Cookie chunk removes a non-primary transport and a following DATA chunk stores the same transport pointer back into association state. After the receive reference is released and the transport is freed through RCU, a later FWD-TSN or delayed SACK can dereference the dangling pointer. Some paths may also write to the freed transport and link it into the live transport list, creating a stronger memory corruption primitive than a crash-only invalid read. A malicious SCTP peer can trigger the condition through protocol traffic sent from a non-primary address while the association is in COOKIE-ECHOED state. For the CVSS the PR:N value is used because no capabilities or authenticated local user context are required when the attacker can communicate with the exposed SCTP endpoint. The practical attack surface is limited to systems that accept SCTP traffic and permit the required association and address state. Impact is at least remote denial of service through a kernel crash. The confirmed use-after-free write path also supports possible confidentiality and integrity impact and requires manual security review. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 9.8 and the production ActionableScore returned 7 with recommendation Strong Important candidate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. final reconciliation stage IMPORTANT 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 produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and TEMP_KPANIC_MERGE_IMPORTANT_WINS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 5.3 and ActionableScore 5 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. 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 Strong Important candidate (with actual score 7) MAYBE READ DANGER UAF NETWORK LINUS PACKET INCREASED_TO_HIGH_BASED_ON_GUESSCVSS TEMP_KPANIC_MERGE_IMPORTANT_WINS KPANIC_UNSUPPORTED_REMOVED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=7 PrimitiveClass=CLASS_B RealUAF=YES HostAdminRequired=NO NamespaceScopedPrivilege=NO ImportantCandidate=YES ManualReview=REQUIRED AutoDowngradeAllowed=NO RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw conservative score: 7 * Conservative score: 7 * Raw paranoid score: 7 * Paranoid score: 7 * No floor or cap changed either score. * Final recommended bucket: **Strong Important candidate**:: 2. Signal breakdown Positive signals: * +2 Real UAF. The association retains `last_data_from` after the referenced `sctp_transport` is RCU-freed, and later processing dereferences it. * +2 Direct network reachability. A malicious SCTP peer can trigger the sequence using bundled stale-cookie ERROR and DATA chunks from a non-primary address. * +1 Reliable kernel crash. The supplied reproducer produced a KASAN slab-use-after-free in the SCTP receive path. * +1 Attacker-influenced active-object lifetime. The peer controls the SCTP association, non-primary transport, packet ordering, and protocol transition that removes the transport and restores its pointer into association state. * +1 Structured stale-object leverage. Later paths read transport fields, may write through the freed object, and may link it into the live transport list. This is meaningfully stronger than an immediate stale-pointer fault. Not awarded: * No +2 strong exploitation leverage. Controlled reclaim, attacker-shaped replacement contents, directed write destination, callback control, or arbitrary kernel read/write is not established. * No separate integrity point. The post-free write and live-list insertion are already represented by structured stale-object leverage. * No additional availability point. The same crash cannot be counted twice. * No confidentiality point. No disclosure path is demonstrated. * No race penalty. The vulnerable behavior follows an attacker-controlled ordered packet sequence rather than a narrow, externally uncontrollable timing race. * No repeated-release/refcount point. There is no demonstrated second `put`, release, or refcount mutation through the exact dangling reference. * NEW52 B5-R does not apply. This is not a retry-accumulated heap OOB write. * NEW53 B3-PENDING does not apply. No shared file-backed or COW backing write is involved. * NEW54 B4-PENDING does not apply. No authorization decision or omitted mandatory guard is involved. 3. Reachability analysis A remote malicious SCTP peer can trigger the bug if it can participate in an SCTP association, reach `COOKIE-ECHOED`, use a non-primary address, and send the required bundled chunks. The commit also explicitly identifies a local unprivileged user acting as the peer, with no capabilities required. No host UID 0, host-admin capability, namespace capability, special hardware, or separate privileged control-plane operation is required for the documented trigger. An SCTP endpoint and suitable association state must exist, but this is protocol exposure rather than evidence of an attacker-required privileged setup. The trigger is deterministic packet-state manipulation, not a conventional race. The practical exposure is limited to hosts accepting relevant SCTP traffic, but this does not negate direct network reachability. 4. Primitive and severity interpretation The primitive is an active structured-object UAF: 1. `SCTP_CMD_DEL_NON_PRIMARY` removes the arrival transport. 2. A bundled DATA chunk copies the removed `chunk->transport` back into `asoc->peer.last_data_from`. 3. The receive reference is dropped and the transport is RCU-freed. 4. FWD-TSN or the delayed SACK timer later dereferences the dangling pointer. 5. Additional paths may write into the freed transport and insert it into the live transport list. This is broader active use, not cleanup-only dereferencing or fixed stale storage. The demonstrated consequence is a remotely triggerable kernel UAF and crash. The unresolved consequence is exploitation of the post-free writes and list manipulation after allocator reuse. The issue remains CLASS_B because controlled reclaim, directed corruption, and a completed privilege-escalation primitive are not demonstrated. It is nevertheless a strong Important candidate because the attacker controls the network sequence, the stale object survives in association-lifetime state, and privileged kernel consumers later perform nontrivial reads and possible writes through it. `ImportantCandidate=YES` and `AutoDowngradeAllowed=NO` preserve this unresolved remote active-object corruption for manual exploitation-leverage analysis. This does not assert proven remote code execution, privilege escalation, or confidentiality compromise. 5. One-sentence report phrase A malicious SCTP peer can preserve a pointer to an RCU-freed transport through bundled stale-cookie and DATA processing, causing a remote kernel use-after-free with subsequent field access, possible post-free writes, live-list corruption, and denial of service. 6. Manual review recommendation MANUAL CHECK REQUIRED The bug is remotely reachable without capabilities and establishes an active structured-object UAF with a possible write to freed storage and insertion into a live kernel list. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: sctp: discard the rest of the packet on a stale-cookie error Commit: c0af3fd150fdc43821df3bb77e5ec74faf4dfc7a Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0af3fd150fdc43821df3bb77e5ec74faf4dfc7a Commit description: When an association is in COOKIE-ECHOED state and the peer sends a bundled [ERROR(Stale Cookie)][DATA] packet from one of its non-primary addresses, processing the ERROR chunk takes the non-fatal stale-cookie retry path sctp_sf_do_5_2_6_stale(), which queues SCTP_CMD_DEL_NON_PRIMARY while keeping the association alive. sctp_cmd_del_non_primary() removes every non-primary transport - including the very transport this packet arrived on, which is still referenced by the receive lookup and shared by all chunks of the packet via chunk->transport. sctp_assoc_rm_peer() does redirect asoc->peer.last_data_from away from the removed transport, but right afterwards the bundled DATA chunk makes sctp_assoc_bh_rcv() re-register asoc->peer.last_data_from = chunk->transport unconditionally, undoing the redirection with the just-removed transport. Once the packet is done, the receive reference is dropped and the transport is RCU-freed, while the surviving association keeps the dangling last_data_from. A later FWD-TSN (or the delayed SACK timer) makes sctp_gen_sack() dereference it (->param_flags and friends), and sctp_make_sack()/sctp_outq_select_transport() may write to the freed object and link it into the live transport list. This is a use-after-free triggerable by any malicious SCTP peer (or a local unprivileged user acting as one) with no capabilities required: BUG: KASAN: slab-use-after-free in sctp_do_sm+0x498a/0x5660 Read of size 4 at addr ffff88800e1e356c by task poc/115 Call Trace: sctp_do_sm <- sctp_assoc_bh_rcv <- sctp_inq_push <- sctp_rcv <- ip_protocol_deliver_rcu <- ip_rcv Allocated: sctp_transport_new <- sctp_assoc_add_peer <- sctp_process_init (INIT-ACK processing) Freed: kfree <- sctp_transport_destroy_rcu <- rcu_core (call_rcu queued by sctp_transport_put at end of sctp_rcv) The buggy address is located 364 bytes inside of freed 1024-byte region [ffff88800e1e3400, ffff88800e1e3800), cache kmalloc-1k Note that commit 03a9d10 ("sctp: drop a chunk if its transport was removed") only covers the window between the receive lookup and the chunk processing (e.g. an ASCONF DEL-IP racing the socket backlog); here the transport is removed *while* the packet is being processed, by an earlier chunk of the same packet, so the drop in sctp_inq_push() does not reach this path. Verified with the bundled [ERROR(Stale Cookie)][DATA] + FWD-TSN reproducer: the KASAN report above still fires with that commit applied, and is gone with this patch on top. Fix it by discarding the rest of the packet on this path, as suggested by Xin. After the stale-cookie ERROR has sent the association back to COOKIE-WAIT and removed the non-primary transports, the remaining chunks of the packet can only run against the restarted handshake while referencing the removed arrival transport through chunk->transport: besides the last_data_from registration above, sctp_cmd_setup_t2() and the sctp_make_*() reply builders would also copy that pointer into association-lifetime state that sctp_assoc_rm_peer() has already sanitized. Let the peer retransmit them, in line with what sctp_inq_push() does for chunks whose transport was removed before processing. Fixes: 1da177e ("Linux-2.6.12-rc2") Suggested-by: Xin Long Reported-by: TencentOS Corvus AI Cc: stable@vger.kernel.org Signed-off-by: Aohan Mei Acked-by: Xin Long Link: https://patch.msgid.link/20260921093707.1432184-1-ljp1205831794@gmail.com Signed-off-by: Jakub Kicinski Signed-off-by: Greg Kroah-Hartman Changed files: net/sctp/sm_statefuns.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=c0af3fd150fdc43821df3bb77e5ec74faf4dfc7a ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98548 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98548 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:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 9.8 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: unknown Paranoid ActionableScore: 7 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).