From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98393][LOW] net: ipv4: Fix UDP length overflow with PMTU discover and big MTU [ Upstream Date: Wed, 14 Oct 2026 23:26:23 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98393 X-AL-KERNEL-Priority: LOW X-AL-KERNEL-Severity: LOW X-AL-KERNEL-Base-Severity: LOW X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 2 X-AL-KERNEL-ActionableScore-Lower: 1 X-AL-KERNEL-Commit: 83fedc4741210f75be8ca5b487985a242bf6f18d List-Id: CVE: CVE-2026-98393 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: net: ipv4: Fix UDP length overflow with PMTU discover and big MTU [ Upstream Commit: 83fedc4741210f75be8ca5b487985a242bf6f18d Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=83fedc4741210f75be8ca5b487985a242bf6f18d Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98393 Analysis date: Wed, 14 Oct 2026 23:26:23 -0400 ActionableScore: 2 ActionableScore lower bound: 1 Actionable bucket: Ordinary Moderate Manual review required: NO Summary: An oversized IPv4 PMTU combined with IP_PMTUDISC_PROBE can overflow the 16-bit UDP length and trigger a warning, or a kernel panic on panic-on-warn systems, without establishing memory corruption or a remotely reachable attack. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98393 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98393 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: LOW Manual review required: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-98393 LOW https://www.kernelcve.org/list/?q=CVE-2026-98393 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT LOW AEGIS_AS=0 LEGACY=LOW RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both LOW) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-190;*CWE-20;CWE-1284;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'An integer overflow in the IPv4 UDP transmit path can occur when IP_PMTUDISC_PROBE is enabled and the route uses an unusually large device MTU. Sending a 65528 byte UDP payload can produce a total UDP length of 65536, which does not fit in the 16 bit UDP length field and triggers a WARN in udp_set_len_short. For the CVSS the PR:L reflects that a local unprivileged process may trigger the affected send path after the oversized MTU has already been configured, while CAP_NET_ADMIN is normally required to create that configuration. AC:H reflects the nondefault MTU and socket option prerequisites. The issue is not remotely triggered by an incoming network packet. The demonstrated impact is a warning and limited availability degradation, while systems configured with panic_on_warn may experience a kernel panic and host denial of service. No UAF, out of bounds write, information disclosure, or privilege escalation primitive is supported by the affected code path. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE to LOW because the ActionableScore threshold rule was applied. final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic. 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. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 0.0 and ActionableScore 0 with primitive class CLASS_C and manual review NO. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE REMOTE DANGER NETWORK HARDWARE SYZBOT PACKET DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 KPANIC_UNSUPPORTED_REMOVED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=2 ActionableScoreLower=1 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=UNKNOWN NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=NO ManualReview=NO AutoDowngradeAllowed=YES RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO ## 1. ActionableScore * Raw conservative score: 1 * Conservative score: 1 * Raw paranoid score: 2 * Paranoid score: 2 * No floor or cap was applied. * Final recommended bucket: **Ordinary Moderate**:: ## 2. Signal breakdown Conservative signals: * +1 Local unprivileged trigger. After a suitable oversized-MTU route exists, an ordinary process can set `IP_PMTUDISC_PROBE` and send the triggering UDP payload. * Total: 1. Additional paranoid signal: * +1 Reliable kernel crash or strong DoS. The demonstrated `WARN` becomes fatal on systems configured with `panic_on_warn`. * Total: 2. Not awarded: * No corruption bonus. The overflow affects the 16-bit UDP length representation, not an allocation bound or memory offset. * No B5-R signal. There is no heap OOB write, retry-accumulated offset, or later consumption of heap-corrupted structured state. * No remote-reachability bonus. Incoming network traffic does not directly reach the vulnerable transmit construction path. * No authorization or security-boundary signal. * No confidentiality or integrity signal. * No additional availability point. The possible panic is the same consequence already counted. * No uncommon-configuration penalty. The oversized MTU is unusual, but the available evidence does not completely resolve whether an attacker can create the required device and route within namespaces. ## 3. Reachability analysis The direct trigger is a local UDP send using `IP_PMTUDISC_PROBE` and a 65528-byte payload over a route whose device MTU exceeds `IP_MAX_MTU + 20`. The socket operations themselves are ordinarily available without privilege. Configuring the oversized MTU requires `CAP_NET_ADMIN` over the relevant device. The supplied evidence does not establish whether this must be host `CAP_NET_ADMIN`, whether a suitable device can be created entirely inside attacker-controlled user and network namespaces, or whether a separate administrator must provide the route. Therefore `HostAdminRequired` and `NamespaceScopedPrivilege` remain `UNKNOWN`. No special hardware or race is required. The unusual MTU is a deterministic environmental prerequisite. The issue cannot be triggered directly by sending an incoming malformed UDP packet. ## 4. Primitive and severity interpretation The exact defect is a numeric overflow of the UDP protocol length. A calculated length of 65536 cannot be represented in the 16-bit UDP length field and reaches the warning in `udp_set_len_short()`. This is not a heap or slab OOB write, UAF, stale-object operation, arbitrary write, or retry-accumulated corruption case. The demonstrated consequence is a warning and malformed or rejected packet construction. A host panic is possible when warnings are configured as fatal. The primitive is therefore `CLASS_C`. There is no exploitation leverage beyond the availability consequence supported by the supplied evidence. `ImportantCandidate=NO` and `AutoDowngradeAllowed=YES` because no unresolved memory-corruption, ownership, authorization, or security-boundary primitive remains. B4-PENDING is inapplicable because the patch does not change an authorization decision. B3-PENDING is also inapplicable because no shared-backing write occurs. ## 5. One-sentence report phrase An oversized IPv4 PMTU combined with IP_PMTUDISC_PROBE can overflow the 16-bit UDP length and trigger a warning, or a kernel panic on panic-on-warn systems, without establishing memory corruption or a remotely reachable attack. ## 6. Manual review recommendation NO MANUAL CHECK NEEDED The issue is a deterministic protocol-length overflow with warning or conditional DoS impact, and the patch provides no evidence of an OOB memory write, UAF, authorization bypass, or integrity primitive. ## 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: net: ipv4: Fix UDP length overflow with PMTU discover and big MTU [ Upstream Commit: 83fedc4741210f75be8ca5b487985a242bf6f18d Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=83fedc4741210f75be8ca5b487985a242bf6f18d Changed files: net/ipv4/ip_output.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=83fedc4741210f75be8ca5b487985a242bf6f18d ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98393 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98393 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:L The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 4.7 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: 1 Paranoid ActionableScore: 2 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: LOW KPANIC detected for this report: NO Published priority for this report (same as in Subject): LOW 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).