From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-53183][MODERATE REGULAR] mptcp: allow subflow rcv wnd to shrink Date: Thu, 25 Jun 2026 05:22:56 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-53183 X-AL-KERNEL-Priority: MODERATE REGULAR X-AL-KERNEL-Severity: MODERATE REGULAR X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 4 X-AL-KERNEL-ActionableScore-Lower: 3 X-AL-KERNEL-Commit: bf364b0f10b27679140699821f88af7f01e2a6e3 List-Id: CVE: CVE-2026-53183 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: mptcp: allow subflow rcv wnd to shrink Commit: bf364b0f10b27679140699821f88af7f01e2a6e3 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf364b0f10b27679140699821f88af7f01e2a6e3 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53183 Analysis date: Thu, 25 Jun 2026 05:22:56 -0400 ActionableScore: 4 ActionableScore lower bound: 3 Actionable bucket: Borderline / manual review recommended, but not Strong Important candidate Manual review required: YES Summary: An MPTCP receive-window accounting bug may let a remote peer exceed the receiver rcvbuf limit under out-of-order or backlog conditions, causing network-triggered resource pressure or denial of service without evidence of memory corruption or privilege escalation. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-53183 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53183 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-2026-53183 MODERATE CHECK 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-400;CWE-770;CWE-682;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'An MPTCP receive-window accounting bug can allow a TCP subflow receive window to be artificially inflated when incoming data is acknowledged at the TCP level but is out-of-order in the MPTCP sequence space or is held in backlog. A remote peer can cause traffic accepted by the receiver to exceed the configured receive buffer limit, creating memory pressure or degraded availability on systems where an MPTCP endpoint is reachable. For the CVSS the PR:N is used because the attacker does not need local code execution or an account on the victim, only network reachability to an MPTCP capable service. The issue is network reachable but depends on MPTCP being enabled and on connection patterns that create the affected out-of-order or backlog state. Impact is denial of service through receive buffer overrun and resource pressure, with no concrete evidence of confidentiality loss, integrity loss, UAF, or arbitrary memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended, but not Strong Important candidate (with actual score 4) YES REMOTE SIMPLEFIX NETWORK SKB LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 YES NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=4 ActionableScoreLower=3 ## 1. ActionableScore * Conservative score: 3 * Paranoid score: 4 * Final recommended bucket: **Borderline / manual review recommended, but not Strong Important candidate**:: ## 2. Signal breakdown Conservative signals: * Remote reachable / network-triggerable: +2 MPTCP is a network protocol path, and a peer connected to an MPTCP-capable endpoint can influence receive ordering and backlog behavior. * Reliable kernel crash / strong DoS: +1 The commit states that incoming traffic can exceed the receiver rcvbuf size. This is a resource-accounting DoS concern, but not a demonstrated immediate panic or memory corruption. * Hard or special triggering conditions: -1 The issue depends on MPTCP being used and on data being acknowledged at TCP level while out-of-order in MPTCP sequence space or held in backlog. * Availability impact realistic: +1 in paranoid score only In the stronger interpretation, sustained remote traffic could create meaningful memory pressure or service degradation beyond a minor local failure. No memory corruption signals are applied: * No UAF. * No OOB write. * No double-free. * No page-cache corruption. * No privilege escalation primitive. * No confidentiality or integrity primitive. ## 3. Reachability analysis A remote peer can potentially trigger the bug if it can connect to an MPTCP-enabled service or otherwise participate in an MPTCP connection with the victim. No local account or code execution on the victim is required, so PR:N is appropriate for CVSS-style reasoning. The path is not a generic TCP path for all deployments. It requires MPTCP support and an MPTCP-capable endpoint, which reduces practical exposure compared with ordinary TCP receive-path bugs. Containers and namespaces do not significantly change the main attacker model unless they expose or depend on MPTCP services. Realistic exploitation is a network DoS/resource-pressure scenario, not a memory-corruption or LPE scenario. ## 4. Severity interpretation This behaves like an actionable resource-exhaustion Moderate, not an Important-class kernel memory corruption issue. The theoretical worst case is degraded availability from receive-buffer accounting failure and memory pressure. The realistic evidence from the patch supports DoS only. There is no concrete primitive for attacker-controlled memory corruption, privilege escalation, data disclosure, or integrity violation. The score should not be auto-closed as trivial because the affected path is network reachable with PR:N when MPTCP is exposed. But it also should not be escalated to Important without additional evidence of system-wide reliable exhaustion, panic, or corruption. ## 5. One-sentence report phrase An MPTCP receive-window accounting bug may let a remote peer exceed the receiver rcvbuf limit under out-of-order or backlog conditions, causing network-triggered resource pressure or denial of service without evidence of memory corruption or privilege escalation. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Because the issue is PR:N and network reachable when MPTCP is exposed, it should receive manual review. However, the patch supports a DoS/resource-accounting classification only, so manual review is recommended rather than required for Important-class escalation. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: mptcp: allow subflow rcv wnd to shrink Commit: bf364b0f10b27679140699821f88af7f01e2a6e3 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf364b0f10b27679140699821f88af7f01e2a6e3 Commit description: In MPTCP connection, the `window` field in the TCP header refers to the MPTCP-level rcv_nxt and it's right edge should not move backward. Such constraint is enforced at DSS option generation time. At the same time, the TCP stack ensures independently that the TCP-level rcv wnd right's edge does not move backward. That in turn causes artificial inflating of the MPTCP rcv window when the incoming data is acked at the TCP level and is OoO in the MPTCP sequence space (or lands in the backlog). As a consequence, the incoming traffic can exceed the receiver rcvbuf size even when the sender is not misbehaving. Prevent such scenario forcibly allowing the TCP subflow to shrink the TCP-level rcv wnd regardless of the current netns setting. Fixes: f3589be ("mptcp: never shrink offered window") Cc: stable@vger.kernel.org Signed-off-by: Paolo Abeni Reviewed-by: Matthieu Baerts (NGI0) Signed-off-by: Matthieu Baerts (NGI0) Link: https://patch.msgid.link/20260602-net-mptcp-misc-fixes-7-1-rc7-v2-4-856831229976@kernel.org Signed-off-by: Jakub Kicinski Signed-off-by: Greg Kroah-Hartman Changed files: net/mptcp/options.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=bf364b0f10b27679140699821f88af7f01e2a6e3 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-53183 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53183 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:H/PR:N/UI:N/S:U/C:N/I:N/A:L 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: 3 Paranoid ActionableScore: 4 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 alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).