From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-63896][MODERATE 7.0] usb: gadget: composite: fix integer underflow in WebUSB GET_URL handling Date: Sun, 19 Jul 2026 23:16:12 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-63896 X-AL-KERNEL-Priority: MODERATE 7.0 X-AL-KERNEL-Severity: MODERATE 7.0 X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 6 X-AL-KERNEL-ActionableScore-Lower: 4 X-AL-KERNEL-Commit: 046870ff6b6f7b743c953c061043a9b30700d491 List-Id: CVE: CVE-2026-63896 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: usb: gadget: composite: fix integer underflow in WebUSB GET_URL handling Commit: 046870ff6b6f7b743c953c061043a9b30700d491 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=046870ff6b6f7b743c953c061043a9b30700d491 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63896 Analysis date: Sun, 19 Jul 2026 23:16:12 -0400 ActionableScore: 6 ActionableScore lower bound: 4 Actionable bucket: Actionable Moderate at minimum / Strong Important candidate for manual review Manual review required: YES Summary: A malicious USB host can trigger an integer underflow in WebUSB GET_URL handling on configured USB gadgets, causing a huge memcpy into the gadget request buffer and resulting in kernel memory corruption and likely device crash. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63896 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63896 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 7.0 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-2026-63896 MODERATE CHECK WITH IMPACT FROM ORIG NN LOW Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';CWE-191;*CWE-787;CWE-122;Other CVSS 'AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.8';DESCR 'WebUSB GET_URL handling in usb gadget composite has an integer underflow when a USB host sends wLength smaller than the URL descriptor header. The underflow makes landing_page_length wrap and can drive a very large memcpy from cdev landing_page into cdev req buf, producing a slab out of bounds write and a kernel crash. For the CVSS the PR:N is used because the attacking USB host does not need privileges on the gadget system beyond access to the USB control endpoint. The attack is not Internet reachable and requires a malicious or compromised USB host connected to a gadget with webusb use enabled and landingPage configured. Impact is at least denial of service, and because this is an out of bounds write in kernel memory, the paranoid interpretation should treat confidentiality and integrity impact as possible and require manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum / Strong Important candidate for manual review (with actual score 5) YES WRITE KASAN OOB USB LINUS PACKET KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS YES NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=6 ActionableScoreLower=4 ## 1. ActionableScore * Conservative score: 4 * Paranoid score: 6 * Final recommended bucket: **Actionable Moderate at minimum / Strong Important candidate for manual review**:: ## 2. Signal breakdown Conservative signals: * Generic memory corruption: +2. The patch fixes an unsigned integer underflow that can drive a huge `memcpy()` into `cdev->req->buf`, producing a slab out-of-bounds write. * Reliable kernel crash / strong DoS: +1. KASAN reports slab-out-of-bounds and FORTIFY_SOURCE traps the oversized memcpy. * Privileged kernel/device-management memory corruption path: +1. The corruption occurs in the USB gadget control request handling path, which is a trusted kernel device-management context influenced by an external USB host. * Physical-only condition: -2. The attacker must act as a USB host connected to the gadget device. * Configuration-dependent exposure: -1. The gadget must have `webusb/use=1` and a configured `landingPage`. * Availability impact realistic: +1. A single malformed SETUP request can trigger a host-wide kernel crash on affected gadget configurations. Paranoid additional signals: * Confidentiality impact plausible: +1. The oversized copy reads far beyond the intended landing page buffer before writing, so unintended kernel memory movement is plausible even if no direct disclosure path is shown. * Integrity impact plausible: +1. The destination is kernel memory beyond the intended USB request buffer, so kernel memory corruption is plausible. * Weak LPE concern: +1. This is not a demonstrated LPE primitive, but an attacker-triggered OOB write in kernel memory is enough to require manual review. ## 3. Reachability analysis The bug is reachable by a malicious or compromised USB host sending a malformed WebUSB GET_URL SETUP packet with `wLength` smaller than the URL descriptor header. No privileges are required inside the attacked gadget system, so from the gadget OS perspective the triggering side is PR:N. This is not network reachable over IP and should not be scored as remote network exposure. The realistic deployment requirement is a Linux USB gadget configuration with WebUSB enabled and a landing page configured, which is not universal but is a valid production mode for embedded and device-side USB systems. Namespaces and containers are not the main factor here because the attacker is external to the gadget OS and interacts through the physical USB control endpoint. Call-site confidence: high. The vulnerable arithmetic and the subsequent memcpy are both shown in the provided patch context. ## 4. Severity interpretation This is more serious than an ordinary Moderate crash bug because the failure mode is an integer underflow leading to a very large kernel memcpy and a slab out-of-bounds write. Realistically, the most demonstrated impact is a reliable local-physical DoS of the gadget device. Theoretical confidentiality or integrity impact is plausible because this is kernel memory corruption, but the patch does not show attacker-controlled target selection, controlled payload placement, object reclaim, callback control, or a demonstrated privilege-escalation chain. Therefore the conservative score remains Borderline/manual-review, while the paranoid score reaches Actionable Moderate. ## 5. One-sentence report phrase A malicious USB host can trigger an integer underflow in WebUSB GET_URL handling on configured USB gadgets, causing a huge memcpy into the gadget request buffer and resulting in kernel memory corruption and likely device crash. ## 6. Manual review recommendation MANUAL CHECK REQUIRED This should not be auto-closed because it is an externally triggerable kernel OOB write, even though the exposure is physical USB rather than network and practical exploitation beyond DoS is not demonstrated. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: usb: gadget: composite: fix integer underflow in WebUSB GET_URL handling Commit: 046870ff6b6f7b743c953c061043a9b30700d491 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=046870ff6b6f7b743c953c061043a9b30700d491 Commit description: The WebUSB GET_URL handler in composite_setup() narrows landing_page_length to fit the host-supplied wLength using landing_page_length = w_length - WEBUSB_URL_DESCRIPTOR_HEADER_LENGTH + landing_page_offset; If wLength is smaller than WEBUSB_URL_DESCRIPTOR_HEADER_LENGTH the unsigned subtraction wraps, and the subsequent memcpy(url_descriptor->URL, cdev->landing_page + landing_page_offset, landing_page_length - landing_page_offset); ends up copying close to UINT_MAX bytes from cdev->landing_page into cdev->req->buf. KASAN reports a slab-out-of-bounds in composite_setup on the kmalloc-2k gadget_info allocation, and FORTIFY_SOURCE traps the memcpy as a 4294967293-byte field-spanning write into url_descriptor->URL (size 252). A USB host can reach this from a single SETUP packet against any gadget that has webusb/use=1 and a landingPage configured. Handle the small-wLength case before the math: when the host requested fewer bytes than the URL descriptor header, only the header is meaningful and no URL bytes need to be copied. Setting landing_page_length to landing_page_offset makes the existing memcpy a no-op and leaves the descriptor returned to the host unchanged for all larger wLength values. Fixes: 93c4739 ("usb: gadget: add WebUSB landing page support") Cc: stable Signed-off-by: Jeremy Erazo Link: https://patch.msgid.link/20260512160530.352318-1-mendozayt13@gmail.com Signed-off-by: Greg Kroah-Hartman Signed-off-by: Greg Kroah-Hartman Changed files: drivers/usb/gadget/composite.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=046870ff6b6f7b743c953c061043a9b30700d491 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63896 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63896 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:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 6.8 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: 4 Paranoid ActionableScore: 6 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: YES Published priority for this report (same as in Subject): MODERATE 7.0 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).