From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-63957][MODERATE 7.0] USB: serial: safe_serial: fix memory corruption with small endpoint Date: Sun, 19 Jul 2026 18:03:18 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-63957 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: 5 X-AL-KERNEL-Commit: e3a1d6eee25dc96b1d2db0ecd9d8741e92056476 List-Id: CVE: CVE-2026-63957 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: USB: serial: safe_serial: fix memory corruption with small endpoint Commit: e3a1d6eee25dc96b1d2db0ecd9d8741e92056476 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e3a1d6eee25dc96b1d2db0ecd9d8741e92056476 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63957 Analysis date: Sun, 19 Jul 2026 18:03:18 -0400 ActionableScore: 6 ActionableScore lower bound: 5 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: A malicious USB serial device can report an undersized bulk-out endpoint, causing user-controlled slab corruption in `safe_serial` safe mode and potentially leading to kernel crash or privilege escalation on systems that attach the device. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63957 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63957 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-2026-63957 MODERATE CHECK WITH IMPACT FROM ORIG NN IMPORTANT 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-787;CWE-122;*CWE-20;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H';BEST CVSS score: '6.8';DESCR 'A malicious USB serial device can report a bulk out endpoint size smaller than the safe mode write format requires. In safe mode the driver appends a two byte trailer to outgoing data, so a too small bulk out buffer can lead to user controlled slab corruption when the port is used for writes. For the CVSS the PR:N is used for the paranoid case because the attacker does not need an account on the target host if they can present or attach the malicious USB device. The issue is not network reachable and requires physical USB device exposure or a user attaching the device. Impact may include kernel memory corruption, denial of service, and potential privilege escalation on the host, so this should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6) YES WRITE OOB USB PRINTF-TTY SIMPLEFIX HARDWARE LINUS KPANIC KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=6 ActionableScoreLower=5 ## 1. ActionableScore * Conservative score: **5** * Paranoid score: **6** * Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: ## 2. Signal breakdown Triggered signals: * **Firmware-mediated external influence: +1** A malicious USB device can influence endpoint descriptors and report an unsafe bulk-out size. * **Memory corruption, strong corruption primitive: +2** The commit explicitly says this avoids **user-controlled slab corruption** in safe mode. This is stronger than a generic validation-only fix. * **Privilege escalation plausible: +1** Slab corruption in kernel memory can plausibly lead beyond DoS, but the patch does not show reclaim control, target selection, function pointer overwrite, or a demonstrated exploit chain. Therefore use weak-to-moderate LPE concern, not full +2. * **Reliable kernel crash / strong DoS: +1** Kernel slab corruption from a malicious device can realistically crash the host. * **Privileged kernel/device-management memory corruption path: +1** The corruption is driven by USB device-provided metadata in a kernel driver path. * **Physical-only or rare hardware-only condition: -2** Exploitation requires physical USB exposure or an equivalent ability to present a malicious USB device. * **Rare AND difficult-to-reach subsystem/configuration: -1** `safe_serial` is not a broad default kernel attack surface compared with core networking, filesystems, or common USB classes. Paranoid adjustment: * Add practical C/I concern due to explicit user-controlled slab corruption, but do not raise above 6 because the vector is physical and the patch does not demonstrate a concrete arbitrary write or LPE exploit chain. ## 3. Reachability analysis The attacker needs to present a malicious USB serial device that binds to `safe_serial` and reports a bulk-out endpoint size smaller than required by safe mode. This is not network reachable. No local Linux account is required if the attacker can physically attach the device, but a real-world attack depends on USB device exposure or user-assisted device connection. Namespaces and containers generally do not reduce the physical-device requirement. If USB device passthrough is delegated to a VM or container, the impact may shift to that environment, but the host risk depends on where the vulnerable driver is bound. The path is not a broad default remote surface. It is a USB serial driver path with a specific malicious-device precondition. ## 4. Severity interpretation This is more serious than ordinary Moderate because the commit explicitly identifies **user-controlled slab corruption**, not just a warning, leak, or validation issue. Realistic exploitation is constrained by physical access and a relatively narrow driver path, so it is not a clear High/Critical remote-style issue. It should be treated as **Actionable Moderate at minimum**, with **Important-candidate manual review** due to kernel memory corruption. Theoretical impact includes privilege escalation or broader kernel compromise. Realistic evidence from the patch supports memory corruption and DoS, but not a demonstrated arbitrary write or reliable LPE chain. ## 5. One-sentence report phrase A malicious USB serial device can report an undersized bulk-out endpoint, causing user-controlled slab corruption in `safe_serial` safe mode and potentially leading to kernel crash or privilege escalation on systems that attach the device. ## 6. Manual review recommendation **MANUAL CHECK REQUIRED** Reason: explicit user-controlled kernel slab corruption is present, even though exploitation is physical-device constrained and not network reachable. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: USB: serial: safe_serial: fix memory corruption with small endpoint Commit: e3a1d6eee25dc96b1d2db0ecd9d8741e92056476 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e3a1d6eee25dc96b1d2db0ecd9d8741e92056476 Commit description: Make sure that the bulk-out buffer size is at least eight bytes to avoid user-controlled slab corruption in "safe" mode should a malicious device report a smaller size. Fixes: 1da177e ("Linux-2.6.12-rc2") Cc: stable@vger.kernel.org Reviewed-by: Greg Kroah-Hartman Signed-off-by: Johan Hovold Signed-off-by: Greg Kroah-Hartman Changed files: drivers/usb/serial/safe_serial.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=e3a1d6eee25dc96b1d2db0ecd9d8741e92056476 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63957 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63957 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:R/S:U/C:H/I:H/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: 5 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).