CVE-2026-74681 Overview
CVE-2026-74681 is a Linux kernel vulnerability in the usb: misc: usbio driver. The flaw resides in the usbio_bulk_msg() function, which handles bulk USB message transfers. The EMSGSIZE check for the receive buffer length (ibuf_len) incorrectly compares it against txbuf_len, the bulk OUT (transmit) endpoint size, instead of rxbuf_len, the bulk IN endpoint size. Because these two values come from independent endpoints in usbio_probe(), the validation is incorrect when the buffer sizes differ. The fix replaces the incorrect comparison with rxbuf_len so the size check matches the buffer that actually holds inbound response data.
Critical Impact
A mismatched buffer-size check in the usbio driver can allow oversized inbound USB bulk transfers to bypass validation, creating conditions for out-of-bounds memory access in kernel space.
Affected Products
- Linux kernel versions containing the usb: misc: usbio driver prior to the fix commits
- Linux stable branches receiving backports referenced in the kernel Git commits
- Distributions shipping vulnerable kernel builds with the usbio driver enabled
Discovery Timeline
- 2026-08-22 - CVE-2026-74681 published to NVD
- 2026-08-22 - Last updated in NVD database
Technical Details for CVE-2026-74681
Vulnerability Analysis
The vulnerability is an input validation error in the Linux kernel USB I/O driver located in drivers/usb/misc/usbio. The usbio_bulk_msg() function transfers data across a bulk USB endpoint pair. Each direction has its own endpoint and its own maximum packet size, established during usbio_probe(). The driver stores these as txbuf_len for the OUT (transmit) direction and rxbuf_len for the IN (receive) direction.
When validating the size of an inbound receive buffer (ibuf_len), the driver compares the value against txbuf_len and returns EMSGSIZE only when the OUT endpoint size is exceeded. If rxbuf_len is smaller than txbuf_len, an inbound transfer larger than the actual receive buffer can pass the check. Because the response data is written to the buffer sized by rxbuf_len, the mismatch creates an out-of-bounds write condition in kernel memory during bulk IN completion. The corrected code compares ibuf_len against rxbuf_len, aligning the check with the buffer that receives the response.
Root Cause
The root cause is an incorrect variable reference in the boundary check. The developer used txbuf_len in an EMSGSIZE comparison that should have used rxbuf_len. This is a classic improper input validation defect where the wrong bound is enforced against user-supplied or device-supplied length values.
Attack Vector
Exploitation requires interaction with the USB stack through a device that speaks the usbio protocol. A malicious or malformed USB device advertising asymmetric bulk endpoint sizes can trigger the incorrect path. When rxbuf_len < txbuf_len, inbound bulk data that exceeds the receive buffer bypasses the length check and reaches the completion handler. See the Kernel Git Commit 7e22c9f, Kernel Git Commit 9ad0164f, and Kernel Git Commit ebfd1e82 for the patch contents.
Detection Methods for CVE-2026-74681
Indicators of Compromise
- Kernel log entries showing bulk USB transfer errors, memory corruption warnings, or KASAN out-of-bounds reports referencing the usbio driver
- Unexpected system crashes or oops events correlated with USB device attachment on hosts using the usbio driver
- Presence of unknown or unauthorized USB devices enumerating with asymmetric bulk endpoint descriptors
Detection Strategies
- Audit the running kernel version against the patched commits listed in the kernel.org references to confirm remediation status
- Enable CONFIG_KASAN in kernel builds used for testing to surface out-of-bounds writes originating from the usbio driver
- Monitor dmesg and journald for usbio driver messages combined with USB enumeration events from newly connected devices
Monitoring Recommendations
- Collect USB device enumeration telemetry, including vendor IDs, product IDs, and endpoint descriptors, for baseline comparison
- Alert on kernel panics or WARN_ON events referencing drivers/usb/misc/usbio in centralized logging
- Track physical USB port usage on sensitive endpoints and correlate anomalies with authentication and session telemetry
How to Mitigate CVE-2026-74681
Immediate Actions Required
- Apply the upstream Linux kernel patches referenced in the kernel.org commits 7e22c9f, 9ad0164f, and ebfd1e82 or install vendor-provided kernel updates that include these fixes
- Restrict physical and virtual USB access on servers, workstations, and shared systems where untrusted devices could be attached
- Disable or unload the usbio kernel module on systems that do not require it
Patch Information
The fix replaces the incorrect txbuf_len comparison with rxbuf_len inside usbio_bulk_msg(), ensuring the length check matches the buffer that receives inbound bulk data. The patch is available in the following stable kernel commits: Kernel Git Commit 7e22c9f, Kernel Git Commit 9ad0164f, and Kernel Git Commit ebfd1e82. Rebuild or update to a kernel image that includes these commits.
Workarounds
- Blocklist the usbio module using /etc/modprobe.d/ entries on systems that do not need the driver
- Apply USB device authorization policies via udev or USBGuard to prevent unknown devices from enumerating
- Limit console and physical access to hosts running unpatched kernels until updates are deployed
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
