CVE-2026-64339 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
usb: misc: usbio: bound bulk IN response length to the received transfer
usbio_bulk_msg() copies bpkt_len = le16_to_cpu(bpkt->len) bytes out of the bulk IN buffer (usbio->rxbuf, allocated with size usbio->rxbuf_len) into the caller's buffer. bpkt_len is fully controlled by the device and is only checked against ibuf_len; ibuf_len in turn is checked against usbio->txbuf_len, not against rxbuf_len:
if ((obuf_len > (usbio->txbuf_len - sizeof(*bpkt))) || (ibuf_len > (usbio->txbuf_len - sizeof(*bpkt)))) return -EMSGSIZE;
txbuf_len and rxbuf_len are taken independently from the bulk OUT and bulk IN endpoint wMaxPacketSize in usbio_probe(). A malicious or malfunctioning device that advertises a large bulk OUT endpoint and a small bulk IN endpoint (e.g. by claiming one of the quirk-free IDs such as the Lattice NX33U, 0x2ac1:0x20cb) therefore makes ibuf_len, and hence the device-supplied bpkt_len, exceed rxbuf_len. memcpy() then reads up to txbuf_len - rxbuf_len bytes past the end of the rxbuf slab object. The over-read bytes are handed back to the i2c layer and on to user space through i2c-dev, disclosing adjacent slab memory; with KASAN this is reported as a slab-out-of-bounds read.
The number of bytes actually received is already known: act equals the URB actual_length and is bounded by rxbuf_len. Reject any response that claims more payload than was received, mirroring the existing "act < sizeof(*bpkt)" check just above.
The control path (usbio_ctrl_msg()) is not affected: it uses a single buffer (ctrlbuf) for both directions, so its analogous copy can never leave the allocation.
Found by code review. The out-of-bounds read was confirmed under AddressSanitizer with a faithful userspace model of usbio_bulk_msg()'s receive path (an rxbuf_len-sized buffer, the same act/ibuf_len/bpkt_len checks and the memcpy). A USB raw-gadget + dummy_hcd reproducer is also available.
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Analysis
by VulDB Data Team • 07/25/2026
This vulnerability exists within the linux kernel's usbio driver implementation where a critical bounds checking flaw allows for out-of-bounds memory reads during bulk input operations. The issue stems from improper validation of device-supplied packet lengths in the usbio_bulk_msg() function, which processes bulk IN transfers from USB devices. When a malicious or malfunctioning device presents inconsistent endpoint specifications, particularly with large bulk OUT endpoints and small bulk IN endpoints, the system fails to properly validate that the reported payload length does not exceed the allocated receive buffer size.
The technical flaw occurs due to the separation of buffer management for input and output operations within the usbio driver. During initialization, the driver allocates distinct buffers for bulk OUT and bulk IN operations based on endpoint descriptors obtained from device enumeration. The bulk IN buffer size is determined independently from the bulk OUT buffer size through endpoint wMaxPacketSize values, creating a potential mismatch where device descriptors can specify a much smaller IN endpoint than OUT endpoint. This design allows a crafted device to supply a packet length value that exceeds the actual allocated receive buffer capacity.
The specific vulnerability manifests when the function copies bpkt_len = le16_to_cpu(bpkt->len) bytes from usbio->rxbuf into caller buffers, where bpkt_len originates from device-supplied data and is only validated against ibuf_len. However, ibuf_len itself is compared against txbuf_len instead of rxbuf_len, creating an unchecked path that allows memory corruption. The memcpy operation then reads beyond the allocated receive buffer boundaries, potentially accessing up to txbuf_len - rxbuf_len bytes past the end of the rxbuf slab object.
This vulnerability directly maps to CWE-129 and CWE-787 within the Common Weakness Enumeration framework, representing improper input validation and out-of-bounds read conditions. The security implications are significant as the over-read memory contents can contain sensitive kernel data structures, stack values, or other confidential information from adjacent memory regions. With KASAN enabled, this issue manifests as a slab-out-of-bounds read error, while AddressSanitizer confirms the vulnerability through controlled testing environments.
The operational impact extends beyond simple memory disclosure to potential information leakage that could aid in advanced exploitation techniques such as kernel address space layout randomization (ASLR) bypasses or privilege escalation attacks. The vulnerability affects any system using the usbio driver with devices that present inconsistent endpoint descriptors, particularly those using quirk-free device IDs like the Lattice NX33U device mentioned in the analysis.
The mitigation strategy involves implementing proper bounds checking by comparing the device-supplied packet length against the actual receive buffer size rather than allowing unchecked comparisons. The fix requires modifying the validation logic to ensure that any response claiming more payload than was actually received is rejected, similar to the existing check for cases where actual transfer length is less than packet header size. This approach mirrors the existing defensive pattern already present in the codebase and prevents the out-of-bounds memory access while maintaining compatibility with legitimate device behavior.
The control path (usbio_ctrl_msg()) remains unaffected because it uses a single buffer for both directions, eliminating the possibility of mismatched buffer sizes that could cause similar issues. This distinction highlights the importance of consistent buffer management practices across different code paths within kernel drivers. The vulnerability was identified through careful code review and confirmed using both AddressSanitizer for memory error detection and custom reproducer testing with USB raw-gadget functionality combined with dummy_hcd emulation to accurately model the problematic receive path behavior.