CVE-2026-31617
MediumCVSS 5.5Exploitation Probability (EPSS)
Low risk3th percentile - higher than 3% of all known CVEs
Summary
In the Linux kernel, the USB f_ncm gadget driver (CDC NCM network function) lacks validation of the minimum block_len in ncm_unwrap_ntb(). When block_len is smaller than the NDP size, arithmetic underflow occurs, bypassing bounds checks and allowing a malicious USB host to copy adjacent kernel memory into the network skb.
Risk Assessment
An attacker with physical USB access (or remotely via USB device emulation) can exploit this vulnerability to read sensitive kernel memory or potentially escalate privileges, posing a serious risk to system confidentiality and integrity.
Recommendation
Immediately update the Linux kernel to a version containing the fix (commit addressing the vulnerability). If patching is not possible, temporarily disable CDC NCM support in USB gadget configuration or restrict physical access to USB ports.
Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: usb: gadget: f_ncm: validate minimum block_len in ncm_unwrap_ntb() The block_len read from the host-supplied NTB header is checked against ntb_max but has no lower bound. When block_len is smaller than opts->ndp_size, the bounds check of: ndp_index > (block_len - opts->ndp_size) will underflow producing a huge unsigned value that ndp_index can never exceed, defeating the check entirely. The same underflow occurs in the datagram index checks against block_len - opts->dpe_size. With those checks neutered, a malicious USB host can choose ndp_index and datagram offsets that point past the actual transfer, and the skb_put_data() copies adjacent kernel memory into the network skb. Fix this by rejecting block lengths that cannot hold at least the NTB header plus one NDP. This will make block_len - opts->ndp_size and block_len - opts->dpe_size both well-defined. Commit 8d2b1a1ec9f5 ("CDC-NCM: avoid overflow in sanity checking") fixed a related class of issues on the host side of NCM.