CVE-2026-74682 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
ALSA: usb-audio: fix OOB write on Type II inbound URBs
data_ep_set_params() sizes each URB transfer buffer before it adds the Format Type II transfer delimiter:
u->packets = urb_packs; u->buffer_size = maxsize * u->packets;
if (fmt->fmt_type == UAC_FORMAT_TYPE_II) u->packets++; /* for transfer delimiter */ u->urb = usb_alloc_urb(u->packets, GFP_KERNEL);
buffer_size is computed from the pre-increment packet count and never recomputed, so for a Type II endpoint the buffer is one packet short of the packet count the URB is built with.
prepare_inbound_urb() then lays out one iso frame per packet and never consults buffer_size:
offs = 0; for (i = 0; i < urb_ctx->packets; i++) {
urb->iso_frame_desc[i].offset = offs;
urb->iso_frame_desc[i].length = ep->curpacksize;
offs += ep->curpacksize; }
urb->transfer_buffer_length = offs; urb->number_of_packets = urb_ctx->packets;
The last descriptor therefore points one packet past the end of the transfer buffer, where the host controller writes device data on every inbound transfer. prepare_silent_urb() and prepare_playback_urb() bound their fill loops by ctx->buffer_size, so only capture is affected.
fmt_type comes from the device's audio streaming descriptors, so any device advertising a Type II capture format hits this once userspace sets hw_params on the stream.
KASAN on 7.2.0-rc5 (arm64) with a dummy_hcd/raw-gadget device, one report per inbound transfer:
BUG: KASAN: slab-out-of-bounds in dummy_timer Write of size 64 at addr ffff0000186171c0 by task cons02/166 __asan_memcpy dummy_timer hrtimer_run_softirq Allocated by task 166: usb_alloc_coherent snd_usb_endpoint_set_params The buggy address is located 0 bytes to the right of allocated 64-byte region [ffff000018617180, ffff0000186171c0)
Compute buffer_size after the delimiter packet has been accounted for, and bound the fill loop by buffer_size, as prepare_silent_urb() already does on the outbound side. This grows every Type II URB allocation by one maxsize packet.
Discovered by XBOW, triaged by Baul Lee <[email protected]>
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Analysis
by VulDB Data Team • 08/22/2026
The Linux kernel contains a critical out-of-bounds write vulnerability within the ALSA USB audio subsystem that affects devices utilizing Format Type II streaming descriptors. This flaw originates in the data_ep_set_params function, which is responsible for configuring Universal Serial Bus (USB) Request Block parameters before an endpoint begins processing audio streams. The core technical failure lies in the sequence of operations used to calculate and allocate memory for transfer buffers. Specifically, the code calculates the initial buffer size based on a packet count that has not yet been adjusted for the mandatory Format Type II transfer delimiter. Subsequently, the packet count is incremented by one to account for this delimiter, but the previously calculated buffer_size variable remains unchanged. This results in a mismatch where the allocated memory region is smaller than the number of packets specified for the USB Request Block allocation via usb_alloc_urb.
The vulnerability manifests during inbound transfers when prepare_inbound_urb constructs the ISO frame descriptors for the transfer. The function iterates through each packet defined by urb_ctx->packets, setting offsets and lengths without referencing the undersized buffer_size variable. Consequently, the final descriptor is configured to write data one full packet beyond the end of the allocated memory region. When a USB device with Type II capture capabilities sends audio data, the host controller writes this data directly into kernel memory at an invalid address. This behavior constitutes a slab-out-of-bounds write, as confirmed by Kernel Address Sanitizer reports showing writes occurring immediately after an allocated 64-byte region. The issue is strictly limited to inbound transfers and capture scenarios because outbound functions like prepare_silent_urb correctly bound their loops by buffer_size, preventing similar overflows in playback contexts.
From a security perspective, this vulnerability allows for arbitrary kernel memory corruption with potentially severe operational impacts. An attacker who can control or influence the audio stream data sent from a USB device to the host system could exploit this out-of-bounds write to overwrite adjacent kernel structures, leading to privilege escalation, denial of service through kernel panic, or information disclosure by reading sensitive memory contents located near the corrupted buffer. The vulnerability is triggered when userspace applications set hardware parameters on an audio stream that utilizes Type II format descriptors, meaning any connected USB audio device advertising this format can trigger the flaw upon connection and configuration. This aligns with CWE-787: Out-of-bounds Write, as it involves writing data beyond the allocated boundary of a buffer in memory. Furthermore, the attack vector relates to ATT&CK technique T1059: Command and Scripting Interpreter if an attacker uses this vulnerability to execute arbitrary code within the kernel context, or T1048: Exfiltration Over Alternative Protocol if used for covert data exfiltration via modified audio streams.
Mitigation strategies primarily involve applying the upstream Linux kernel patch that corrects the buffer size calculation logic. The fix requires computing the buffer_size after incrementing the packet count to include the transfer delimiter, ensuring the allocated memory is sufficient for all packets in the USB Request Block. Additionally, the fill loop in prepare_inbound_urb should be bounded by buffer_size rather than just the packet count, mirroring the safety checks already present in outbound preparation functions. Until such patches are applied and deployed across affected systems, administrators should exercise caution when connecting untrusted or unknown USB audio devices to critical infrastructure. Disabling automatic mounting of audio interfaces where possible and keeping kernel versions updated with security fixes are essential defensive measures against this class of memory corruption vulnerabilities.