CVE-2026-74499 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
ALSA: usb-audio: fix OOB write in snd_usbmidi_akai_output()
snd_usbmidi_akai_output() computes its fill-loop bound
buf_end = ep->max_transfer - MAX_AKAI_SYSEX_LEN - 1;
as a signed int, so a small device-advertised bulk-OUT max_transfer makes buf_end negative. The loop guard then compares the u32 urb->transfer_buffer_length against that negative int: the usual arithmetic conversion turns buf_end into a large unsigned value, so the guard stays true and each iteration keeps appending SysEx framing and payload bytes past the end of the URB transfer buffer, which is only max_transfer bytes long.
A USB device that advertises a tiny bulk-OUT endpoint can therefore trigger an attacker-length- and content-controlled heap out-of-bounds write when a process writes to the created /dev/snd/midiC*D* node.
Return early when there is no room for even one SysEx, so the loop is never entered with a bound that would wrap. The loop is the last statement of the function, so bailing out is equivalent to it not running.
Discovered by XBOW, triaged by Baul Lee <[email protected]>
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Analysis
by VulDB Data Team • 08/16/2026
The vulnerability in question affects the Linux kernel's ALSA USB audio subsystem, specifically within the snd_usbmidi_akai_output() function that handles Akai-specific SysEx message processing. This represents a critical out-of-bounds write condition that can be exploited through malicious USB device enumeration to achieve heap corruption. The flaw stems from improper handling of buffer boundary calculations where a signed integer computation results in negative values when processed against unsigned transfer buffer lengths, creating a dangerous arithmetic conversion scenario.
The technical implementation of this vulnerability involves a specific calculation pattern that computes the loop bound using a signed integer approach. When a USB device advertises an unusually small bulk-out endpoint maximum transfer size, the calculation buf_end = ep->max_transfer - MAX_AKAI_SYSEX_LEN - 1 produces a negative value due to integer underflow. This negative signed integer is then compared against an unsigned urb->transfer_buffer_length value through standard arithmetic conversion rules that promote the signed value to unsigned, effectively creating a massive positive comparison value that bypasses all bounds checking mechanisms.
The operational impact of this vulnerability extends beyond simple memory corruption as it enables attackers to control both the length and content of the heap overflow. When processes write to the corresponding /dev/snd/midiCD device nodes, they trigger the vulnerable code path where SysEx framing and payload bytes are continuously appended past the legitimate URB transfer buffer boundaries. The buffer size is strictly limited to max_transfer bytes, but the loop continues indefinitely due to the corrupted bounds check, leading to systematic heap corruption that can potentially be leveraged for privilege escalation or system instability.
This vulnerability maps directly to CWE-129 and CWE-787 within the CWE catalog, representing improper input validation and out-of-bounds write conditions respectively. The attack vector aligns with ATT&CK technique T1059.007 for command and scripting interpreter usage, while the exploitation pathway follows T1203 for legitimate credentials and T1068 for local privilege escalation through kernel memory corruption. The fix implemented addresses this by introducing an early return condition that prevents entry into the vulnerable loop when insufficient buffer space exists for even a single SysEx message, effectively eliminating the arithmetic conversion issue.
The security implications of this vulnerability extend to systems running Linux kernels with USB audio support, particularly those that process MIDI data from potentially malicious USB devices. The exploitation requires minimal privileges since it operates through legitimate device node access but can result in significant system compromise through heap corruption attacks. System administrators should prioritize kernel updates and implement proper USB device access controls as mitigation measures. The vulnerability demonstrates the critical importance of proper integer handling in kernel code, particularly when dealing with hardware-provided boundary values that may not meet expected ranges or assumptions.
The fix for this vulnerability represents a defensive programming approach that prevents the problematic arithmetic conversion by validating buffer capacity before entering potentially dangerous loop constructs. This solution prevents the wraparound condition that would otherwise allow attackers to control heap memory layout through carefully crafted USB device descriptors and subsequent SysEx message transmission patterns. The implementation ensures that when the calculated buffer bounds are insufficient for even basic SysEx processing, the function exits gracefully rather than proceeding with corrupted boundary calculations that could lead to arbitrary code execution or system crashes.