CVE-2026-31758 Overview
CVE-2026-31758 is a use-after-free vulnerability in the Linux kernel's USB Test and Measurement Class (USBTMC) driver. The flaw resides in the usbtmc_release function, which previously failed to flush or kill pending anchored USB Request Blocks (URBs) before releasing driver state. Anchored URBs completing later through the Host Controller Driver (HCD) giveback path could reference freed memory, leading to memory corruption or local privilege escalation.
The upstream fix calls usbtmc_draw_down() during release to allow anchored URBs to complete safely.
Critical Impact
A local user with access to a USBTMC device node can trigger a use-after-free in kernel context, potentially escalating privileges or crashing the system.
Affected Products
- Linux kernel usb/usbtmc driver (multiple stable branches)
- Distributions shipping vulnerable Linux kernel versions prior to the referenced stable commits
- Systems exposing USBTMC instrument device nodes to unprivileged users
Discovery Timeline
- 2026-05-01 - CVE-2026-31758 published to NVD
- 2026-05-03 - Last updated in NVD database
Technical Details for CVE-2026-31758
Vulnerability Analysis
The USBTMC driver tracks asynchronous bulk transfers using a usb_anchor structure. Anchored URBs may still be in flight when a user process closes the character device. The original usbtmc_release path tore down driver state without ensuring those URBs had completed or been killed.
When the HCD later invoked the URB completion (giveback) callback, it dereferenced driver context that had already been freed. This produces a classic use-after-free condition in kernel memory, exploitable by local users who can open /dev/usbtmcN and trigger asynchronous I/O before closing the descriptor.
Root Cause
The root cause is missing lifetime management for anchored URBs in the release handler. The fix introduces a call to usbtmc_draw_down(), which kills outstanding anchored URBs and waits for completion before freeing associated structures. This guarantees that no in-flight URB references stale driver state during giveback.
Attack Vector
Exploitation requires local access and permission to open a USBTMC device node. An attacker submits asynchronous I/O requests that anchor URBs, then closes the file descriptor while transfers are pending. Successful exploitation can corrupt kernel memory, leak information, or yield local privilege escalation. Network exploitation is not possible.
No public proof-of-concept code is available. The vulnerability is described in prose only, with technical details available in the upstream Linux kernel commits referenced below.
Detection Methods for CVE-2026-31758
Indicators of Compromise
- Unexpected kernel oops or panic messages referencing usbtmc, usb_anchor, or HCD giveback paths in dmesg and /var/log/kern.log
- KASAN reports flagging use-after-free reads or writes inside drivers/usb/class/usbtmc.c
- Unprivileged processes opening /dev/usbtmc* device nodes followed by abnormal process termination or system instability
Detection Strategies
- Enable CONFIG_KASAN on test or canary systems to surface use-after-free conditions during USBTMC interactions
- Audit running kernel versions against the fixed stable commits (7fa8f61, 8a76855, 959ef32, 95e09b0, 977b632, d13318d, d40198d, e189d44)
- Hunt for processes interacting with USBTMC device nodes that are not legitimate instrument-control software
Monitoring Recommendations
- Forward kernel ring buffer logs to a centralized logging or SIEM platform for alerting on usbtmc faults
- Monitor auditd for open() syscalls against /dev/usbtmc* from unexpected user accounts
- Track installed kernel package versions across the fleet and flag hosts that lag behind the patched stable releases
How to Mitigate CVE-2026-31758
Immediate Actions Required
- Update to a Linux kernel build that includes one of the upstream stable fixes referenced in the kernel.org commits
- Restrict permissions on /dev/usbtmc* device nodes so that only trusted users or groups can open them
- Unload the usbtmc kernel module on systems that do not require USB instrument control
Patch Information
The fix adds a usbtmc_draw_down() call to usbtmc_release so that anchored URBs are flushed or killed before driver state is freed. Patched stable commits include Kernel Git Commit 7fa8f61, Kernel Git Commit 8a76855, Kernel Git Commit 959ef32, Kernel Git Commit 95e09b0, Kernel Git Commit 977b632, Kernel Git Commit d13318d, Kernel Git Commit d40198d, and Kernel Git Commit e189d44.
Workarounds
- Blacklist the usbtmc module on hosts that do not connect to test and measurement instruments
- Tighten udev rules to restrict /dev/usbtmc* access to a dedicated trusted group
- Apply USB device filtering policies that block unauthorized USBTMC class devices from binding
# Blacklist the usbtmc module to remove the attack surface
echo "blacklist usbtmc" | sudo tee /etc/modprobe.d/blacklist-usbtmc.conf
sudo rmmod usbtmc 2>/dev/null || true
# Restrict device node access via udev
cat <<'EOF' | sudo tee /etc/udev/rules.d/60-usbtmc.rules
KERNEL=="usbtmc[0-9]*", MODE="0660", GROUP="usbtmc"
EOF
sudo udevadm control --reload-rules && sudo udevadm trigger
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
