A logic bug in the Linux kernel’s Kvaser USB CAN driver has been fixed after being assigned CVE-2025-68308—a subtle off-by-one handling error in the command parsers that could cause an infinite parsing loop and result in a local denial-of-service on systems that interact with affected Kvaser USB CAN hardware.
Background
The vulnerability affects the Linux kernel driver for Kvaser USB CAN adapters, specifically code paths in the kvaser_usb leaf implementation that parse device commands delivered over USB. The two routines implicated are
kvaser_usb_leaf_wait_cmd and
kvaser_usb_leaf_read_bulk_callback. Both contain logic intended to skip so-called
zero-length commands that are used as placeholders to align a data stream to the USB endpoint’s
wMaxPacketSize boundaries.
When a zero-length command is encountered exactly on a packet boundary (for example when the buffer position
pos is already a multiple of
wMaxPacketSize), the code used to align to the next packet boundary relied directly on a
round_up calculation which could return the same value as
pos. That means the parser would not advance the buffer position, remain stuck on the same byte, and enter an infinite loop while parsing the input stream. The fix applied ensures the alignment calculation is performed on
pos + 1 instead of
pos, forcing the rounding to move forward to the next packet boundary and thus guaranteeing progress in the parser.
This issue was publicly disclosed in December 2025 and tracked under CVE-2025-68308. The vulnerability is considered a local availability-impact flaw: it does not directly expose memory corruption, privilege escalation, or information disclosure, but it can result in a denial-of-service by causing the driver or related kernel processing to spin or block indefinitely.
Overview of the technical root cause
USB packet alignment and zero-length placeholders
USB endpoints have a maximum packet size, commonly referred to as
wMaxPacketSize. Protocols layered over USB sometimes use
zero-length packets or special zero-length command markers to align logical messages to that packet boundary. The driver code in the Kvaser implementation recognizes and skips these placeholders by advancing its internal buffer index to the next packet boundary.
A helper macro or function such as
round_up(x, align) is typically used to compute the smallest multiple of
align that is greater than or equal to
x. The behavior of such a rounding function is well-defined: if
x is already a multiple of
align,
round_up(x, align) returns
x.
Off-by-one logic failure
The problematic code path relied on
round_up(pos, wMaxPacketSize) to skip past a zero-length placeholder. If
pos is already on a packet boundary,
round_up returned the same
pos, so the code attempted to skip a placeholder by setting
pos = round_up(pos, wMaxPacketSize) — but
pos did not change. The next loop iteration examined the same data again, found the same zero-length placeholder, and attempted the same skip. The loop therefore never made forward progress.
The fix replaces the rounding operand with
pos + 1, i.e.,
pos = round_up(pos + 1, wMaxPacketSize). This guarantees that even when
pos is on a boundary, the call to
round_up returns the next multiple of the packet size, ensuring
pos advances and the parser escapes the placeholder.
Why this is more than a cosmetic bug
At first glance an off-by-one adjustment might seem trivial. In kernel device drivers that handle stream parsing, however, progress guarantees are essential. A tight infinite loop inside a kernel context can translate into:
- A CPU-hogging kernel thread or interrupt handler that saturates a CPU core.
- A hung I/O operation that prevents further device operation or consumes kernel resources.
- A scenario in which drivers are unresponsive, causing dependent subsystems to block or degrade.
Because these parsing routines run as part of USB bulk read handling and command waiting logic, the bug can manifest as a local denial-of-service, particularly harmful on embedded systems and appliances that rely on CAN hardware for real-time control or telemetry.
What was changed in the patch
The minimal, targeted change replaced
round_up(pos, wMaxPacketSize) with
round_up(pos + 1, wMaxPacketSize) and applied the same correction consistently across the two affected parsing locations. The change is intentionally small and conservative: it preserves the original alignment strategy while adding a guaranteed forward step when encountering a zero-length placeholder located on a packet boundary.
This approach avoids more invasive redesigns of the parser while eliminating the infinite-loop failure mode. The maintainers applied the change to the relevant file within the Linux kernel CAN driver tree responsible for Kvaser USB devices.
Affected systems and versions
- The vulnerability affects Linux kernel trees that include the split Kvaser USB driver files (kvaser_usb_core.c and kvaser_usb_leaf.c) introduced by earlier driver refactors.
- Distribution security trackers and advisories list multiple kernel package versions that required updates; downstream distributions have issued fixed kernel packages in their security streams.
- The flaw is present wherever the kvaser_usb leaf parser code with the original alignment logic exists, so any Linux-based system with an affected kernel version and the Kvaser USB driver enabled is potentially affected.
Because kernel driver backports vary across distributions, the practical exposure is distribution- and release-dependent. Some distributions issued security fixes and updated kernel packages; systems running older kernel builds or vendor kernels that had not yet integrated the upstream patch remained vulnerable until updated.
Exploitability and attacker model
How an attacker could trigger the issue
- The bug is local in nature. An attacker must supply crafted USB packet sequences to a host where the vulnerable driver is active.
- There are two feasible scenarios for exploitation:
- A malicious or compromised USB device presenting itself as a Kvaser-compatible adapter and returning specially crafted packet sequences designed to place zero-length placeholders exactly on packet boundaries.
- A local user or process with access to the device interface who can inject crafted inputs to the driver (this is less likely unless unprivileged access to the device is permitted).
What an exploit achieves
- The primary impact is availability: the device driver may enter an infinite loop or spin, causing CPU consumption or blocking further I/O with the physical CAN adapter.
- The vulnerability does not directly imply memory safety issues such as buffer overflows or arbitrary code execution.
- Because the attack vector is local (or requires direct USB-level control), the ease of exploitation is constrained by physical access or the ability to attach a malicious/peripheral device.
Practical considerations
- Systems in industrial, automotive, or embedded environments that use Kvaser devices for CAN connectivity may be most at risk if they accept untrusted USB devices or operate in environments where device integrity cannot be guaranteed.
- Systems that allow users to plug USB devices or that operate with untrusted peripherals could see denial-of-service effects if a compromised or malicious Kvaser device (or emulator) is attached.
- Virtualized or container environments that expose USB devices to guests via pass-through may extend the risk surface if guest OSs contain the vulnerable kernel driver.
Severity assessment
- The vulnerability is best characterized as a local denial-of-service with a limited impact scope: it affects availability rather than confidentiality or integrity.
- Because exploitation requires local access to USB traffic (physical access, malicious peripheral, or privileged local software), the overall risk to well-hardened servers that do not expose USB to untrusted users is relatively low.
- For edge devices, instrumentation systems, or industrial controllers that interact with CAN hardware and may accept USB devices, the risk is materially higher.
Operational impact depends on whether the affected machine performs critical roles tied to the Kvaser device. In embedded, industrial control, or automated test environments, even a transient denial-of-service can cause production disruptions or safety hazards.
Mitigation and remediation
Immediate steps for system administrators and users
- Install vendor or distribution kernel updates that include the patch. The fix is small and has been applied to the upstream Linux kernel; distributions have incorporated the change in security updates. Keeping kernels up to date through your platform’s standard security channels is the primary mitigation.
- Avoid connecting untrusted USB devices to hosts that handle CAN traffic until the kernel has been patched.
- Apply device-level policies: where feasible, enforce USB device authorization and process-level access controls to prevent unprivileged users from interacting with the CAN adapter device file.
- Reboot after applying kernel updates when required by your distribution to ensure the new kernel is running and the driver change is active.
For distribution and kernel maintainers
- Backport the upstream patch into supported kernel branches where the vulnerable code exists, and ship security updates with the fix included.
- Consider adding defense-in-depth: bounding iteration counts in parsing loops or adding explicit progress checks to avoid any future regressions leading to live-lock or infinite loops.
- Review similar alignment or placeholder-handling logic across other USB drivers to detect analogous off-by-one conditions.
For embedded and industrial deployments
- Inventory hosts that use Kvaser USB adapters and identify their kernel versions.
- Schedule maintenance windows to deploy patched kernels and reboot devices safely.
- Where downtime is unacceptable, evaluate temporary mitigations such as disabling the kvaser_usb driver (unload the module) or preventing the use of affected devices until a patch can be applied.
- Use USB device whitelisting, secure boot, and device access controls to reduce the risk of malicious peripherals in the field.
Recommended hardening and long-term protections
- Hardened USB/device policies: implement administratorside restrictions on which USB device classes can be enumerated or accessed, using udev rules, group permissions, or vendor ID whitelisting.
- Kernel-level safe parsing patterns: driver authors should enforce forward progress invariants in all parsing loops and include fail-safe break conditions that avoid infinite loops when encountering unexpected input.
- Fuzz-testing and protocol verification: adopt rigorous fuzzing for USB driver stacks and use coverage-guided fuzzers that can exercise boundary conditions such as zero-length packets and exact packet-boundary placements.
- Defensive coding guidelines: treat zero-length packets and alignment placeholders as untrusted input and validate all derived indices and lengths before use.
Why the fix is the right balance
The patch’s single-line change (aligning on
pos + 1 rather than
pos) is intentionally narrow and conservative. It avoids wholesale redesign of the parser while directly removing the infinite-loop hazard. The modification is easy to review, minimizes regression risk, and addresses the root cause—namely the reliance on
round_up semantics when already at an aligned boundary.
Alternative or complementary approaches could have been taken—adding an explicit
if (pos == round_up(pos, wMaxPacketSize) pos++; guard, or centralizing alignment and loop-termination checks. But the selected fix preserves earlier behavior and semantics for all non-boundary cases and keeps the code simple.
Limitations and residual risks
- The fix mitigates the infinite-loop scenario but does not change the fact that zero-length placeholders are part of the stream protocol; if other parsing edge cases exist, they may still cause unexpected behavior.
- Systems that cannot be updated quickly (for example, proprietary embedded appliances with slow vendor-update cycles) may remain vulnerable; in such environments, administrators should assess compensating controls like physical device management and runtime monitoring for driver hangs.
- Because the vulnerability does not involve memory-corruption, exploit code cannot trivially escalate to arbitrary code execution; nevertheless, a persistent denial-of-service can be as damaging as other higher-severity effects in safety-critical systems.
Practical checklist for administrators (quick actions)
- Update: apply your vendor or distribution kernel security updates that include the kvaser_usb patch.
- Reboot: when necessary, reboot machines into the updated kernel.
- Audit: identify systems with Kvaser USB adapters and document kernel versions.
- Restrict: block untrusted USB peripherals and restrict access to device files via udev and permissions.
- Monitor: watch for signs of CPU starvation, hung kernel threads, or driver errors in dmesg/logs that might indicate attempted exploitation or remaining issues.
Broader lesson: parsing correctness in device drivers
Device drivers frequently interact with noisy, hardware-generated data. Aligning behavior across packet boundaries and handling of special-case markers is a classic source of subtle bugs. This vulnerability reiterates several enduring lessons for kernel and driver authors:
- Always enforce progress invariants—parsers must guarantee forward progress on each loop to avoid live-locks.
- Treat alignment helpers and rounding functions with caution when their input may already be aligned—explicitly test and document edge cases.
- Keep patches small, focused, and easy to reason about; minimal fixes are less likely to regress unrelated behavior.
- Use fuzzing and targeted unit tests for boundary conditions such as zero-length packets and exact alignment scenarios.
Conclusion
CVE-2025-68308 is a textbook example of a small, subtle logic error producing a real-world denial-of-service risk in kernel space. The fix—changing the rounding operand to
pos + 1—is minimal and effective, restoring a forward-progress guarantee to the Kvaser USB CAN parser and preventing infinite loops when zero-length placeholders occur on packet boundaries.
For administrators and operators, the practical response is straightforward: apply distributed kernel security updates or upstream patches as soon as feasible, restrict untrusted USB devices, and ensure monitoring for driver hang symptoms is in place. For driver authors and maintainers, the incident is a useful reminder of the importance of robust parsing invariants, comprehensive boundary testing, and the value of clearly documented alignment semantics when working with USB packetized protocols.
Source: MSRC
Security Update Guide - Microsoft Security Response Center