CVE-2026-80566 in Linux
Summary
by MITRE • 08/26/2026
In the Linux kernel, the following vulnerability has been resolved:
Input: hynitron_cstxxx - validate touch count and finger IDs
The driver allocates max_touch_num input slots, which are indexed from zero through max_touch_num - 1. The current check allows a finger ID equal to max_touch_num to reach cst3xx_report_contact(). While the input core ignores out-of-range slot indices, reporting touch data without a valid slot change corrupts the touch state of the previously active slot.
The touch count is read from the controller's report and is used to index the fixed-size report buffer without first checking its range. Reject counts larger than the supported number of touch slots before checking the trailing byte or parsing touch data.
Reject finger IDs equal to or greater than max_touch_num, and return immediately when an invalid finger ID is encountered so that corrupt touch frames are discarded instead of reporting partial contact state.
The V821 Avaota F1 board configures the vendor driver with one touch slot, so finger ID 1 is already invalid on that device.
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Analysis
by VulDB Data Team • 08/26/2026
This vulnerability resides within the Linux kernel input subsystem, specifically affecting the hynitron_cstxxx touchscreen driver used in devices such as the V821 Avaota F1 board. The core issue stems from insufficient boundary validation when processing multi-touch data reported by the hardware controller. Modern touchscreens often support multiple simultaneous contact points, and drivers must map these physical contacts to internal software slots for further processing by the input layer. In this specific implementation, the driver allocates a fixed number of input slots based on the max_touch_num parameter but fails to rigorously enforce bounds checking during the parsing phase. This oversight allows malformed or maliciously crafted touch reports from the hardware interface to bypass critical safety checks, leading to potential state corruption within the kernel's input handling subsystem.
The technical flaw manifests in two distinct areas of data validation: finger identification and contact counting. First, when iterating through reported contacts, the driver does not verify that a given finger ID is strictly less than max_touch_num before attempting to access the corresponding slot index. While the Linux input core generally ignores out-of-range indices during final reporting, this safety net only prevents immediate crashes or memory corruption in some contexts; it does not prevent logical errors within the driver itself. Specifically, if an invalid finger ID reaches the cst3xx_report_contact function, it can corrupt the touch state of a previously active slot because the internal logic assumes valid indexing for state management operations such as tracking pressure and coordinates over time. Second, the total number of touches reported by the controller is read directly into a buffer index without verifying that this count does not exceed max_touch_num. This allows an attacker or faulty hardware to trigger out-of-bounds reads or writes if subsequent parsing logic relies on this unvalidated count to determine how many bytes to consume from the report stream.
The operational impact of these flaws extends beyond simple denial of service, although system instability is a likely outcome. By allowing invalid finger IDs and excessive touch counts to propagate through the driver stack, the integrity of the multi-touch state machine is compromised. This can result in ghost touches being reported to user-space applications, where coordinates or pressure values from one slot are incorrectly attributed to another active contact point. In security terms, this represents a potential avenue for local privilege escalation if an unprivileged process can exploit the resulting memory corruption or logic errors to gain unauthorized access to kernel resources. Furthermore, on devices like the V821 Avaota F1 which are configured with only one touch slot, any finger ID greater than zero is inherently invalid. The failure to reject such IDs immediately means that even minor hardware glitches or maliciously crafted input events can destabilize the entire touchscreen subsystem, leading to unresponsive interfaces or kernel panics depending on how the corrupted state interacts with other kernel modules.
From a classification perspective, this vulnerability aligns closely with CWE-125 Out-of-bounds Read and CWE-787 Out-of-bounds Write, as the lack of range checking allows access beyond allocated memory boundaries for touch slot data. It also relates to CWE-20 Improper Input Validation, specifically regarding the failure to validate integer values against expected limits before use in array indexing or loop control structures. In terms of adversarial tactics, this could be leveraged within an ATT&CK framework context under T1564.003 Hidden Windows and Processes if the corruption leads to unexpected behavior that obscures system state, though more directly it fits into exploitation patterns involving memory safety violations common in kernel drivers. The vulnerability highlights a critical gap in defensive programming practices where trust is placed in hardware-reported data without sufficient sanitization at the driver level.
Mitigation strategies must focus on rigorous input validation within the driver code itself rather than relying solely on higher-level subsystem protections. Developers should implement strict bounds checking for both the total touch count and individual finger IDs before any array access or state update occurs. Specifically, the logic in cst3xx_report_contact must be updated to immediately return an error if a finger ID is greater than or equal to max_touch_num, ensuring that corrupt frames are discarded entirely rather than processed partially. Additionally, the loop iterating through touch data should verify that the reported count does not exceed the allocated slot limit before attempting to parse subsequent bytes from the report buffer. For system administrators and users of affected devices such as the V821 Avaota F1, applying kernel updates that include this patch is essential to restore proper input handling integrity. Until patches are applied, monitoring for unusual touchscreen behavior or system instability may serve as an indicator of potential exploitation attempts targeting this specific driver flaw.