CVE-2026-99108 in Linux
Summary
by MITRE • 10/28/2026
In the Linux kernel, the following vulnerability has been resolved:
ntfs: fix FITRIM range alignment
ntfs_trim_fs() aligns the start of a free extent up to the device discard granularity, but derives the discard length by aligning the original extent length down. When the free extent start is not discard-aligned, adding that length to the aligned start can extend the discard past the free extent and into allocated clusters.
For example, with 4 KiB clusters and 32 KiB discard granularity, the free extent [4 KiB, 36 KiB) becomes the discard range [32 KiB, 64 KiB), so
28 KiB beyond the free extent may be discarded.
Align the absolute end of the free extent down and derive the length from the two aligned endpoints. Skip extents that contain no full discard unit.
Reproduced with a 4 KiB-cluster NTFS filesystem on scsi_debug configured for 32 KiB discard granularity and read-zero-after-trim. Before this change, FITRIM zeroed seven allocated 4 KiB clusters following an unaligned 32 KiB hole. With this change, the same data remains intact across FITRIM and remount.
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Analysis
by VulDB Data Team • 10/28/2026
The vulnerability identified in the Linux kernel's NTFS filesystem driver involves a critical logic error within the ntfs_trim_fs function, which handles file system trimming operations via the FITRIM ioctl interface. This operation is designed to inform storage devices about unused blocks so they can be optimized for performance or power consumption, typically by sending discard commands such as TRIM on SSDs. The core technical flaw lies in how the kernel calculates the boundaries of these discard ranges when dealing with free extents that are not naturally aligned to the device's discard granularity. Specifically, the code aligns the starting address of a free extent up to the nearest multiple of the discard granularity but incorrectly derives the length by simply rounding down the original extent size. This approach fails to account for the shift in position caused by the upward alignment of the start point, leading to a miscalculation that extends the discard range beyond the actual boundaries of the free space and into regions containing allocated data clusters.
This misalignment results in severe operational impacts where valid user data is inadvertently destroyed. For instance, on a system with 4 KiB cluster sizes and a storage device configured for 32 KiB discard granularity, an unaligned free extent might be processed such that the calculated discard range covers not only the intended empty space but also subsequent allocated clusters. In practical terms, this means that data residing in those adjacent clusters is zeroed out by the underlying storage hardware due to read-zero-after-trim behavior or similar mechanisms. This constitutes a significant integrity failure, as it leads to silent data corruption without any warning to the operating system or applications relying on that file system. The issue was reproducible using scsi_debug with specific configurations, confirming that unaligned holes in NTFS volumes could trigger this destructive behavior during standard maintenance operations like FITRIM calls.
From a security and standards perspective, this vulnerability is best classified under CWE-787: Out-of-bounds Write, as the operation effectively writes zeros to memory addresses or storage blocks outside the intended logical boundary of free space. Although it manifests through I/O commands rather than direct buffer overflows in traditional software terms, the effect is identical to an out-of-bound write that corrupts adjacent data structures. In the context of the MITRE ATT&CK framework, this could be associated with T1485: Data Destruction, as the flaw allows for the unauthorized and unintended deletion or alteration of stored information. The root cause stems from insufficient validation of range boundaries relative to hardware constraints, highlighting a gap in defensive programming practices where assumptions about alignment are not rigorously enforced against actual device capabilities.
The resolution implemented by the Linux kernel maintainers addresses this issue by fundamentally changing how discard ranges are calculated. Instead of aligning only the start and deriving length independently, the fix now aligns both the absolute end of the free extent down to the discard granularity boundary and then derives the valid length from these two properly aligned endpoints. This ensures that the resulting discard range remains strictly within the bounds of the actual free space. Additionally, the patch introduces a check to skip any extents that do not contain at least one full discard unit, preventing partial or meaningless operations. These changes guarantee that FITRIM operations preserve allocated data integrity even when dealing with unaligned free spaces on devices with specific granularity requirements.
To mitigate this vulnerability in environments running affected kernel versions, it is imperative to apply the latest security patches and updates provided by your Linux distribution vendor. System administrators should verify their kernel version against known vulnerable ranges and ensure that ntfs-3g or native NTFS driver modules are updated accordingly. For organizations relying on NTFS volumes for critical data storage, regular backups remain essential until patching is complete to protect against potential accidental data loss during routine file system maintenance tasks. Furthermore, monitoring logs for unexpected FITRIM calls can help detect if such operations are being triggered in production environments where the vulnerability might still be present.