CVE-2026-68388
Received Received - Intake

Heap Overflow in Linux Kernel SMB Client

Publication date: 2026-08-10

Last updated on: 2026-08-10

Assigner: kernel.org

Description
In the Linux kernel, the following vulnerability has been resolved: smb/client: handle overlapping allocated ranges in fallocate smb3_simple_fallocate_range() can skip holes when an allocated range returned by the server starts before the current fallocate offset. The skipped hole is not zero-filled, but fallocate still returns success. A later write to that hole may therefore fail with ENOSPC. The function queries allocated ranges so that it can preserve existing contents and write zeroes only into holes. However, the server may return a range that starts before the current fallocate offset. For example, assume the fallocate request is [100, 400) and the only allocated range returned by the server is [0, 200): Request: [100, 400) Server range: [ 0, 200) allocated Correct: [100, 200) allocated data, skip [200, 400) hole, zero-fill Current: [100, 300) skipped [300, 400) zero-filled afterwards The current code adds the full server range length, 200, to the current offset 100 and moves to 300. As a result, the hole in [200, 300) is skipped without being zero-filled. Fix this by advancing only over the part of the allocated range that overlaps the current fallocate offset. Ignore ranges that end before the current offset and reject ranges whose end offset overflows. This also prevents a malformed range length from causing an out-of-bounds zero-buffer read.

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Meta Information

Published
2026-08-10
Last Modified
2026-08-10
Generated
2026-08-10
AI Q&A
2026-08-10
EPSS Evaluated
N/A
NVD
CVE-2026-68388
EUVD
EUVD-2026-55574

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
linux linux_kernel *

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Exploitability

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CWE ID Description
CWE-UNKNOWN

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Executive Summary

This vulnerability in the Linux kernel involves a flaw in the SMB client's fallocate function. When handling allocated ranges, the function may skip zero-filling holes in a file if the server returns an allocated range starting before the current fallocate offset. This can lead to uninitialized data being left in the file, potentially causing ENOSPC errors on subsequent writes.

Detection Guidance

This vulnerability is specific to the Linux kernel's SMB client implementation and may not have direct network detection methods. Check kernel logs for SMB-related errors or ENOSPC failures during fallocate operations. Use commands like dmesg | grep -i smb or journalctl -k | grep -i smb to inspect kernel logs for related issues.

Impact Analysis

This vulnerability could cause data corruption or unexpected ENOSPC errors when writing to files on SMB shares. Applications relying on fallocate to pre-allocate space may encounter failures or leave uninitialized data in files, leading to potential application crashes or incorrect behavior.

Mitigation Strategies

Apply the latest Linux kernel security patches that address this issue. If using a distribution with delayed updates, consider temporarily disabling SMB client functionality or restricting SMB operations until patched. Monitor vendor advisories for kernel updates.

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