CVE-2026-68388 smb/client: handle overlapping allocated ranges in fallocate
A flaw in the Linux kernel SMB client allows skipping zero-filling of holes during fallocate, potentially causing write failures and out-of-bounds reads.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-68388?
CVE-2026-68388 is a critical memory corruption vulnerability in Linux, published on 10 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-125
Which versions are affected by CVE-2026-68388?
How do I fix CVE-2026-68388?
Apply the vendor's patch.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2026-68388 being exploited?
There is no known exploitation of CVE-2026-68388 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-68388, DEBIAN-CVE-2026-68388, GHSA-h6fv-x4pg-r56c, UBUNTU-CVE-2026-68388.