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CVE-2026-68388
smb/client: handle overlapping allocated ranges in fallocate
Published 2026-08-10 (2 months ago) by the Linux kernel CNA. Subsystem fs/smb. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-68388 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixedDLA-4777-1 · 2026-09-08 · text | 6.1.187-1 | Update the kernel and reboot. DLA-4777-1 shipped the fix on 2026-09-08, in 6.1.187-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.101-1 | Update the kernel and reboot. Fixed in 6.12.101-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.6-1 | Update the kernel and reboot. Fixed in 7.1.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.6-1 | Update the kernel and reboot. Fixed in 7.1.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4745-1 · 2026-08-18 · text | 6.12.101-1~deb12u1 | Update the kernel and reboot. DLA-4745-1 shipped the fix on 2026-08-18, in 6.12.101-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
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.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
fs/smb/client/smb2ops.c
- What this is
- COMMON INTERNET FILE SYSTEM CLIENT (CIFS and SMB3)MAINTAINERS section matched on fs/smb/client/ · linux-cifs@vger.kernel.org · Supported
- Kernel module
- cifs
Check it on the machine
grep -wE 'CONFIG_CIFS|CONFIG_SMBFS' /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
lsmod | grep -w cifsPrints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.
- bookworm, trixie, forky, sid, bookworm · linux-6.12m
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switches
- CONFIG_CIFS, CONFIG_SMBFS
- Reads as
- all on means the file is compiled; any one off means it is not.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-08-10 (2 months ago)source
- CVSS
- 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.51%, higher than 42.5% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- fs/smb
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.10.50 | 5.10.265 | aeb58a4eb39a |
| 5.14 | 5.15.216 | 017198832355 |
| 5.14 | 6.1.183 | f47c7277c03a |
| 5.14 | 6.6.148 | 437637f5ff3f |
| 5.14 | 6.12.101 | 377fe3e583e4 |
| 5.14 | 6.18.42 | 7e08ab7a061b |
| 5.14 | 7.1.6 | a4a09e514283 |
| 5.14 | 7.2 | b09ae45d85dc |
| 5.12.17 | 0 | 0 |
| 5.13.2 | 0 | 0 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4777-1 | 2026-09-08 | bookworm 6.1.187-1 | tracker |
| DLA-4745-1 | 2026-08-18 | bookworm 6.12.101-1~deb12u1 | tracker |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:NAttack vector, network
- The malformed input is the FSCTL_QUERY_ALLOCATED_RANGES response parsed by the SMB client from a remote peer, so a malicious, compromised, or MITM'd SMB server exploits this purely over the network (TCP/445) against the mounted client.
- AC:LAttack complexity, low
- The server fully and deterministically controls the returned range file_offset/length fields; a single record with bit 63 set in length reliably inflates the loop's len past the 1 MB buffer with no race, timing, or memory-layout precondition.
- PR:NPrivileges required, none
- The attacker is the remote SMB server (or a network MITM) and needs no account, credentials, or privileges on the victim client; only the pre-existing SMB session the client itself established is required.
- UI:NUser interaction, none
- Against an already-mounted share (fstab/autofs/systemd automount) a compromised or MITM'd server needs no victim action, and fallocate/posix_fallocate is issued routinely by ordinary applications such as databases, VM images, and download managers.
- S:UScope, unchanged
- The out-of-bounds read, the corrupted write offsets, and the resulting crash all stay within the kernel of the client host; no VM, IOMMU, or sandbox boundary is crossed.
- C:HConfidentiality impact, high
- Because smb3_simple_fallocate_write_range() advances buf by every 64 KB chunk written, the inflated length walks far past the 1 MB kvzalloc'd zero buffer and transmits adjacent kernel heap memory straight back to the attacker's server, yielding a large sequential kernel memory disclosure.
- I:HIntegrity impact, high
- The negative length drives off backwards and lets attacker-chosen offsets be zero-filled over ranges of the victim's file that hold real data, and the original defect makes fallocate report success while leaving holes unallocated, silently breaking the allocation guarantee applications rely on.
- A:HAvailability impact, high
- The out-of-bounds read walks potentially gigabytes past the allocation and will fault on unmapped or guard pages, causing a kernel oops/panic, while the inflated length also drives an extremely long 64 KB-at-a-time write loop that stalls the caller.