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CVE-2026-89634
smb: client: fix ALIGN() overflow in symlink_data() error context loop
Published 2026-09-11 (37 days 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-2026-89634 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-4841-1 · 2026-10-17 · text | 6.1.190-1 | Update the kernel and reboot. DLA-4841-1 shipped the fix on 2026-10-17, in 6.1.190-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6528-1 · 2026-09-29 · text | 6.12.111-1 | Update the kernel and reboot. DSA-6528-1 shipped the fix on 2026-09-29, in 6.12.111-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4817-1 · 2026-10-04 · text | 6.12.111-1~deb12u1 | Update the kernel and reboot. DLA-4817-1 shipped the fix on 2026-10-04, in 6.12.111-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
The check added by commit 7d9a7f1f96cd ("smb/client: fix possible infinite loop and oob read in symlink_data()") compared the post-ALIGN length against the remaining buffer, but ALIGN() itself can overflow: for ErrorDataLength near UINT32_MAX (e.g. 0xFFFFFFF9), ALIGN(x, 8) wraps to 0, so the subsequent bounds check passes, and the loop advances by zero bytes leaving 'p' pointing into stale data. Fix by checking the raw ErrorDataLength against the remaining space before applying ALIGN(), then checking again after. Since raw_len is bounded by the buffer, raw_len + 7 cannot overflow, so the second check is an exact post-alignment bounds guard.
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/smb2file.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
- 2026-09-11 (37 days ago)source
- CVSS
- 9.1 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.69%, higher than 52.0% 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 |
|---|---|---|
| 6.1 | 6.1.188 | 2b4a25baa0b9 |
| 6.1 | 6.6.157 | b89d678f9eaf |
| 6.1 | 6.12.109 | 657372ba6ea5 |
| 6.1 | 6.18.50 | 1f824f61d1df |
| 6.1 | 7.2.4 | 62a3025983c8 |
| 6.1 | 7.3 | 62656b024efc |
| 6.0.16 | 0 | 0 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4841-1 | 2026-10-17 | bookworm 6.1.190-1 | tracker |
| DLA-4817-1 | 2026-10-04 | bookworm 6.12.111-1~deb12u1 | tracker |
| DSA-6528-1 | 2026-09-29 | trixie 6.12.111-1 | 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 flaw is in the SMB client parser of a STATUS_STOPPED_ON_SYMLINK CREATE error returned over TCP/445; a malicious, compromised, or MITM SMB server supplies the crafted ErrorDataLength in the network response.
- AC:LAttack complexity, low
- The attacker fully controls ErrorContextCount, ErrorId, and ErrorDataLength in a single SMB2 error response; setting ErrorDataLength near UINT32_MAX (e.g. 0xFFFFFFF9) makes ALIGN() wrap to 0 and bypasses the bounds check with no race or attacker-uncontrolled condition.
- PR:NPrivileges required, none
- The attacker is the remote SMB peer and needs no account or capability on the victim client; guest/null sessions and already-established mounts are enough for the client to parse the malformed error.
- UI:NUser interaction, none
- Persistent fstab, autofs, and systemd automounts plus reconnect-driven path walks already open/lookup files on the share, so a compromised or MITM'd server can return the crafted symlink error during ordinary client activity with no extra user action.
- S:UScope, unchanged
- Impact stays in the client kernel SMB parser and its memory; this is not a VM escape, IOMMU bypass, or cross-scope sandbox escape.
- C:HConfidentiality impact, high
- The ALIGN overflow bypasses the ErrorDataLength bounds check added to stop out-of-bounds reads in this loop (CVE-2026-52967/CVE-2026-31613); a stale overlapping context pointer can then be parsed as symlink PathBuffer data and UTF-16-decoded into a userspace target.
- I:NIntegrity impact, none
- The defect only mis-advances a read-side parser pointer after ALIGN() wraps to 0; it does not write out of bounds or provide a kernel memory-modification primitive.
- A:HAvailability impact, high
- With the skip length wrapped to 0 the error-context loop fails to consume the declared payload and can stall or walk a stale pointer; that is the remaining hole in the 32-bit infinite-loop/OOB class in this function and can hang or oops the client kernel.