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CVE-2026-90153
ksmbd: bound smb_check_perm_dacl() ACE walks by DACL size
Published 2026-09-17 (31 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-2026-90153 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 | vulnerable | 6.1.190-1 | No fix published anywhere yet, neither in Debian nor upstream for the series bookworm tracks. Watch the Debian tracker page for this CVE. Advisories for bookworm have historically landed a median of 23 days after publication. |
| 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
smb_check_perm_dacl() validates that the DACL fits inside the NT security descriptor, but then bounds its two ACE walks by the remaining NTSD length (acl_size) rather than the DACL's declared size (pdacl_size). When pdacl->size is smaller than the trailing NTSD buffer, bytes after the declared DACL boundary - still inside the stored security descriptor - are parsed as ACEs during access checks. A crafted DACL can place an access-granting ACE beyond pdacl->size, and the current code accepts it during SMB2_CREATE access validation, while parse_dacl() and smb_inherit_dacl() stop at pdacl_size. Bound both ACE walks by pdacl_size to match the DACL boundary semantics used elsewhere in the server. Validation: - semantic KUnit harness shows the post-boundary ACE is selected before the fix and rejected (EACCES) after it - linux master (7.2-rc6), x86_64
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/server/smbacl.c
- What this is
- KERNEL SMB3 SERVER (KSMBD)MAINTAINERS section matched on fs/smb/server/ · linux-cifs@vger.kernel.org · Maintained
- Kernel module
- ksmbd
Check it on the machine
grep -wE 'CONFIG_SMBFS|CONFIG_SMB_SERVER' /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 ksmbdPrints 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_SMBFS, CONFIG_SMB_SERVER
- 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
- 2026-09-17 (31 days ago)source
- CVSS
- 8.1 high
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Nsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.48%, higher than 39.8% 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 |
|---|---|---|
| 6.0 | 6.6.157 | a2051ffe452a |
| 6.0 | 6.12.110 | a29f57a14c93 |
| 6.0 | 6.18.52 | a4a307d14932 |
| 6.0 | 7.2.6 | 8e4f75e979c1 |
| 6.0 | 7.3 | 79decd88dd3f |
| 5.15.62 | 0 | 0 |
| 5.18.18 | 0 | 0 |
| 5.19.2 | 0 | 0 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| 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 crafted NTSD is carried in SMB2_SET_INFO (SMB2_O_INFO_SECURITY) or an SMB2_CREATE SMB2_CREATE_SD_BUFFER context; set_info_sec()/ksmbd_vfs_set_sd_xattr() store that buffer, and a later SMB2_CREATE in smb2_open() loads it into smb_check_perm_dacl() from ksmbd_conn_handler_loop() on TCP/445.
- AC:LAttack complexity, low
- The attacker sets pdacl->size smaller than the NTSD tail, inflates num_aces, and places an ACCESS_ALLOWED ACE after the declared DACL; smb_check_perm_dacl() then walks those bytes on the next SMB2_CREATE with no race or attacker-uncontrollable layout.
- PR:LPrivileges required, low
- __handle_ksmbd_work() requires check_user_session()/get_ksmbd_tcon() before smb2_open()/smb2_set_info(); planting the NTSD needs FILE_WRITE_DAC/FILE_WRITE_OWNER in smb2_set_info_sec() or create rights for SMB2_CREATE_SD_BUFFER on an acl-xattr share, not a pre-auth path.
- UI:NUser interaction, none
- The attacker alone sends the SMB2_SET_INFO or SMB2_CREATE SD_BUFFER that stores the NTSD and the SMB2_CREATE that calls smb_check_perm_dacl(); no victim mount, open, or other interactive step is required.
- S:UScope, unchanged
- smb_check_perm_dacl() only mis-evaluates the stored DACL for ksmbd file opens; the resulting access grant stays inside the kernel SMB server and does not cross a VM, IOMMU, or sandbox boundary.
- C:HConfidentiality impact, high
- The SID-matching ACE walk in smb_check_perm_dacl() can select a post-pdacl->size ACCESS_ALLOWED ACE for the caller or Everyone and apply its access_req, so SMB2_CREATE is authorized for FILE_READ_DATA that the declared DACL would have denied with -EACCES.
- I:HIntegrity impact, high
- The same extra ACE supplies attacker-controlled ace->access_req; ACCESS_ALLOWED with FILE_WRITE_DATA/GENERIC_ALL makes smb_check_perm_dacl() accept a write SMB2_CREATE that parse_dacl() and smb_inherit_dacl() never grant from the bounded DACL.
- A:NAvailability impact, none
- Both walks stay inside the NTSD (acl_size from pntsd_size); the bug only returns success instead of -EACCES and does not oops, panic, hang, or corrupt memory.