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CVE-2026-89677
nfsd: fix possible fh_compose of wrong dentry in nfsd4_create_file()
Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem fs/nfsd. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-89677 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 | not affected | 6.1.190-1 | Nothing to do. bookworm never shipped the vulnerable code. |
| Debian 13trixie | not affected | 6.12.113-1 | Nothing to do. trixie never shipped the vulnerable code. |
| 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. |
The vulnerability
What the bug is
dentry_create() can hypothetically provide a different dentry than the one passed in. This could happen, for example, if the exported filesystem is NFS, and the server returned to OPEN a filehandle which matched a directory that was already in the dcache. Clearly this would not be expected! If this were to happen the dentry (child) that was already stored in resfhp could be freed and later dereferenced. We shouldn't call fh_compose() until we are certain that we have the final dentry, so this patch moved the fh_compose() call to two places: one for the case where the target already exists, and one after dentry_create() where it was created.
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/nfsd/nfs4proc.c
- What this is
- KERNEL NFSD, SUNRPC, AND LOCKD SERVERSMAINTAINERS section matched on fs/nfsd/ · linux-nfs@vger.kernel.org · Supported
- Kernel module
- nfsd
Check it on the machine
grep -wE 'CONFIG_NFSD|CONFIG_NFSD_V4' /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 nfsdPrints 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, sidm
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switches
- CONFIG_NFSD, CONFIG_NFSD_V4
- 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
- 9.8 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
- 0.55%, higher than 45.0% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- Subsystem
- fs/nfsd
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 7.0 | 7.2.4 | 033e783709ce |
| 7.0 | 7.3 | 3e2c79360c6a |
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 nfsd4_create_file(), reached from a remote NFSv4 COMPOUND OPEN(CREATE) on in-kernel nfsd (TCP/2049) via nfsd4_proc_compound(), nfsd4_open(), and do_open_lookup(), so a network NFS client can hit it with no local access on the server.
- AC:LAttack complexity, low
- On nfsd re-exports of filesystems with ->atomic_open (NFS, CIFS, FUSE, Ceph, GFS2), a client can populate the dcache with directory lookups and then issue OPEN(CREATE) so dentry_create()/atomic_open()/d_splice_alias() returns a different dentry than fh_compose() stored; no race or layout beyond the attacker's requests is required.
- PR:NPrivileges required, none
- In the highest reasonable deployment a writable NFS export uses AUTH_SYS, AUTH_NULL, or host-based access, so a remote client can spoof a create-capable identity and reach nfsd4_create_file() without a real server account, Kerberos, or init-namespace privilege.
- UI:NUser interaction, none
- Exploitation is solely the attacker's own NFSv4 OPEN(CREATE) compound after nfsd and the export are already running; no victim user must mount, open a file, or otherwise interact.
- S:UScope, unchanged
- The use-after-free corrupts dentry state in the NFS server kernel under the same host security authority and does not cross a VM, IOMMU, or guest-to-host boundary.
- C:HConfidentiality impact, high
- fh_compose() stores the lookup dentry in resfhp, then atomic_open()/d_splice_alias() can drop that object so later nfsd_create_setattr(), nfsd_check_obj_isreg(), and nfsd4_process_open2() dereference a freed dentry, which is a use-after-free read primitive over kernel heap.
- I:HIntegrity impact, high
- A freed struct dentry can be reallocated and sprayed, corrupting d_inode, d_op, and related VFS objects used on the OPEN path, which is a write/control-flow hijack primitive equivalent to other kernel dentry use-after-frees.
- A:HAvailability impact, high
- The stale resfhp->fh_dentry is used in the same request (including d_inode() in nfsd_check_obj_isreg() and fh_getattr()), so the use-after-free oopses or panics nfsd and can take down the host kernel.