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CVE-2026-90104
NFSv4.1: zero referring call lists before decoding
Published 2026-09-17 (31 days ago) by the Linux kernel CNA. Subsystem fs/nfs. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-90104 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 | vulnerable | 6.12.113-1 | No fix published anywhere yet, neither in Debian nor upstream for the series trixie tracks. Watch the Debian tracker page for this CVE. Advisories for trixie have historically landed a median of 11 days after publication. |
| 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
decode_cb_sequence_args() allocates csa_rclists with kmalloc_objs(), so each referring_call_list starts uninitialized. decode_rc_list() assigns rcl_refcalls only when rcl_nrefcalls is nonzero. A valid list with zero referring calls therefore leaves the pointer uninitialized, and nfs4_callback_sequence() later passes stale slab contents to kfree(). Allocate csa_rclists with kzalloc_objs() so every rcl_refcalls member is NULL from the beginning, including valid empty referring call lists.
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/nfs/callback_xdr.c
- What this is
- NFS, SUNRPC, AND LOCKD CLIENTSMAINTAINERS section matched on fs/nfs/ · linux-nfs@vger.kernel.org · Maintained
- Kernel module
- nfsv4
Check it on the machine
grep -wE 'CONFIG_NFS_FS|CONFIG_NFS_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 nfsv4Prints 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_NFS_FS, CONFIG_NFS_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
- 2026-09-17 (31 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
- 0.61%, higher than 48.4% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- fs/nfs
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 2.6.31 | 6.18.57 | f4d23937e1c4 |
| 2.6.31 | 7.2.6 | f31f3c042e02 |
| 2.6.31 | 7.3 | 8fa4804fe62c |
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
- xs_read_stream_call()/rpcrdma_bc_receive_call() deliver a backchannel RPC_CALL on the established NFSv4.1 TCP or RPC/RDMA session into nfs4_callback_compound → process_op → decode_cb_sequence_args(); the attacker-controlled bytes are the CB_SEQUENCE csa_nrclists and rcl_nrefcalls XDR fields sent by the NFS server.
- AC:LAttack complexity, low
- A single well-formed CB_SEQUENCE with csa_nrclists≥1 and decode_rc_list() seeing rcl_nrefcalls==0 skips the rcl_refcalls assignment and still returns 0; nfs4_callback_sequence() then kfree()s that stale pointer. Empty referring_call_list4 is valid RFC 5661 XDR, so no race or victim-only state is required.
- PR:NPrivileges required, none
- nfs_callback_authenticate() accepts RPC_AUTH_UNIX on the NFSv4.1 backchannel, and decode_cb_sequence_args() runs in process_op() before nfs4_find_client_sessionid(); the connected NFS server needs no account or capability on the client to send CB_SEQUENCE.
- UI:NUser interaction, none
- After nfs4_init_callback() has attached the backchannel (boot-time, autofs, or Kubernetes NFSv4.1 mounts), the server unilaterally sends CB_SEQUENCE; nfs4_callback_sequence() kfree()s rcl_refcalls with no further victim open, mount, or click.
- S:UScope, unchanged
- kfree of the stale rcl_refcalls pointer corrupts the NFS client's kmalloc heap in the same host kernel; it does not cross a VM, IOMMU, or guest/host boundary.
- C:HConfidentiality impact, high
- nfs4_callback_sequence() kfree()s uninitialized rcl_refcalls left by kmalloc_objs() of csa_rclists when rcl_nrefcalls was 0. A prior CB_SEQUENCE that allocated then freed rcl_refcalls can leave that pointer in the reused slab object, a heap double-free/UAF read primitive.
- I:HIntegrity impact, high
- The same kfree of stale rcl_refcalls is a slab double-free of the referring_call array (size chosen by the attacker's earlier nrefcalls). Reclaim or spraying a replacement object over that free yields an arbitrary kernel write / control-flow hijack primitive.
- A:HAvailability impact, high
- kfree of a non-NULL garbage rcl_refcalls pointer from a successful decode_cb_sequence_args() of an empty referring_call_list causes SLUB to BUG, oops, or panic even when the double-free is not fully exploited.