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CVE-2026-63817
f2fs: validate compress cache inode only when enabled
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem fs/f2fs. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-63817 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 | fixed | 6.1.177-1 | Update the kernel and reboot. Fixed in 6.1.177-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.95-1 | Update the kernel and reboot. Fixed in 6.12.95-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.3-1 | Update the kernel and reboot. Fixed in 7.1.3-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.3-1 | Update the kernel and reboot. Fixed in 7.1.3-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
F2FS_COMPRESS_INO() uses NM_I(sbi)->max_nid as the synthetic inode number for the compressed page cache inode. That inode only exists when the compress_cache mount option is enabled. When compress_cache is disabled, max_nid is outside the valid inode range. A corrupted directory entry that points to ino == max_nid should therefore be rejected by f2fs_check_nid_range(). However, is_meta_ino() currently treats F2FS_COMPRESS_INO() as a meta inode unconditionally, so f2fs_iget() bypasses do_read_inode() and its nid range check, and instantiates a fake internal inode instead. Gate the compressed cache inode case on COMPRESS_CACHE, matching f2fs_init_compress_inode(). With compress_cache disabled, ino == max_nid now follows the normal inode path and is rejected as an out-of-range nid.
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/f2fs/inode.c
- What this is
- F2FS FILE SYSTEMMAINTAINERS section matched on fs/f2fs/ · linux-f2fs-devel@lists.sourceforge.net · Maintained
- Kernel module
- f2fs
Check it on the machine
grep -w CONFIG_F2FS_FS /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 f2fsPrints 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, but not on every flavour. At least one release below builds the code on some of its kernel flavours and not on others, so the answer depends on the architecture and flavour installed.
- bookworm, trixie, forky, sidMnot built on amd64/cloud-amd64, arm64/cloud-arm64
- M
- a module where it is built at all; some flavours do not build it
- Build switch
- CONFIG_F2FS_FS
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
- 2026-07-19 (3 months ago)source
- CVSS
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.13%, higher than 2.6% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- fs/f2fs
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.14 | 5.15.212 | fcc051d377a9 |
| 5.14 | 6.1.177 | 29115b8c9172 |
| 5.14 | 6.6.144 | 13e4b59d3a94 |
| 5.14 | 6.12.95 | 16161444c30d |
| 5.14 | 6.18.38 | 77f216ff9ce5 |
| 5.14 | 7.1.3 | 0969926d987b |
| 5.14 | 7.2 | 5073c66a96a9 |
| 5.13.19 | 0 | 0 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4700-1 | 2026-07-26 | bullseye 6.1.177-1~deb11u1 | 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:LAttack vector, local
- The bug is reached through local VFS operations (open/lookup, get_parent, mount-time recovery, and background GC) on a mounted F2FS filesystem; there is no direct network packet handler, though NFS export is a secondary path still requiring local filesystem access to plant corruption.
- AC:LAttack complexity, low
- An attacker can deterministically craft an F2FS image with a directory entry pointing at ino=max_nid (known at image creation) and reliably trigger f2fs_iget() on mount or lookup without depending on uncontrollable runtime conditions.
- PR:LPrivileges required, low
- Triggering requires mounting or writing to F2FS, which needs CAP_SYS_ADMIN but is obtainable by an unprivileged user via user namespaces; planting a corrupted dentry also requires only low-privilege write access to the filesystem.
- UI:NUser interaction, none
- Once a crafted or corrupted filesystem is in place, exploitation is driven entirely by attacker-initiated syscalls (mount, lookup, open) or automatic kernel paths (GC, recovery) with no victim interaction required.
- S:UScope, unchanged
- Impact is confined to kernel memory/filesystem integrity on the host running the affected F2FS code; it does not cross a VM, container, or IOMMU security boundary in the manner of a guest-to-host escape.
- C:HConfidentiality impact, high
- Bypassing inode validation instantiates a mis-typed, uninitialized inode that subsequent f2fs data and node paths operate on, creating a type-confusion condition that can be leveraged for arbitrary kernel memory read primitives.
- I:HIntegrity impact, high
- The fake compress-cache inode is wired with compress address_space operations and skips normal inode teardown, enabling type confusion that can be abused for arbitrary kernel memory writes or control-flow hijacking.
- A:HAvailability impact, high
- Operating on an invalid synthetic inode in GC, recovery, or lookup paths can trigger kernel oops, EFSCORRUPTED error handling, or filesystem panic (errors=panic), causing denial of service or system crash.