Look up another CVE · open CVE-2026-63808 in the interactive tracker
CVE-2026-63808
exfat: fix potential use-after-free in exfat_find_dir_entry()
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem fs/exfat. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-63808 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
In exfat_find_dir_entry(), the buffer_head obtained from exfat_get_dentry() is released with brelse(bh) before the fall-through TYPE_EXTEND branch reads the directory entry through ep (which points into bh->b_data): brelse(bh); if (entry_type == TYPE_EXTEND) { ... len = exfat_extract_uni_name(ep, entry_uniname); ... } After brelse() drops our reference, nothing guarantees that the underlying page backing bh->b_data remains valid for the subsequent exfat_extract_uni_name() read. This is the same pattern fixed in commit fc961522ddbd ("exfat: Fix potential use after free in exfat_load_upcase_table()"). Move brelse(bh) so it runs after ep is no longer dereferenced on each branch. Confirmed on QEMU x86_64 with CONFIG_KASAN=y + CONFIG_DEBUG_PAGEALLOC=y + CONFIG_PAGE_POISONING=y on linux-next, using a crafted exFAT image (long filename with same-hash collisions forcing the TYPE_EXTEND path). With a debug-only invalidate_bdev() inserted between brelse(bh) and the ep read to make the stale-deref window deterministic, the unpatched kernel faults: BUG: KASAN: use-after-free in exfat_find_dir_entry+0x133b/0x15a0 BUG: unable to handle page fault for address: ffff88801a5fa0c2 Oops: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI RIP: 0010:exfat_find_dir_entry+0x1188/0x15a0 With this patch applied, the same instrumented harness completes cleanly under the same sanitizer stack. I have not reproduced a crash on an uninstrumented kernel under ordinary reclaim; the instrumented A/B establishes the lifetime violation and that the patch closes it, not an unaided triggerability claim.
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/exfat/dir.c
- What this is
- EXFAT FILE SYSTEMMAINTAINERS section matched on fs/exfat/ · linux-fsdevel@vger.kernel.org · Maintained
- Kernel module
- exfat
Check it on the machine
grep -w CONFIG_EXFAT_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 exfatPrints 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 switch
- CONFIG_EXFAT_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
- 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
- 0.53%, higher than 43.4% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- fs/exfat
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.7 | 5.10.260 | e6f1a11cfb80 |
| 5.7 | 5.15.211 | e48f413c2815 |
| 5.7 | 6.1.177 | 06c4e1e9967d |
| 5.7 | 6.6.144 | 8e0abc17fbd7 |
| 5.7 | 6.12.95 | 4d101016d5e5 |
| 5.7 | 6.18.38 | adfacfbaeae2 |
| 5.7 | 7.1.3 | 708b97e79294 |
| 5.7 | 7.2 | 3f5f8ee9917c |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4717-1 | 2026-08-05 | bullseye 5.10.262-1 | tracker |
| 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:NAttack vector, network
- exfat_find_dir_entry() is invoked during VFS directory lookups on mounted exFAT volumes, reachable remotely when that volume is exported via ksmbd or nfsd (common on NAS/router USB shares). A network client resolving paths on the share triggers server-side exfat_lookup without local shell access.
- AC:LAttack complexity, low
- The attacker fully controls the crafted exFAT image to force the TYPE_EXTEND code path and can repeatedly trigger lookups (including concurrent SMB/NFS requests and memory pressure) to win the post-brelse buffer reclaim window. UAF lifetime violations are treated as low complexity per kernel guidance.
- PR:NPrivileges required, none
- Exploitation requires only access to the mounted exFAT filesystem, not real root; guest/anonymous SMB shares on consumer NAS devices and auto-mounted removable media grant unprivileged attackers filesystem access without elevated kernel credentials.
- UI:NUser interaction, none
- Once a crafted exFAT image is present on a shared or auto-mounted volume, triggering the bug requires only automated path lookups (SMB OPEN/LOOKUP, openat, statx) with no additional victim interaction beyond normal filesystem access.
- S:UScope, unchanged
- Successful exploitation compromises kernel memory within the same security authority; this is a standard kernel memory corruption issue, not a cross-boundary escape such as VM guest-to-host breakout.
- C:HConfidentiality impact, high
- The UAF reads freed buffer_head page cache memory through exfat_extract_uni_name(), enabling out-of-bounds kernel heap reads and information disclosure from attacker-influenced reclaimed data.
- I:HIntegrity impact, high
- Use-after-free on buffer_head data provides a foundation for heap grooming and arbitrary memory corruption primitives that can be leveraged for kernel code execution and integrity compromise.
- A:HAvailability impact, high
- The bug is a confirmed kernel use-after-free that produced a KASAN fault and page fault oops; even without full exploitation it can cause kernel crashes and denial of service.