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CVE-2026-90203
Squashfs: check block offset is not negative
Published 2026-09-17 (31 days ago) by the Linux kernel CNA. Subsystem fs/squashfs. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-90203 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 | fixedDLA-4841-1 · 2026-10-17 · text | 6.1.190-1 | Update the kernel and reboot. DLA-4841-1 shipped the fix on 2026-10-17, in 6.1.190-1. A running kernel keeps the old code until the machine restarts. |
| 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
If a negative offset is read off disk (for example the offset into the decompressed fragment block), this will cause squashfs_copy_data() to perform an out of bounds access. Fix by checking if offset is negative, and returning 0. This matches existing behaviour where an offset beyond the block returns 0 bytes copied. To trigger this out of bounds access requires a crafted Squashfs filesystem and CAP_SYS_ADMIN to mount it. Unprivileged users will not be able to mount such a filesystem, but once mounted, an unprivileged user can trigger the out of bounds access by reading the crafted file with the negative offset.
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/squashfs/cache.c
- What this is
- SQUASHFS FILE SYSTEMMAINTAINERS section matched on fs/squashfs/ · squashfs-devel@lists.sourceforge.net · Maintained
- Kernel module
- squashfs
Check it on the machine
grep -w CONFIG_SQUASHFS /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 squashfsPrints 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 switch
- CONFIG_SQUASHFS
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
- 7.1 high
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.17%, higher than 6.1% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- fs/squashfs
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 2.6.29 | 5.10.270 | b169185d5c67 |
| 2.6.29 | 5.15.221 | c2a126fca820 |
| 2.6.29 | 6.1.188 | 95dadf366c11 |
| 2.6.29 | 6.6.157 | 3d2f0cb66c90 |
| 2.6.29 | 6.12.110 | bbb2218eb072 |
| 2.6.29 | 6.18.52 | d0a3729d464f |
| 2.6.29 | 7.2.6 | e4afd90bc7bf |
| 2.6.29 | 7.3 | e300eb500292 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4841-1 | 2026-10-17 | bookworm 6.1.190-1 | tracker |
| 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:LAttack vector, local
- The bad bytes are squashfs_reg_inode.offset/squashfs_lreg_inode.offset, stored by squashfs_read_inode() as fragment_offset and used by squashfs_read_folio()/squashfs_readahead_fragment() → squashfs_copy_data() on a loop/block-backed image. That field is on-disk fragment metadata, not a network protocol message; squashfs_fs_type is FS_REQUIRES_DEV.
- AC:LAttack complexity, low
- squashfs_read_inode() assigns le32_to_cpu(sqsh_ino->offset) to signed int fragment_offset with no range check, so a high-bit-set on-disk value is negative. read() of that file's tail (generic_file_read_iter → squashfs_read_folio) then calls squashfs_copy_data() with that offset; there is no race.
- PR:NPrivileges required, none
- squashfs_fs_type has no FS_USERNS_MOUNT, so mount_capable() needs init-namespace CAP_SYS_ADMIN, but the attacker who crafts the image needs no account. After that privileged mount, an unprivileged read of the crafted file is enough; the mount is scored as User Interaction, not Privileges Required.
- UI:RUser interaction, required
- The negative fragment_offset exists only in an attacker-supplied Squashfs image, and squashfs_copy_data() runs only after that image is mounted and the crafted file is read. The mount is the required victim action; a standing snapd or live-image setup does not remove it.
- S:UScope, unchanged
- squashfs_copy_data() indexes the fragment cache's kmalloc'd data[] array and memcpy's into the file's own page-cache folio in the same kernel. This is not a KVM/Xen guest-to-host escape or an IOMMU/DMA bypass.
- C:HConfidentiality impact, high
- A negative offset makes squashfs_copy_data() evaluate entry->data[offset/PAGE_SIZE] before the kcalloc'd pointer array, then memcpy up to PAGE_SIZE bytes from that address into the folio returned by read(). That is an unbounded OOB kernel read into userspace, not a few-byte leak.
- I:NIntegrity impact, none
- The memcpy destination is the kmap of the crafted file's own folio in squashfs_fill_page()/squashfs_readahead_fragment(); the source is the OOB pointer. Length is capped by avail (≤PAGE_SIZE), so this path does not write kernel objects out of bounds.
- A:HAvailability impact, high
- Indexing entry->data[] with a negative offset/PAGE_SIZE and memcpy from the resulting pointer is the same squashfs_copy_data() out-of-bounds access that produced a general protection fault in CVE-2026-23388; an unmapped address oopses the kernel.