Cloudanix VulnDB

CVE-2026-98959

Critical · 9.8

CVSS

9.8

EPSS

0.6%

Published

Oct 23, 2026

CISA due date

—

Cloudanix analysis

Next patch cycle SSVC: Track

A bug in the Linux kernel's btrfs filesystem can corrupt internal file metadata when certain extended attributes are changed, potentially causing crashes or data access problems on affected machines.

Exploitation
Local Privilege
Attacker needs
Local User
Complexity
Moderate
Impact
Data Tampering
Component
OS kernel
Class
Memory Corruption
Remediation
Upgrade
Effort
Medium
Patch risk
Low

What it actually does

Uninitialized btrfs_dir_item structure created during xattr value replacement when multiple xattrs share a crc32c hash and are packed into the same dir_item, leaving name, name length, key, flags, and transaction ID fields unset and triggering tree-checker corruption on extent buffer writeback.

Why it matters to the business

Filesystem metadata corruption on btrfs volumes can cause mount failures, transaction aborts, and potential loss of extended attributes or data integrity for any systems using the affected kernel versions.

How an attack unfolds

An attacker with local access creates a file on a btrfs filesystem, sets two xattrs whose names collide on crc32c hash so they pack into one btrfs_dir_item, then replaces one xattr value with a different-sized value; this triggers deletion and re-insertion that leaves the new dir_item partially uninitialized, detected as corruption on writeback.

How to fix it

Upgrade to a kernel containing the referenced btrfs xattr fix; rebuild and reboot systems using btrfs; verify the patch is present by checking the commit that initializes all btrfs_dir_item fields before insertion.

How to confirm the fix worked

Run the provided reproduction script after patching and confirm no transaction abort or tree-checker warnings appear in dmesg; inspect extent buffers with btrfs inspect-internal to verify all dir_item fields are populated.

Typical attack path

  1. Obtain local shell or process on target system
  2. Identify or create btrfs filesystem mount point
  3. Create a file and apply two xattrs with colliding crc32c names
  4. Replace one xattr value with a significantly different size
  5. Trigger metadata writeback or unmount to surface corruption

How to detect it

Search kernel logs for BTRFS warning messages containing report_eb_range, access to eb bytenr out of range, or transaction abort during unmount; monitor for setxattr syscalls followed immediately by listxattr failures on the same file.

  • BTRFS warning (device
  • report_eb_range
  • access to eb bytenr
  • out of range start
  • transaction abort

What your security team should do

  • Prioritize kernel updates on all btrfs-using systems within the next maintenance window
  • Add btrfs transaction-abort and tree-checker warning signatures to SIEM alerting
  • Audit systems for custom xattr usage that may increase exposure to hash collisions

The record

In the Linux kernel, the following vulnerability has been resolved: btrfs: fix xattr replace when multiple xattrs are packed in the same item If we have a btrfs_dir_item item that packs multiple xattrs and then we replace the value of one of them (with the setxattr(2) family of syscalls) with another value of a different size, we end up not having a fully initialized btrfs_dir_item, resulting in a corruption that the tree checker will detect at extent buffer writeback time. This is because in btrfs_setxattr() when we find a btrfs_dir_item with multiple xattrs (due to the crc32c hash of their name being the same) we delete one of the xattr items (btrfs_dir_item) and then insert a new one, but the deletion and insertion results in shifting existing data in the leaf and therefore when the new value of a xattr has a different size, the new btrfs_dir_item is placed in a leaf section that was not initialized and we only copy the value's data and set the value's length in the new btrfs_dir_item, without setting the name, the name's length, the key (which must be all zeroes for xattrs), flags (BTRFS_FT_XATTR) and transaction ID. The following script reproduces the issue: $ cat test.sh #!/bin/bash DEV=/dev/sdi MNT=/mnt/sdi mkfs.btrfs -f $DEV mount $DEV $MNT touch $MNT/testfile # Add two xattrs that, on btrfs, have the same hash (crc32c) for their # name and therefore are packed into the same btrfs_dir_item. setfattr -n user.foobar -v 123 $MNT/testfile setfattr -n user.WvG1c1Td -v qwerty $MNT/testfile # Verify the xattrs are present. echo "xattrs before:" getfattr --absolute-names --dump $MNT/testfile # Now replace the value of the foobar xattr with a significantly larger # value. setfattr -n user.foobar -v abcdefghijklmnopqrstuvwxyz $MNT/testfile # Check the xattrs have the expected values. echo "xattrs after:" getfattr --absolute-names --dump $MNT/testfile umount $MNT Running it: $ ./test.sh (...) xattrs before: # file: /mnt/sdi/testfile user.WvG1c1Td="qwerty" user.foobar="123" xattrs after: # file: /mnt/sdi/testfile user.WvG1c1Td="qwerty" So the "user.foobar" xattr is missing and there was a transaction abort when unmounting the fs with the following traces in dmesg: $ dmesg [869800.159271] BTRFS warning (device sdi): access to eb bytenr 30474240 len 16384 out of range start 16015 len 25964 [869800.159293] ------------[ cut here ]------------ [869800.159296] WARNING: fs/btrfs/extent_io.c:4408 at report_eb_range+0x44/0x60 [btrfs], CPU#8: getfattr/2605179 [869800.168961] Modules linked in: btrfs dm_thin_pool (...) [869800.190527] CPU: 8 UID: 0 PID: 2605179 Comm: getfattr Tainted: G W 7.3.0-rc3-btrfs-next-244+ #1 PREEMPT(full) [869800.193726] Tainted: [W]=WARN [869800.194502] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014 [869800.197576] RIP: 0010:report_eb_range+0x44/0x60 [btrfs] [869800.198985] Code: 48 8b 7b 18 (...) [869800.203531] RSP: 0018:ffffce4541937d58 EFLAGS: 00010246 [869800.204609] RAX: 0000000000000000 RBX: ffff8dde054b8738 RCX: 0000000000000000 [869800.206137] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffffffffc04c92a0 [869800.207645] RBP: 0000000000003e8f R08: 0000000000000000 R09: 3fffffffffefffff [869800.226175] R10: ffffce4541937a88 R11: 0000000000000003 R12: 000000000000656c [869800.227594] R13: ffff8dde14be800f R14: 000000000000656c R15: 0000000000000069 [869800.229097] FS: 00007f59023bb780(0000) GS:ffff8de5788ed000(0000) knlGS:0000000000000000 [869800.231137] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [869800.232321] CR2: 0000558dc45e6a78 CR3: 0000000765b16004 CR4: 0000000000370ef0 [869800.233783] Call Trace: [869800.234330] <TASK> [869800.234791] read_extent_buffer+0x4d/0x100 [btrfs] [869800.235898] btrfs_listxattr+0x199/0x240 [btrfs] [869800.236912] vfs_listxattr+0x51/0 ---truncated---

Affected packages

Ecosystem Package Introduced Fixed