CVE-2026-98959 in Linux
Summary
by MITRE • 10/23/2026
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---
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Analysis
by VulDB Data Team • 10/23/2026
The vulnerability in the Linux kernel's Btrfs filesystem driver involves a critical data corruption issue arising from improper handling of extended attributes when multiple items are packed into a single directory item. This flaw occurs specifically during the replacement of an existing extended attribute value with one that differs significantly in size, leading to incomplete initialization of the resulting btrfs_dir_item structure and subsequent detection by the tree checker during extent buffer writeback operations. The root cause lies within the btrfs_setxattr function logic when it encounters a collision where two distinct extended attributes share the same crc32c hash for their names, causing them to be stored together in a single btrfs_dir_item container.
When such a packed item is encountered and one of its constituent xattrs needs to have its value replaced with data of a different length, the kernel performs a deletion of the original specific xattr entry followed by an insertion of the new one. This sequence triggers internal leaf shifting mechanisms within the B-tree structure that rearrange existing data blocks to accommodate the size change. However, during this reallocation and shift process, the code fails to properly initialize several critical fields in the newly placed btrfs_dir_item structure. Specifically, while the value's data is copied and its length field is updated correctly, essential metadata components remain uninitialized or incorrectly set. These missing elements include the attribute name string itself, the length of that name, the key which must be zeroed out for extended attributes to distinguish them from other file types, the flags indicating it is an xattr type (BTRFS_FT_XATTR), and the transaction ID required for consistency tracking.
The operational impact of this defect manifests as silent data corruption until a subsequent read operation or filesystem unmount triggers validation checks. As demonstrated in reproduction scenarios, after replacing a small value with a larger one under these specific collision conditions, the affected extended attribute becomes inaccessible during listing operations because its metadata is malformed. More severely, when the filesystem attempts to write back modified extent buffers to disk, the tree checker detects that parts of the buffer are out of range or improperly structured due to the uninitialized memory regions being treated as valid directory entries. This results in kernel warnings regarding access violations and can lead to transaction aborts during unmounting, potentially causing data loss or requiring filesystem repair tools like btrfs check to resolve inconsistencies before the system remains stable.
From a security perspective, this vulnerability aligns with CWE-908, which describes use of uninitialized resource, as well as CWE-787, out-of-bounds write access if the shifting logic inadvertently overwrites adjacent memory structures due to incorrect boundary calculations during leaf manipulation. In terms of ATT&CK mapping, while not directly exploitable for remote code execution without further chaining with other flaws, this represents a potential Denial of Service vector through resource exhaustion or system instability caused by repeated corruption events, falling under T1490: Inhibit System Recovery if the resulting filesystem errors prevent normal booting or data access. The lack of proper initialization also touches upon CWE-672, use of inherently unsafe function, in the context of memory management within kernel space where unchecked assumptions about buffer states lead to structural integrity failures.
Mitigation strategies primarily involve applying the upstream Linux kernel patch that corrects the btrfs_setxattr logic to ensure all fields are explicitly initialized before inserting the new directory item into the leaf after shifting operations complete. Administrators should verify their systems are running patched versions of the Btrfs driver where this specific race condition and initialization gap have been addressed. For environments unable to immediately update kernels, avoiding scenarios that trigger multiple extended attribute collisions with significant size variations can reduce exposure risk until patches are deployed. Regular filesystem checks using btrfs check --repair may be necessary for volumes already affected by this corruption to restore consistency before further writes occur.