Linux Kernel up to 6.17-rc1 /fs/btrfs/extent-tree.c num_bytes privilege escalation

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6.1	$0-$5k	0.00

Summaryinfo

A vulnerability classified as problematic has been found in Linux Kernel up to 6.17-rc1. This impacts the function num_bytes of the file /fs/btrfs/extent-tree.c. The manipulation leads to an unknown weakness. This vulnerability is documented as CVE-2025-39738. There is not any exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in Linux Kernel up to 6.17-rc1. This issue affects the function num_bytes of the file /fs/btrfs/extent-tree.c. The impact remains unknown. The summary by CVE is:

In the Linux kernel, the following vulnerability has been resolved: btrfs: do not allow relocation of partially dropped subvolumes [BUG] There is an internal report that balance triggered transaction abort, with the following call trace: item 85 key (594509824 169 0) itemoff 12599 itemsize 33 extent refs 1 gen 197740 flags 2 ref#0: tree block backref root 7 item 86 key (594558976 169 0) itemoff 12566 itemsize 33 extent refs 1 gen 197522 flags 2 ref#0: tree block backref root 7 ... BTRFS error (device loop0): extent item not found for insert, bytenr 594526208 num_bytes 16384 parent 449921024 root_objectid 934 owner 1 offset 0 BTRFS error (device loop0): failed to run delayed ref for logical 594526208 num_bytes 16384 type 182 action 1 ref_mod 1: -117 ------------[ cut here ]------------ BTRFS: Transaction aborted (error -117) WARNING: CPU: 1 PID: 6963 at ../fs/btrfs/extent-tree.c:2168 btrfs_run_delayed_refs+0xfa/0x110 [btrfs] And btrfs check doesn't report anything wrong related to the extent tree. [CAUSE] The cause is a little complex, firstly the extent tree indeed doesn't have the backref for 594526208. The extent tree only have the following two backrefs around that bytenr on-disk: item 65 key (594509824 METADATA_ITEM 0) itemoff 13880 itemsize 33 refs 1 gen 197740 flags TREE_BLOCK tree block skinny level 0 (176 0x7) tree block backref root CSUM_TREE item 66 key (594558976 METADATA_ITEM 0) itemoff 13847 itemsize 33 refs 1 gen 197522 flags TREE_BLOCK tree block skinny level 0 (176 0x7) tree block backref root CSUM_TREE But the such missing backref item is not an corruption on disk, as the offending delayed ref belongs to subvolume 934, and that subvolume is being dropped: item 0 key (934 ROOT_ITEM 198229) itemoff 15844 itemsize 439 generation 198229 root_dirid 256 bytenr 10741039104 byte_limit 0 bytes_used 345571328 last_snapshot 198229 flags 0x1000000000001(RDONLY) refs 0 drop_progress key (206324 EXTENT_DATA 2711650304) drop_level 2 level 2 generation_v2 198229 And that offending tree block 594526208 is inside the dropped range of that subvolume. That explains why there is no backref item for that bytenr and why btrfs check is not reporting anything wrong. But this also shows another problem, as btrfs will do all the orphan subvolume cleanup at a read-write mount. So half-dropped subvolume should not exist after an RW mount, and balance itself is also exclusive to subvolume cleanup, meaning we shouldn't hit a subvolume half-dropped during relocation. The root cause is, there is no orphan item for this subvolume. In fact there are 5 subvolumes from around 2021 that have the same problem. It looks like the original report has some older kernels running, and caused those zombie subvolumes. Thankfully upstream commit 8d488a8c7ba2 ("btrfs: fix subvolume/snapshot deletion not triggered on mount") has long fixed the bug. [ENHANCEMENT] For repairing such old fs, btrfs-progs will be enhanced. Considering how delayed the problem will show up (at run delayed ref time) and at that time we have to abort transaction already, it is too late. Instead here we reject any half-dropped subvolume for reloc tree at the earliest time, preventing confusion and extra time wasted on debugging similar bugs.

It is possible to read the advisory at git.kernel.org. The identification of this vulnerability is CVE-2025-39738 since 04/16/2025. The exploitation is known to be difficult. Technical details of the vulnerability are known, but there is no available exploit.

The vulnerability scanner Nessus provides a plugin with the ID 264733 (Linux Distros Unpatched Vulnerability : CVE-2025-39738), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 5.15.190, 6.1.149, 6.6.103, 6.12.43, 6.15.11, 6.16.2 or 6.17-rc2 eliminates this vulnerability. Applying a patch is able to eliminate this problem. The bugfix is ready for download at git.kernel.org. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the databases at Tenable (264733), EUVD (EUVD-2025-28973) and CERT Bund (WID-SEC-2025-2040). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Affected

• Google Container-Optimized OS
• Debian Linux
• Google Cloud Platform
• Amazon Linux 2
• Red Hat Enterprise Linux
• Fedora Linux
• Ubuntu Linux
• SUSE Linux
• Oracle Linux
• SUSE openSUSE
• RESF Rocky Linux
• Open Source Linux Kernel
• Proxmox Virtual Environment
• Proxmox Backup Server
• Dell Secure Connect Gateway
• IBM QRadar SIEM

Productinfo

Type

• Operating System

Vendor

• Linux

Name

• Kernel

Version

• 5.15.189
• 6.1.148
• 6.6.102
• 6.12.0
• 6.12.1
• 6.12.2
• 6.12.3
• 6.12.4
• 6.12.5
• 6.12.6
• 6.12.7
• 6.12.8
• 6.12.9
• 6.12.10
• 6.12.11
• 6.12.12
• 6.12.13
• 6.12.14
• 6.12.15
• 6.12.16
• 6.12.17
• 6.12.18
• 6.12.19
• 6.12.20
• 6.12.21
• 6.12.22
• 6.12.23
• 6.12.24
• 6.12.25
• 6.12.26
• 6.12.27
• 6.12.28
• 6.12.29
• 6.12.30
• 6.12.31
• 6.12.32
• 6.12.33
• 6.12.34
• 6.12.35
• 6.12.36
• 6.12.37
• 6.12.38
• 6.12.39
• 6.12.40
• 6.12.41
• 6.12.42
• 6.15.0
• 6.15.1
• 6.15.2
• 6.15.3
• 6.15.4
• 6.15.5
• 6.15.6
• 6.15.7
• 6.15.8
• 6.15.9
• 6.15.10
• 6.16.0
• 6.16.1
• 6.17-rc1

License

• open-source

Website

• Vendor: https://www.kernel.org/

CPE 2.3info

• 🔒
• 🔒
• 🔒

CPE 2.2info

• 🔒
• 🔒
• 🔒

CVSSv4info

VulDB Vector: 🔒
VulDB Reliability: 🔍

CVSSv3info

VulDB Meta Base Score: 6.2
VulDB Meta Temp Score: 6.1

VulDB Base Score: 4.6
VulDB Temp Score: 4.4
VulDB Vector: 🔒
VulDB Reliability: 🔍

NVD Base Score: 7.8
NVD Vector: 🔒

CVSSv2info

AV	AC	Au	C	I	A
💳	💳	💳	💳	💳	💳
💳	💳	💳	💳	💳	💳
💳	💳	💳	💳	💳	💳

Vector	Complexity	Authentication	Confidentiality	Integrity	Availability
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VulDB Base Score: 🔒
VulDB Temp Score: 🔒
VulDB Reliability: 🔍

Exploitinginfo

Class: Privilege escalation
CWE: Unknown
CAPEC: 🔒
ATT&CK: 🔒

Physical: Partially
Local: Yes
Remote: Partially

Availability: 🔒
Status: Not defined

EPSS Score: 🔒
EPSS Percentile: 🔒

Price Prediction: 🔍
Current Price Estimation: 🔒

0-Day	Unlock	Unlock	Unlock	Unlock
Today	Unlock	Unlock	Unlock	Unlock

Nessus ID: 264733
Nessus Name: Linux Distros Unpatched Vulnerability : CVE-2025-39738

Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: Upgrade
Status: 🔍

0-Day Time: 🔒

Upgrade: Kernel 5.15.190/6.1.149/6.6.103/6.12.43/6.15.11/6.16.2/6.17-rc2
Patch: git.kernel.org

Timelineinfo

04/16/2025 CVE reserved
09/11/2025 +148 days Advisory disclosed
09/11/2025 +0 days VulDB entry created
01/10/2026 +121 days VulDB entry last update

Sourcesinfo

Vendor: kernel.org

Advisory: git.kernel.org
Status: Confirmed

CVE: CVE-2025-39738 (🔒)
GCVE (CVE): GCVE-0-2025-39738
GCVE (VulDB): GCVE-100-323651
EUVD: 🔒
CERT Bund: WID-SEC-2025-2040 - Linux Kernel: Mehrere Schwachstellen ermöglichen Denial of Service

Entryinfo

Created: 09/11/2025 19:30
Updated: 01/10/2026 07:35
Changes: 09/11/2025 19:30 (56), 09/12/2025 11:41 (7), 09/14/2025 22:06 (2), 09/15/2025 11:01 (1), 11/07/2025 13:06 (1), 11/15/2025 20:25 (1), 01/09/2026 21:03 (11), 01/10/2026 07:35 (1)
Complete: 🔍
Cache ID: 216::103

Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Discussion

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