NVD / CVE Program· CNA Linux

CVE-2025-21731: nbd: don't allow reconnect after disconnect

Noroxi assessment

Priority: Monitor (action score 31/100)

The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
Probability of exploitation in the next 30 days: 0.24% (FIRST EPSS).
Someone with local access to the system can trigger it. A low-privileged account is enough. No user action is required. No special conditions are required; it is repeatable.
Fixed version: Linux Linux 6bef6222a3f6c7adb6396f77f25a3579d821b09a, 844b8cdc681612ff24df62cdefddeab5772fadf1, 9793bd5ae4bdbdb2dde401a3cab94a6bfd05e302; cbl2 kernel 5.15.176.3-3 on CBL Mariner 2.0 5.15.180.1-1; cbl2 kernel 5.15.176.3-3 on CBL-Mariner 2.0 5.15.180.1-1.

Low priority for now. Upgrade to Linux Linux 6bef6222a3f6c7adb6396f77f25a3579d821b09a, 844b8cdc681612ff24df62cdefddeab5772fadf1, 9793bd5ae4bdbdb2dde401a3cab94a6bfd05e302; cbl2 kernel 5.15.176.3-3 on CBL Mariner 2.0 5.15.180.1-1; cbl2 kernel 5.15.176.3-3 on CBL-Mariner 2.0 5.15.180.1-1 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

In the Linux kernel, the following vulnerability has been resolved: nbd: don't allow reconnect after disconnect Following process can cause nbd_config UAF: 1) grab nbd_config temporarily; 2) nbd_genl_disconnect() flush all recv_work() and release the initial reference: nbd_genl_disconnect nbd_disconnect_and_put nbd_disconnect flush_workqueue(nbd->recv_workq) if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF, ...)) nbd_config_put -> due to step 1), reference is still not zero 3) nbd_genl_reconfigure() queue recv_work() again; nbd_genl_reconfigure config = nbd_get_config_unlocked(nbd) if (!config) -> succeed if (!test_bit(NBD_RT_BOUND, ...)) -> succeed nbd_reconnect_socket queue_work(nbd->recv_workq, &args->work) 4) step 1) release the reference; 5) Finially, recv_work() will trigger UAF: recv_work nbd_config_put(nbd) -> nbd_config is freed atomic_dec(&config->recv_threads) -> UAF Fix the problem by clearing NBD_RT_BOUND in nbd_genl_disconnect(), so that nbd_genl_reconfigure() will fail.

HighCVSS 7.8 · v3.1—No exploit Fix available

Memory safety

BDU:2025-03899

Published: Feb 27, 2025
Updated: Oct 15, 2026
EPSS: 0.2% · 15th percentile
CWE: CWE-416

Report tools

JSON

Action score

Monitor

Low priority for now.

CVSS: 31 / 40 · 7.8 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 0.2%

CISA SSVC decision

Exploitation: none
Automatable: no
Technical impact: total

Vulnrichment: CISA's decision-tree inputs.

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Not in KEV: no record of exploitation in the wild
No public exploit code on record
A fix has been released

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

Vendor	Product	CPE
linux	linux kernel	cpe:2.3:o:linux:linux_kernel

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

linux linux kernel4.12 and later · before 5.4.291
linux linux kernel5.5 and later · before 5.10.235
linux linux kernel5.11 and later · before 5.15.179
linux linux kernel5.16 and later · before 6.1.129
linux linux kernel6.2 and later · before 6.6.76
linux linux kernel6.7 and later · before 6.12.13
linux linux kernel6.13 and later · before 6.13.2

Versions reported by the vendor

Affected version ranges reported by the assigning authority (Linux). Independent of NVD's CPE analysis and usually ahead of it.

Linux Linux
- 4.12affected
- b7aa3d39385dc2d95899f9e379623fef446a2acd and later · before e70a578487a47d7cf058904141e586684d1c3381affected · git
- b7aa3d39385dc2d95899f9e379623fef446a2acd and later · before 6bef6222a3f6c7adb6396f77f25a3579d821b09aaffected · git
- b7aa3d39385dc2d95899f9e379623fef446a2acd and later · before e3be8862d73cac833e0fb7602636c19c6cb94b11affected · git
- b7aa3d39385dc2d95899f9e379623fef446a2acd and later · before e7343fa33751cb07c1c56b666bf37cfca357130eaffected · git
- b7aa3d39385dc2d95899f9e379623fef446a2acd and later · before d208d2c52b652913b5eefc8ca434b0d6b757f68faffected · git
- b7aa3d39385dc2d95899f9e379623fef446a2acd and later · before a8ee6ecde2b7bfb58c8a3afe8a9d2b848f580739affected · git
- b7aa3d39385dc2d95899f9e379623fef446a2acd and later · before 9793bd5ae4bdbdb2dde401a3cab94a6bfd05e302affected · git
- b7aa3d39385dc2d95899f9e379623fef446a2acd and later · before 844b8cdc681612ff24df62cdefddeab5772fadf1affected · git
- before 4.12not affected · semver

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

Ecosystem	Package	Affected range	Fix
Debian:12	linux	before 6.1.129-1	6.1.129-1
Debian:13	linux	before 6.12.13-1	6.12.13-1

Same product

linux: all records

Other highest-scoring records for the same primary product.

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CVE-2021-22555Heap Out-Of-Bounds Write in Netfilter IP6T_SO_SET_REPLACEKEV
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Published before and after this one in the same product

← EarlierCVE-2025-21728Feb 27, 2025 · bpf: Send signals asynchronously if !preemptible
22Monitor
← EarlierCVE-2025-21729Feb 27, 2025 · wifi: rtw89: fix race between cancel_hw_scan and hw_scan completion
31Monitor
Later →CVE-2024-52559Feb 27, 2025 · drm/msm/gem: prevent integer overflow in msm_ioctl_gem_submit()
22Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / package	Fixed version	Source
Linux Linux	6bef6222a3f6c7adb6396f77f25a3579d821b09a	Vendor (CNA)
Linux Linux	844b8cdc681612ff24df62cdefddeab5772fadf1	Vendor (CNA)
Linux Linux	9793bd5ae4bdbdb2dde401a3cab94a6bfd05e302	Vendor (CNA)
Linux Linux	a8ee6ecde2b7bfb58c8a3afe8a9d2b848f580739	Vendor (CNA)
Linux Linux	d208d2c52b652913b5eefc8ca434b0d6b757f68f	Vendor (CNA)
Linux Linux	e3be8862d73cac833e0fb7602636c19c6cb94b11	Vendor (CNA)
Linux Linux	e70a578487a47d7cf058904141e586684d1c3381	Vendor (CNA)
Linux Linux	e7343fa33751cb07c1c56b666bf37cfca357130e	Vendor (CNA)
cbl2 kernel 5.15.176.3-3 on CBL Mariner 2.0	5.15.180.1-1 · CBL-Mariner Releases	Microsoft (MSRC)
cbl2 kernel 5.15.176.3-3 on CBL-Mariner 2.0	5.15.180.1-1 · CBL-Mariner Releases	Microsoft (MSRC)
cbl2 kernel 5.15.180.1-1 on CBL Mariner 2.0	5.15.180.1-1 · CBL-Mariner Releases	Microsoft (MSRC)
debian:linux	6.1.129-1 · Debian:12	Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

Someone with local access to the system can trigger it.
A low-privileged account is enough.
No user action is required.
No special conditions are required; it is repeatable.

If successful

Confidentiality: high · data can be read
Integrity: high · data or configuration can be modified
Availability: high · the service can be disrupted

Attack vector: Local
Attack complexity: Low
Privileges required: Low
User interaction: None
Scope: Unchanged
Confidentiality impact: High
Integrity impact: High
Availability impact: High

Weakness class (CWE)

CWE-416 · Use After Free

CVSS vector

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

nvd-secondary

Root cause

After a memory region is freed, another reference pointing to it is still used. By then, the same region may have been allocated for different data.

The bug in code

The whole CWE-416 class

A representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.

Vulnerable

1free(dev->buf);
2/* ... another thread is still running ... */
3process(dev->buf);

Fixed

1lock(&dev->lock);
2free(dev->buf);
3dev->buf = NULL;
4unlock(&dev->lock);

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

Fix✗ → ✓Sep 28, 2026, 10:10 PM UTC

For records you follow, these changes also arrive as notifications. →

References

git.kernel.org/stable/c/6bef6222a3f6c7adb6396f77f25a3579d821b09a

Show all references (12 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records