# CVE-2025-21862: drop_monitor: fix incorrect initialization order

> A race condition in the Linux kernel drop_monitor module causes a spinlock error, leading to a system crash or denial of service.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 12 Mar 2025
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-58502, AZL-58578, BELL-CVE-2025-21862, DEBIAN-CVE-2025-21862, GHSA-fm6j-mfpf-prfq, ROOT-OS-UBUNTU-2404-CVE-2025-21862, UBUNTU-CVE-2025-21862

## What is CVE-2025-21862?

CVE-2025-21862 is a medium-severity vulnerability in Linux kernel, published on 12 Mar 2025. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

drop_monitor: fix incorrect initialization order

Syzkaller reports the following bug:

BUG: spinlock bad magic on CPU#1, syz-executor.0/7995
 lock: 0xffff88805303f3e0, .magic: 00000000, .owner: <none>/-1, .owner_cpu: 0
CPU: 1 PID: 7995 Comm: syz-executor.0 Tainted: G            E     5.10.209+ #1
Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 11/12/2020
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x119/0x179 lib/dump_stack.c:118
 debug_spin_lock_before kernel/locking/spinlock_debug.c:83 [inline]
 do_raw_spin_lock+0x1f6/0x270 kernel/locking/spinlock_debug.c:112
 __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:117 [inline]
 _raw_spin_lock_irqsave+0x50/0x70 kernel/locking/spinlock.c:159
 reset_per_cpu_data+0xe6/0x240 [drop_monitor]
 net_dm_cmd_trace+0x43d/0x17a0 [drop_monitor]
 genl_family_rcv_msg_doit+0x22f/0x330 net/netlink/genetlink.c:739
 genl_family_rcv_msg net/netlink/genetlink.c:783 [inline]
 genl_rcv_msg+0x341/0x5a0 net/netlink/genetlink.c:800
 netlink_rcv_skb+0x14d/0x440 net/netlink/af_netlink.c:2497
 genl_rcv+0x29/0x40 net/netlink/genetlink.c:811
 netlink_unicast_kernel net/netlink/af_netlink.c:1322 [inline]
 netlink_unicast+0x54b/0x800 net/netlink/af_netlink.c:1348
 netlink_sendmsg+0x914/0xe00 net/netlink/af_netlink.c:1916
 sock_sendmsg_nosec net/socket.c:651 [inline]
 __sock_sendmsg+0x157/0x190 net/socket.c:663
 ____sys_sendmsg+0x712/0x870 net/socket.c:2378
 ___sys_sendmsg+0xf8/0x170 net/socket.c:2432
 __sys_sendmsg+0xea/0x1b0 net/socket.c:2461
 do_syscall_64+0x30/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x62/0xc7
RIP: 0033:0x7f3f9815aee9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f3f972bf0c8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f3f9826d050 RCX: 00007f3f9815aee9
RDX: 0000000020000000 RSI: 0000000020001300 RDI: 0000000000000007
RBP: 00007f3f981b63bd R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 000000000000006e R14: 00007f3f9826d050 R15: 00007ffe01ee6768

If drop_monitor is built as a kernel module, syzkaller may have time
to send a netlink NET_DM_CMD_START message during the module loading.
This will call the net_dm_monitor_start() function that uses
a spinlock that has not yet been initialized.

To fix this, let's place resource initialization above the registration
of a generic netlink family.

Found by InfoTeCS on behalf of Linux Verification Center
(linuxtesting.org) with Syzkaller.

Weakness: CWE-908 (Use of Uninitialized Resource)

## Which versions are affected by CVE-2025-21862?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21862?

Apply the vendor's patch.

## Is CVE-2025-21862 being exploited?

There is no known exploitation of CVE-2025-21862 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-21862 and fix it if it is affected.

What it is: CVE-2025-21862 is a medium-severity vulnerability in Linux kernel, published on 12 Mar 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-21862 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-21862/ (Markdown: https://insights.sbomify.com/CVE-2025-21862/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21862
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21862
- OSV: https://osv.dev/vulnerability/CVE-2025-21862

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21862/
Summaries are generated and may contain mistakes; the official description above is the record.