# CVE-2026-43029: mptcp: fix soft lockup in mptcp_recvmsg()

> A local attacker can cause a soft lockup and system hang by triggering an infinite loop in the MPTCP receive path using specific socket flags.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 1 May 2026
- **Updated:** 5 Aug 2026
- **Also known as:** AZL-87141, BELL-CVE-2026-43029, DEBIAN-CVE-2026-43029, GHSA-fc34-92xq-43pm, ROOT-OS-DEBIAN-13-CVE-2026-43029, ROOT-OS-UBUNTU-2404-CVE-2026-43029, UBUNTU-CVE-2026-43029

## What is CVE-2026-43029?

CVE-2026-43029 is a high-severity denial of service vulnerability in Linux kernel, published on 1 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

mptcp: fix soft lockup in mptcp_recvmsg()

syzbot reported a soft lockup in mptcp_recvmsg() [0].

When receiving data with MSG_PEEK | MSG_WAITALL flags, the skb is not
removed from the sk_receive_queue. This causes sk_wait_data() to always
find available data and never perform actual waiting, leading to a soft
lockup.

Fix this by adding a 'last' parameter to track the last peeked skb.
This allows sk_wait_data() to make informed waiting decisions and prevent
infinite loops when MSG_PEEK is used.

[0]:
watchdog: BUG: soft lockup - CPU#2 stuck for 156s! [server:1963]
Modules linked in:
CPU: 2 UID: 0 PID: 1963 Comm: server Not tainted 6.19.0-rc8 #61 PREEMPT(none)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
RIP: 0010:sk_wait_data+0x15/0x190
Code: 80 00 00 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 41 56 41 55 41 54 49 89 f4 55 48 89 d5 53 48 89 fb <48> 83 ec 30 65 48 8b 05 17 a4 6b 01 48 89 44 24 28 31 c0 65 48 8b
RSP: 0018:ffffc90000603ca0 EFLAGS: 00000246
RAX: 0000000000000000 RBX: ffff888102bf0800 RCX: 0000000000000001
RDX: 0000000000000000 RSI: ffffc90000603d18 RDI: ffff888102bf0800
RBP: 0000000000000000 R08: 0000000000000002 R09: 0000000000000101
R10: 0000000000000000 R11: 0000000000000075 R12: ffffc90000603d18
R13: ffff888102bf0800 R14: ffff888102bf0800 R15: 0000000000000000
FS:  00007f6e38b8c4c0(0000) GS:ffff8881b877e000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055aa7bff1680 CR3: 0000000105cbe000 CR4: 00000000000006f0
Call Trace:
 <TASK>
 mptcp_recvmsg+0x547/0x8c0 net/mptcp/protocol.c:2329
 inet_recvmsg+0x11f/0x130 net/ipv4/af_inet.c:891
 sock_recvmsg+0x94/0xc0 net/socket.c:1100
 __sys_recvfrom+0xb2/0x130 net/socket.c:2256
 __x64_sys_recvfrom+0x1f/0x30 net/socket.c:2267
 do_syscall_64+0x59/0x2d0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x76/0x7e arch/x86/entry/entry_64.S:131
RIP: 0033:0x7f6e386a4a1d
Code: 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 8d 05 f1 de 2c 00 41 89 ca 8b 00 85 c0 75 20 45 31 c9 45 31 c0 b8 2d 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 6b f3 c3 66 0f 1f 84 00 00 00 00 00 41 56 41
RSP: 002b:00007ffc3c4bb078 EFLAGS: 00000246 ORIG_RAX: 000000000000002d
RAX: ffffffffffffffda RBX: 000000000000861e RCX: 00007f6e386a4a1d
RDX: 00000000000003ff RSI: 00007ffc3c4bb150 RDI: 0000000000000004
RBP: 00007ffc3c4bb570 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000103 R11: 0000000000000246 R12: 00005605dbc00be0
R13: 00007ffc3c4bb650 R14: 0000000000000000 R15: 0000000000000000
 </TASK>

Weakness: CWE-667 (Improper Locking)

## Which versions are affected by CVE-2026-43029?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-43029?

Apply the vendor's patch.

## Is CVE-2026-43029 being exploited?

There is no known exploitation of CVE-2026-43029 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-2026-43029 and fix it if it is affected.

What it is: CVE-2026-43029 is a high-severity denial of service vulnerability in Linux kernel, published on 1 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-43029 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-2026-43029/ (Markdown: https://insights.sbomify.com/CVE-2026-43029/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-43029
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-43029
- OSV: https://osv.dev/vulnerability/CVE-2026-43029

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-43029/
Summaries are generated and may contain mistakes; the official description above is the record.