CVE-2026-43080 - Vulnerability Details

- l2tp: Drop large packets with UDP encap

Description

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

l2tp: Drop large packets with UDP encap

syzbot reported a WARN on my patch series [1]. The actual issue is an
overflow of 16-bit UDP length field, and it exists in the upstream code.
My series added a debug WARN with an overflow check that exposed the
issue, that's why syzbot tripped on my patches, rather than on upstream
code.

syzbot's repro:

r0 = socket$pppl2tp(0x18, 0x1, 0x1)
r1 = socket$inet6_udp(0xa, 0x2, 0x0)
connect$inet6(r1, &(0x7f00000000c0)={0xa, 0x0, 0x0, @loopback, 0xfffffffc}, 0x1c)
connect$pppl2tp(r0, &(0x7f0000000240)=@pppol2tpin6={0x18, 0x1, {0x0, r1, 0x4, 0x0, 0x0, 0x0, {0xa, 0x4e22, 0xffff, @ipv4={'\x00', '\xff\xff', @empty}}}}, 0x32)
writev(r0, &(0x7f0000000080)=[{&(0x7f0000000000)="ee", 0x34000}], 0x1)

It basically sends an oversized (0x34000 bytes) PPPoL2TP packet with UDP
encapsulation, and l2tp_xmit_core doesn't check for overflows when it
assigns the UDP length field. The value gets trimmed to 16 bites.

Add an overflow check that drops oversized packets and avoids sending
packets with trimmed UDP length to the wire.

syzbot's stack trace (with my patch applied):

len >= 65536u
WARNING: ./include/linux/udp.h:38 at udp_set_len_short include/linux/udp.h:38 [inline], CPU#1: syz.0.17/5957
WARNING: ./include/linux/udp.h:38 at l2tp_xmit_core net/l2tp/l2tp_core.c:1293 [inline], CPU#1: syz.0.17/5957
WARNING: ./include/linux/udp.h:38 at l2tp_xmit_skb+0x1204/0x18d0 net/l2tp/l2tp_core.c:1327, CPU#1: syz.0.17/5957
Modules linked in:
CPU: 1 UID: 0 PID: 5957 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
RIP: 0010:udp_set_len_short include/linux/udp.h:38 [inline]
RIP: 0010:l2tp_xmit_core net/l2tp/l2tp_core.c:1293 [inline]
RIP: 0010:l2tp_xmit_skb+0x1204/0x18d0 net/l2tp/l2tp_core.c:1327
Code: 0f 0b 90 e9 21 f9 ff ff e8 e9 05 ec f6 90 0f 0b 90 e9 8d f9 ff ff e8 db 05 ec f6 90 0f 0b 90 e9 cc f9 ff ff e8 cd 05 ec f6 90 <0f> 0b 90 e9 de fa ff ff 44 89 f1 80 e1 07 80 c1 03 38 c1 0f 8c 4f
RSP: 0018:ffffc90003d67878 EFLAGS: 00010293
RAX: ffffffff8ad985e3 RBX: ffff8881a6400090 RCX: ffff8881697f0000
RDX: 0000000000000000 RSI: 0000000000034010 RDI: 000000000000ffff
RBP: dffffc0000000000 R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffff520007acf00 R12: ffff8881baf20900
R13: 0000000000034010 R14: ffff8881a640008e R15: ffff8881760f7000
FS: 000055557e81f500(0000) GS:ffff8882a9467000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000033000 CR3: 00000001612f4000 CR4: 00000000000006f0
Call Trace:
<TASK>
pppol2tp_sendmsg+0x40a/0x5f0 net/l2tp/l2tp_ppp.c:302
sock_sendmsg_nosec net/socket.c:727 [inline]
__sock_sendmsg net/socket.c:742 [inline]
sock_write_iter+0x503/0x550 net/socket.c:1195
do_iter_readv_writev+0x619/0x8c0 fs/read_write.c:-1
vfs_writev+0x33c/0x990 fs/read_write.c:1059
do_writev+0x154/0x2e0 fs/read_write.c:1105
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f636479c629
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffffd4241c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000014
RAX: ffffffffffffffda RBX: 00007f6364a15fa0 RCX: 00007f636479c629
RDX: 0000000000000001 RSI: 0000200000000080 RDI: 0000000000000003
RBP: 00007f6364832b39 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f6364a15fac R14: 00007f6364a15fa0 R15: 00007f6364a15fa0
</TASK>

[1]: https://lore.kernel.org/all/20260226201600.222044-1-alice.kernel@fastmail.im/

Published: 2026-05-06

Score: 5.5 Medium

EPSS: < 1% Very Low

KEV: No

Impact:

Action:

Analysis

Impact

Based on the description, an oversized PPPoL2TP packet with UDP encapsulation can be sent, causing the kernel routine that sets the UDP length field to truncate the value to 16 bits. The result is a malformed packet that may crash the transmission path or result in dropped packets, effectively denying service to the affected system. The vulnerability stems from an integer overflow in the packet handling code, an example of a buffer or integer overflow weakness.

Affected Systems

Linux kernel users running the L2TP PPPoL2TP implementation, regardless of distribution version, are potentially impacted if the kernel has not received the fix that drops oversized packets. The advisory refers to generic Linux kernel code, so any Linux system with L2TP over UDP support is at risk.

Risk and Exploitability

Based on the description, it is inferred that an attacker could send a specially crafted PPPoL2TP packet with a UDP length greater than 65535 bytes over an untrusted network or during a PPPoL2TP session. While no public exploits are listed and the EPSS score is < 1%, the nature of the integer overflow indicates a high likelihood of a crash or instability, warranting immediate attention. The vulnerability is not listed in CISA KEV, but the severity of a denial of service in kernel code justifies prompt mitigation. The CVSS score is 5.5, indicating moderate severity.

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions

Linux

unaffected

Version	Status	Constraints
`3557baabf28088f49bdf72a048fd33ab62e205b1`	affected	< 9ccce02d501335f59a02f26c878c5e095b16302f
`3557baabf28088f49bdf72a048fd33ab62e205b1`	affected	< 77c1489398c85a844f90205f5e76fd6bc8bb4089
`3557baabf28088f49bdf72a048fd33ab62e205b1`	affected	< 86534c97abd6365a9a021fd767a2023e63c44469
`3557baabf28088f49bdf72a048fd33ab62e205b1`	affected	< f295fe86e22ff0a2ecebf05e30a387e5cf6f6ddc
`3557baabf28088f49bdf72a048fd33ab62e205b1`	affected	< ebe560ea5f54134279356703e73b7f867c89db13

Linux

affected

Version	Status	Constraints
`2.6.23`	affected	—
`0`	unaffected	< 2.6.23
`6.6.136`	unaffected	≤ 6.6.*
`6.12.83`	unaffected	≤ 6.12.*
`6.18.24`	unaffected	≤ 6.18.*
`6.19.14`	unaffected	≤ 6.19.*
`7.0`	unaffected	≤ *

No data.

Vendor Product Confidence Versions

Linux

Linux Kernel

99%

Version	Status	Scheme	Platform
`[3557baabf28088f49bdf72a048fd33ab62e205b1,9ccce02d501335f59a02f26c878c5e095b16302f)`	affected	code_commit	—
`[3557baabf28088f49bdf72a048fd33ab62e205b1,77c1489398c85a844f90205f5e76fd6bc8bb4089)`	affected	code_commit	—
`[3557baabf28088f49bdf72a048fd33ab62e205b1,86534c97abd6365a9a021fd767a2023e63c44469)`	affected	code_commit	—
`[3557baabf28088f49bdf72a048fd33ab62e205b1,ebe560ea5f54134279356703e73b7f867c89db13)`	affected	code_commit	—
`[3557baabf28088f49bdf72a048fd33ab62e205b1,f295fe86e22ff0a2ecebf05e30a387e5cf6f6ddc)`	affected	code_commit	—

Linux

Linux Kernel

99%

Version	Status	Scheme	Platform
`2.6.23`	affected	semver	—
`[0,2.6.23)`	unaffected	semver	—
`[6.6.136,7.0)`	unaffected	semver	—
`[6.12.83,6.13.0)`	unaffected	semver	—
`[6.18.24,6.19.0)`	unaffected	semver	—
`[6.19.14,6.20.0)`	unaffected	semver	—
`[7.0,*]`	unaffected	semver	—

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Remediation

No vendor fix or workaround currently provided.

OpenCVE Recommended Actions

Apply the latest kernel update that includes the patch for the UDP length overflow in L2TP
If a kernel update is unavailable, disable or block PPPoL2TP over UDP traffic to the affected system
Configure network security controls to drop oversized PPPoL2TP packets and monitor for related warning logs

Generated by OpenCVE AI on May 7, 2026 at 04:11 UTC.

Tracking

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Advisories

No advisories yet.

No CVSS v4.0

Attack Vector Local

Attack Complexity Low

Privileges Required Low

Scope Unchanged

Confidentiality Impact None

Integrity Impact None

Availability Impact High

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

The EPSS score is 0.00024.

Key SSVC decision points have not yet been added.

References

Link	Providers
https://git.kernel.org/stable/c/77c1489398c85a844f90205f5e76fd6bc8bb4089
https://git.kernel.org/stable/c/86534c97abd6365a9a021fd767a2023e63c44469
https://git.kernel.org/stable/c/9ccce02d501335f59a02f26c878c5e095b16302f
https://git.kernel.org/stable/c/ebe560ea5f54134279356703e73b7f867c89db13
https://git.kernel.org/stable/c/f295fe86e22ff0a2ecebf05e30a387e5cf6f6ddc
https://lore.kernel.org/linux-cve-announce/2026050614-CVE-2026-43080-9ace@gregkh/T
https://nvd.nist.gov/vuln/detail/CVE-2026-43080
https://www.cve.org/CVERecord?id=CVE-2026-43080

History

Thu, 07 May 2026 02:45:00 +0000

Type	Values Removed	Values Added
Weaknesses	CWE-119 CWE-20

Thu, 07 May 2026 00:15:00 +0000

Type	Values Removed	Values Added
Weaknesses		CWE-190
References		https://lore.kernel.org/linux-cve-announce/2026050614-CVE-2026-43080-9ace@gregkh/T https://nvd.nist.gov/vuln/detail/CVE-2026-43080 https://www.cve.org/CVERecord?id=CVE-2026-43080
Metrics	threat_severity `None`	cvssV3_1 `{'score': 5.5, 'vector': 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'}` threat_severity `Moderate`

Wed, 06 May 2026 12:15:00 +0000

Type	Values Removed	Values Added
Weaknesses		CWE-119 CWE-20

Wed, 06 May 2026 09:30:00 +0000

Type	Values Removed	Values Added
Description		In the Linux kernel, the following vulnerability has been resolved: l2tp: Drop large packets with UDP encap syzbot reported a WARN on my patch series [1]. The actual issue is an overflow of 16-bit UDP length field, and it exists in the upstream code. My series added a debug WARN with an overflow check that exposed the issue, that's why syzbot tripped on my patches, rather than on upstream code. syzbot's repro: r0 = socket$pppl2tp(0x18, 0x1, 0x1) r1 = socket$inet6_udp(0xa, 0x2, 0x0) connect$inet6(r1, &(0x7f00000000c0)={0xa, 0x0, 0x0, @loopback, 0xfffffffc}, 0x1c) connect$pppl2tp(r0, &(0x7f0000000240)=@pppol2tpin6={0x18, 0x1, {0x0, r1, 0x4, 0x0, 0x0, 0x0, {0xa, 0x4e22, 0xffff, @ipv4={'\x00', '\xff\xff', @empty}}}}, 0x32) writev(r0, &(0x7f0000000080)=[{&(0x7f0000000000)="ee", 0x34000}], 0x1) It basically sends an oversized (0x34000 bytes) PPPoL2TP packet with UDP encapsulation, and l2tp_xmit_core doesn't check for overflows when it assigns the UDP length field. The value gets trimmed to 16 bites. Add an overflow check that drops oversized packets and avoids sending packets with trimmed UDP length to the wire. syzbot's stack trace (with my patch applied): len >= 65536u WARNING: ./include/linux/udp.h:38 at udp_set_len_short include/linux/udp.h:38 [inline], CPU#1: syz.0.17/5957 WARNING: ./include/linux/udp.h:38 at l2tp_xmit_core net/l2tp/l2tp_core.c:1293 [inline], CPU#1: syz.0.17/5957 WARNING: ./include/linux/udp.h:38 at l2tp_xmit_skb+0x1204/0x18d0 net/l2tp/l2tp_core.c:1327, CPU#1: syz.0.17/5957 Modules linked in: CPU: 1 UID: 0 PID: 5957 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014 RIP: 0010:udp_set_len_short include/linux/udp.h:38 [inline] RIP: 0010:l2tp_xmit_core net/l2tp/l2tp_core.c:1293 [inline] RIP: 0010:l2tp_xmit_skb+0x1204/0x18d0 net/l2tp/l2tp_core.c:1327 Code: 0f 0b 90 e9 21 f9 ff ff e8 e9 05 ec f6 90 0f 0b 90 e9 8d f9 ff ff e8 db 05 ec f6 90 0f 0b 90 e9 cc f9 ff ff e8 cd 05 ec f6 90 <0f> 0b 90 e9 de fa ff ff 44 89 f1 80 e1 07 80 c1 03 38 c1 0f 8c 4f RSP: 0018:ffffc90003d67878 EFLAGS: 00010293 RAX: ffffffff8ad985e3 RBX: ffff8881a6400090 RCX: ffff8881697f0000 RDX: 0000000000000000 RSI: 0000000000034010 RDI: 000000000000ffff RBP: dffffc0000000000 R08: 0000000000000003 R09: 0000000000000004 R10: dffffc0000000000 R11: fffff520007acf00 R12: ffff8881baf20900 R13: 0000000000034010 R14: ffff8881a640008e R15: ffff8881760f7000 FS: 000055557e81f500(0000) GS:ffff8882a9467000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000200000033000 CR3: 00000001612f4000 CR4: 00000000000006f0 Call Trace: <TASK> pppol2tp_sendmsg+0x40a/0x5f0 net/l2tp/l2tp_ppp.c:302 sock_sendmsg_nosec net/socket.c:727 [inline] __sock_sendmsg net/socket.c:742 [inline] sock_write_iter+0x503/0x550 net/socket.c:1195 do_iter_readv_writev+0x619/0x8c0 fs/read_write.c:-1 vfs_writev+0x33c/0x990 fs/read_write.c:1059 do_writev+0x154/0x2e0 fs/read_write.c:1105 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7f636479c629 Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007ffffd4241c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000014 RAX: ffffffffffffffda RBX: 00007f6364a15fa0 RCX: 00007f636479c629 RDX: 0000000000000001 RSI: 0000200000000080 RDI: 0000000000000003 RBP: 00007f6364832b39 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 R13: 00007f6364a15fac R14: 00007f6364a15fa0 R15: 00007f6364a15fa0 </TASK> [1]: https://lore.kernel.org/all/20260226201600.222044-1-alice.kernel@fastmail.im/
Title		l2tp: Drop large packets with UDP encap
First Time appeared		Linux Linux linux Kernel
CPEs		cpe:2.3:o:linux:linux_kernel::::::::
Vendors & Products		Linux Linux linux Kernel
References		https://git.kernel.org/stable/c/77c1489398c85a844f90205f5e76fd6bc8bb4089 https://git.kernel.org/stable/c/86534c97abd6365a9a021fd767a2023e63c44469 https://git.kernel.org/stable/c/9ccce02d501335f59a02f26c878c5e095b16302f https://git.kernel.org/stable/c/ebe560ea5f54134279356703e73b7f867c89db13 https://git.kernel.org/stable/c/f295fe86e22ff0a2ecebf05e30a387e5cf6f6ddc

Subscriptions

Linux Linux Kernel

MITRE

Status: PUBLISHED

Assigner: Linux

Published: 2026-05-06T07:40:16.491Z

Updated: 2026-05-11T22:17:20.208Z

Reserved: 2026-05-01T14:12:55.983Z

Link: CVE-2026-43080

Vulnrichment

No data.

NVD

Status : Awaiting Analysis

Published: 2026-05-06T10:16:21.110

Modified: 2026-05-06T13:08:07.970

Link: CVE-2026-43080

Redhat

Severity : Moderate

Publid Date: 2026-05-06T00:00:00Z

Links: CVE-2026-43080 - Bugzilla

OpenCVE Enrichment

Updated: 2026-05-07T04:15:21Z

Weaknesses

CWE-190

Impact

Affected Systems

Risk and Exploitability

Tracking

Attack Vector Local

Attack Complexity Low

Privileges Required Low

Scope Unchanged

Confidentiality Impact None

Integrity Impact None

Availability Impact High

User Interaction None

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