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CVE-2026-43080

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].

Published May 6, 2026 · updated Oct 15, 2026 · linux linux kernel

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

Code

The project this record applies to. Exploit listings stay in References.

Timeline

Detection, updates, and resolution for this record.

  1. May 6, 2026 · NIST NVD

    Detected

    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. sy

  2. Oct 15, 2026 · NIST NVD

    Updated

    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. sy

Affected

  • cpe linux/linux kernelversion unspecified
  • cpe linux/linux kernel7.0

Sources

  • CVE.orgVulnerability recordCVE-2026-43080

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-43080

    30-day exploitation probability 0.2%

    first.org
  • NIST NVDVulnerability recordCVE-2026-43080

    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 WAR

    nvd.nist.gov

References