CVE-2026-43080

/// Technical 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

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References