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

net: ipv6: Fix UDP length overflow with PMTU discover and big MTU

This commit bounds cork->base.fragsize to IP6_MAX_MTU for UDP sockets to
avoid a possible overflow of UDP length that triggers a WARN in
udp_set_len_short when setsockopt IPV6_MTU_DISCOVER is set to
IPV6_PMTUDISC_DO or IPV6_PMTUDISC_PROBE, and a large packet is sent over
a netdev with an unusually large MTU.

Steps to reproduce (included in the new selftest):

1. Set device MTU bigger than IP6_MAX_MTU. cork->base.fragsize will be
set to that MTU in ip6_setup_cork.
2. Set IPV6_MTU_DISCOVER to IPV6_PMTUDISC_PROBE or IPV6_PMTUDISC_DO. It
lets maxnonfragsize be set to device MTU (cork->fragsize) in
__ip6_append_data, rather than to IP6_MAX_MTU.
3. Send 65528 bytes of payload (+8 bytes of UDP header, +40 bytes of
IPv6 header). Device MTU allows it (it's only one byte bigger than
IP6_MAX_MTU, and the device MTU is bigger than that).
4. The UDP length in the built packet is 65536, which overflows the
16-bit length field and triggers the WARN in udp_set_len_short.

To avoid breaking sending UDP jumbograms over raw IPv6 sockets, limit
the change to UDP sockets only.

The original overflow bug with IPv6 and IPV6_PMTUDISC_DO seems to
predate git history (verified reproduction on 2.6.21), was fixed later,
and then reappeared in commit 427faee167bc ("net: ipv6: introduce
ip6_dst_mtu_maybe_forward"), which is chosen as the Fixes tag here. The
overflow with IPV6_PMTUDISC_PROBE reproduces since its introduction in
commit 628a5c561890 ("[INET]: Add IP(V6)_PMTUDISC_RPOBE").
Published: 2026-09-25
Score: n/a
EPSS: n/a
KEV: No
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History

Fri, 25 Sep 2026 14:45:00 +0000

Type Values Removed Values Added
Weaknesses CWE-190

Fri, 25 Sep 2026 10:30:00 +0000

Type Values Removed Values Added
Description In the Linux kernel, the following vulnerability has been resolved: net: ipv6: Fix UDP length overflow with PMTU discover and big MTU This commit bounds cork->base.fragsize to IP6_MAX_MTU for UDP sockets to avoid a possible overflow of UDP length that triggers a WARN in udp_set_len_short when setsockopt IPV6_MTU_DISCOVER is set to IPV6_PMTUDISC_DO or IPV6_PMTUDISC_PROBE, and a large packet is sent over a netdev with an unusually large MTU. Steps to reproduce (included in the new selftest): 1. Set device MTU bigger than IP6_MAX_MTU. cork->base.fragsize will be set to that MTU in ip6_setup_cork. 2. Set IPV6_MTU_DISCOVER to IPV6_PMTUDISC_PROBE or IPV6_PMTUDISC_DO. It lets maxnonfragsize be set to device MTU (cork->fragsize) in __ip6_append_data, rather than to IP6_MAX_MTU. 3. Send 65528 bytes of payload (+8 bytes of UDP header, +40 bytes of IPv6 header). Device MTU allows it (it's only one byte bigger than IP6_MAX_MTU, and the device MTU is bigger than that). 4. The UDP length in the built packet is 65536, which overflows the 16-bit length field and triggers the WARN in udp_set_len_short. To avoid breaking sending UDP jumbograms over raw IPv6 sockets, limit the change to UDP sockets only. The original overflow bug with IPv6 and IPV6_PMTUDISC_DO seems to predate git history (verified reproduction on 2.6.21), was fixed later, and then reappeared in commit 427faee167bc ("net: ipv6: introduce ip6_dst_mtu_maybe_forward"), which is chosen as the Fixes tag here. The overflow with IPV6_PMTUDISC_PROBE reproduces since its introduction in commit 628a5c561890 ("[INET]: Add IP(V6)_PMTUDISC_RPOBE").
Title net: ipv6: Fix UDP length overflow with PMTU discover and big MTU
First Time appeared Linux
Linux linux Kernel
CPEs cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Vendors & Products Linux
Linux linux Kernel
References

Subscriptions

Linux Linux Kernel
cve-icon MITRE

Status: PUBLISHED

Assigner: Linux

Published:

Updated: 2026-09-25T10:23:19.568Z

Reserved: 2026-09-25T10:18:58.208Z

Link: CVE-2026-97984

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Received

Published: 2026-09-25T11:17:26.443

Modified: 2026-09-25T11:17:26.443

Link: CVE-2026-97984

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-09-25T14:30:14Z

Weaknesses