CVE-2026-98393 is a vulnerability in Linux Kernel
Published on October 14, 2026
net: ipv4: Fix UDP length overflow with PMTU discover and big MTU
In the Linux kernel, the following vulnerability has been resolved:
net: ipv4: Fix UDP length overflow with PMTU discover and big MTU
This commit bounds cork->base.fragsize to IP_MAX_MTU to avoid a
possible overflow of UDP length that triggers a WARN in
udp_set_len_short when setsockopt IP_MTU_DISCOVER is set to
IP_PMTUDISC_PROBE, and a large packet is sent over a netdev with an
unusually large MTU.
Steps to reproduce:
1. Set device MTU bigger than IP_MAX_MTU + 20. cork->base.fragsize will
be set to that MTU in ip_setup_cork.
2. Set IP_MTU_DISCOVER to IP_PMTUDISC_PROBE. It lets maxnonfragsize be
set to device MTU (cork->fragsize) in __ip_append_data, rather than
to IP_MAX_MTU.
3. Send 65528 bytes of payload (+8 bytes of UDP header, +20 bytes of
IPv4 header). Device MTU allows it (it's only one byte bigger than
IP_MAX_MTU + IPv4 header, 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.
Note: IP_PMTUDISC_DO with IPv4 is safe, because ip_dst_mtu_maybe_forward
always clamps at IP_MAX_MTU, unlike ip6_dst_mtu_maybe_forward.
The Fixes tag points at the first commit where I could reproduce the
overflow with IPv4 and IP_PMTUDISC_PROBE.
Products Associated with CVE-2026-98393
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Affected Versions
Linux:- Version daba287b299ec7a2c61ae3a714920e90e8396ad5 and below 83fedc4741210f75be8ca5b487985a242bf6f18d is affected.
- Version daba287b299ec7a2c61ae3a714920e90e8396ad5 and below 81f540827d243b8dad76c1f44e70ea31719f393c is affected.
- Version daba287b299ec7a2c61ae3a714920e90e8396ad5 and below b83641e0ab8b20eefcc4cdc5a059f897375291a2 is affected.
- Version 3.13 is affected.
- Before 3.13 is unaffected.
- Version 6.18.53, <= 6.18.* is unaffected.
- Version 7.2.7, <= 7.2.* is unaffected.
- Version 7.3-rc3, <= * is unaffected.