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CVE-2026-74657: ipv4: Fix fib_nlmsg_size() for RTA_VIA nexthops

• Severity: unknown
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-22 · updated 2026-10-14

Description

In the Linux kernel, the following vulnerability has been resolved: ipv4: Fix fib_nlmsg_size() for RTA_VIA nexthops fib_nlmsg_size() still estimates nexthop space as if every gateway is encoded as an IPv4 RTA_GATEWAY attribute. IPv4 routes can also carry an IPv6 gateway, which fib_nexthop_info() dumps as RTA_VIA. As a result, route notifications can allocate an skb that is too small. fib_dump_info() then fails with -EMSGSIZE and rtmsg_fib() hits the WARN_ON() that marks such failures as a fib_nlmsg_size() bug. With panic_on_warn set, this becomes a kernel panic. Mirror the actual nexthop dump layout in fib_nlmsg_size(): account for IPv6 nexthop gateways dumped as RTA_VIA, for the no-header rtnexthop layout used inside RTA_MULTIPATH, and for RTA_FLOW only when it is actually present.

Affected products

• Linux kernel: 5.2 — fixed in 7.2

How to fix

Upgrade to a fixed version: Linux kernel 7.2. Check the vendor advisory for mitigations.

References

• https://git.kernel.org/stable/c/0f0ca602941d0a81ae9514943ca06c55159c6385
• https://git.kernel.org/stable/c/4a5dfbae5179f6574695012a980476254df2d295
• https://git.kernel.org/stable/c/4ff9548d84945d2cbf9e4c207288063a200ea397
• https://git.kernel.org/stable/c/5307a53599fa762c06e475ee4a375252074fd324
• https://git.kernel.org/stable/c/57195f0ab5cfbb5ee0864e5aff15ceb48f5a5e28
• https://git.kernel.org/stable/c/7f80ad373ce4a7af5367ff273cea0f16e91387f3
• https://git.kernel.org/stable/c/9b22f13524fa0de0d963bbd3002df6c28bae3395
• https://git.kernel.org/stable/c/a59edda6eda1252340354322d8ab318b2e9052fb

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