# CVE-2023-53477: ipv6: Add lwtunnel encap size of all siblings in nexthop calculation

> A flaw in Linux kernel IPv6 routing calculation can trigger a kernel warning and potential denial of service when adding routes with mixed lwtunnel siblings.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 1 Oct 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2023-53477, DEBIAN-CVE-2023-53477, GHSA-mrpw-6g2q-pgjw, UBUNTU-CVE-2023-53477

## What is CVE-2023-53477?

CVE-2023-53477 is a medium-severity denial of service vulnerability in Linux kernel, published on 1 Oct 2025. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

ipv6: Add lwtunnel encap size of all siblings in nexthop calculation

In function rt6_nlmsg_size(), the length of nexthop is calculated
by multipling the nexthop length of fib6_info and the number of
siblings. However if the fib6_info has no lwtunnel but the siblings
have lwtunnels, the nexthop length is less than it should be, and
it will trigger a warning in inet6_rt_notify() as follows:

WARNING: CPU: 0 PID: 6082 at net/ipv6/route.c:6180 inet6_rt_notify+0x120/0x130
......
Call Trace:
 <TASK>
 fib6_add_rt2node+0x685/0xa30
 fib6_add+0x96/0x1b0
 ip6_route_add+0x50/0xd0
 inet6_rtm_newroute+0x97/0xa0
 rtnetlink_rcv_msg+0x156/0x3d0
 netlink_rcv_skb+0x5a/0x110
 netlink_unicast+0x246/0x350
 netlink_sendmsg+0x250/0x4c0
 sock_sendmsg+0x66/0x70
 ___sys_sendmsg+0x7c/0xd0
 __sys_sendmsg+0x5d/0xb0
 do_syscall_64+0x3f/0x90
 entry_SYSCALL_64_after_hwframe+0x72/0xdc

This bug can be reproduced by script:

ip -6 addr add 2002::2/64 dev ens2
ip -6 route add 100::/64 via 2002::1 dev ens2 metric 100

for i in 10 20 30 40 50 60 70;
do
	ip link add link ens2 name ipv_$i type ipvlan
	ip -6 addr add 2002::$i/64 dev ipv_$i
	ifconfig ipv_$i up
done

for i in 10 20 30 40 50 60;
do
	ip -6 route append 100::/64 encap ip6 dst 2002::$i via 2002::1
dev ipv_$i metric 100
done

ip -6 route append 100::/64 via 2002::1 dev ipv_70 metric 100

This patch fixes it by adding nexthop_len of every siblings using
rt6_nh_nlmsg_size().

## Which versions are affected by CVE-2023-53477?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2023-53477?

Apply the vendor's patch.

## Is CVE-2023-53477 being exploited?

There is no known exploitation of CVE-2023-53477 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2023-53477 and fix it if it is affected.

What it is: CVE-2023-53477 is a medium-severity denial of service vulnerability in Linux kernel, published on 1 Oct 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2023-53477 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2023-53477/ (Markdown: https://insights.sbomify.com/CVE-2023-53477/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2023-53477
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-53477
- OSV: https://osv.dev/vulnerability/CVE-2023-53477

---
From sbomify Insights: https://insights.sbomify.com/CVE-2023-53477/
Summaries are generated and may contain mistakes; the official description above is the record.