CVE-2026-46099 - Vulnerability Details

- net: ipv6: fix NOREF dst use in seg6 and rpl lwtunnels

Description

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

net: ipv6: fix NOREF dst use in seg6 and rpl lwtunnels

seg6_input_core() and rpl_input() call ip6_route_input() which sets a
NOREF dst on the skb, then pass it to dst_cache_set_ip6() invoking
dst_hold() unconditionally.
On PREEMPT_RT, ksoftirqd is preemptible and a higher-priority task can
release the underlying pcpu_rt between the lookup and the caching
through a concurrent FIB lookup on a shared nexthop.
Simplified race sequence:

ksoftirqd/X higher-prio task (same CPU X)
----------- --------------------------------
seg6_input_core(,skb)/rpl_input(skb)
dst_cache_get()
-> miss
ip6_route_input(skb)
-> ip6_pol_route(,skb,flags)
[RT6_LOOKUP_F_DST_NOREF in flags]
-> FIB lookup resolves fib6_nh
[nhid=N route]
-> rt6_make_pcpu_route()
[creates pcpu_rt, refcount=1]
pcpu_rt->sernum = fib6_sernum
[fib6_sernum=W]
-> cmpxchg(fib6_nh.rt6i_pcpu,
NULL, pcpu_rt)
[slot was empty, store succeeds]
-> skb_dst_set_noref(skb, dst)
[dst is pcpu_rt, refcount still 1]

rt_genid_bump_ipv6()
-> bumps fib6_sernum
[fib6_sernum from W to Z]
ip6_route_output()
-> ip6_pol_route()
-> FIB lookup resolves fib6_nh
[nhid=N]
-> rt6_get_pcpu_route()
pcpu_rt->sernum != fib6_sernum
[W <> Z, stale]
-> prev = xchg(rt6i_pcpu, NULL)
-> dst_release(prev)
[prev is pcpu_rt,
refcount 1->0, dead]

dst = skb_dst(skb)
[dst is the dead pcpu_rt]
dst_cache_set_ip6(dst)
-> dst_hold() on dead dst
-> WARN / use-after-free

For the race to occur, ksoftirqd must be preemptible (PREEMPT_RT without
PREEMPT_RT_NEEDS_BH_LOCK) and a concurrent task must be able to release
the pcpu_rt. Shared nexthop objects provide such a path, as two routes
pointing to the same nhid share the same fib6_nh and its rt6i_pcpu
entry.

Fix seg6_input_core() and rpl_input() by calling skb_dst_force() after
ip6_route_input() to force the NOREF dst into a refcounted one before
caching.
The output path is not affected as ip6_route_output() already returns a
refcounted dst.

Published: 2026-05-27

Score: n/a

EPSS: n/a

KEV: No

Impact:

Action:

Analysis

Impact

This vulnerability is a race condition in the Linux kernel's IPv6 routing code. Concurrent calls to ip6_route_input and dst_cache_set_ip6 can cause a NOREF destination to be released while still in use, leading to a use‑after‑free that triggers a kernel warning and potentially a crash. The flaw can allow an attacker to cause a denial‑of‑service by sending specially crafted IPv6 packets that exercise the vulnerable code paths. The weakness is a classic use‑after‑free (CWE‑665).

Affected Systems

The issue affects all builds of the Linux kernel that include IPv6 support and use seg6 or rpl lightweight tunnels. No specific version numbers are listed, so any kernel before the patch that contains the unfixed path is vulnerable. Auditors should verify kernel versions to determine exposure.

Risk and Exploitability

The CVSS score is not provided, but the vulnerability could lead to a kernel crash, a high‑impact denial‑of‑service scenario, making it a severe problem. EPSS is not available, and the vulnerability is not listed in CISA's KEV catalog. Attack requires concurrent high‑priority activity on the same CPU as ksoftirqd and a crafted IPv6 packet, so exploitation is likely local or remote via network traffic. The race makes exploitation non‑trivial, but the kernel crash outcome would be catastrophic if achieved.

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions

Linux

unaffected

Version	Status	Constraints
`af4a2209b1344939eaac11f269c261d347cbc3ee`	affected	< 6bd17925bd6866027a6555db17905b9fc073d38d
`af4a2209b1344939eaac11f269c261d347cbc3ee`	affected	< 52f9db67f8f35f436366cf4980b4f0a2583d0ef0
`af4a2209b1344939eaac11f269c261d347cbc3ee`	affected	< b778b6d095421619c331fd2d7751143cd5387103
`af4a2209b1344939eaac11f269c261d347cbc3ee`	affected	< 9dd5481f960e337b81d7dfe429529495c1c481c0
`af4a2209b1344939eaac11f269c261d347cbc3ee`	affected	< f9c52a6ba9780bd27e0bf4c044fd91c13c778b6e

Linux

affected

Version	Status	Constraints
`4.12`	affected	—
`0`	unaffected	< 4.12
`6.6.140`	unaffected	≤ 6.6.*
`6.12.86`	unaffected	≤ 6.12.*
`6.18.27`	unaffected	≤ 6.18.*
`7.0.4`	unaffected	≤ 7.0.*
`7.1-rc2`	unaffected	≤ *

No data.

No data available yet.

Remediation

No vendor fix or workaround currently provided.

OpenCVE Recommended Actions

Apply a kernel version that includes the fixed seg6_input_core and rpl_input logic that forces a refcounted destination after ip6_route_input.
If a patched kernel cannot be obtained immediately, disable the seg6 and rpl lightweight tunnel features or unload the associated modules to eliminate the vulnerable code paths.
As a further precaution, consider disabling PREEMPT_RT if it is not required, as the race only occurs when ksoftirqd is preemptible.

Generated by OpenCVE AI on May 27, 2026 at 17:53 UTC.

Tracking

Sign in to view the affected projects.

Advisories

No advisories yet.

No CVSS v4.0

No CVSS v3.1

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

No EPSS score available.

Key SSVC decision points have not yet been added.

References

Link	Providers
https://git.kernel.org/stable/c/52f9db67f8f35f436366cf4980b4f0a2583d0ef0
https://git.kernel.org/stable/c/6bd17925bd6866027a6555db17905b9fc073d38d
https://git.kernel.org/stable/c/9dd5481f960e337b81d7dfe429529495c1c481c0
https://git.kernel.org/stable/c/b778b6d095421619c331fd2d7751143cd5387103
https://git.kernel.org/stable/c/f9c52a6ba9780bd27e0bf4c044fd91c13c778b6e

History

Wed, 27 May 2026 18:15:00 +0000

Type	Values Removed	Values Added
Weaknesses		CWE-665

Wed, 27 May 2026 14:15:00 +0000

Type	Values Removed	Values Added
Description		In the Linux kernel, the following vulnerability has been resolved: net: ipv6: fix NOREF dst use in seg6 and rpl lwtunnels seg6_input_core() and rpl_input() call ip6_route_input() which sets a NOREF dst on the skb, then pass it to dst_cache_set_ip6() invoking dst_hold() unconditionally. On PREEMPT_RT, ksoftirqd is preemptible and a higher-priority task can release the underlying pcpu_rt between the lookup and the caching through a concurrent FIB lookup on a shared nexthop. Simplified race sequence: ksoftirqd/X higher-prio task (same CPU X) ----------- -------------------------------- seg6_input_core(,skb)/rpl_input(skb) dst_cache_get() -> miss ip6_route_input(skb) -> ip6_pol_route(,skb,flags) [RT6_LOOKUP_F_DST_NOREF in flags] -> FIB lookup resolves fib6_nh [nhid=N route] -> rt6_make_pcpu_route() [creates pcpu_rt, refcount=1] pcpu_rt->sernum = fib6_sernum [fib6_sernum=W] -> cmpxchg(fib6_nh.rt6i_pcpu, NULL, pcpu_rt) [slot was empty, store succeeds] -> skb_dst_set_noref(skb, dst) [dst is pcpu_rt, refcount still 1] rt_genid_bump_ipv6() -> bumps fib6_sernum [fib6_sernum from W to Z] ip6_route_output() -> ip6_pol_route() -> FIB lookup resolves fib6_nh [nhid=N] -> rt6_get_pcpu_route() pcpu_rt->sernum != fib6_sernum [W <> Z, stale] -> prev = xchg(rt6i_pcpu, NULL) -> dst_release(prev) [prev is pcpu_rt, refcount 1->0, dead] dst = skb_dst(skb) [dst is the dead pcpu_rt] dst_cache_set_ip6(dst) -> dst_hold() on dead dst -> WARN / use-after-free For the race to occur, ksoftirqd must be preemptible (PREEMPT_RT without PREEMPT_RT_NEEDS_BH_LOCK) and a concurrent task must be able to release the pcpu_rt. Shared nexthop objects provide such a path, as two routes pointing to the same nhid share the same fib6_nh and its rt6i_pcpu entry. Fix seg6_input_core() and rpl_input() by calling skb_dst_force() after ip6_route_input() to force the NOREF dst into a refcounted one before caching. The output path is not affected as ip6_route_output() already returns a refcounted dst.
Title		net: ipv6: fix NOREF dst use in seg6 and rpl lwtunnels
First Time appeared		Linux Linux linux Kernel
CPEs		cpe:2.3:o:linux:linux_kernel::::::::
Vendors & Products		Linux Linux linux Kernel
References		https://git.kernel.org/stable/c/52f9db67f8f35f436366cf4980b4f0a2583d0ef0 https://git.kernel.org/stable/c/6bd17925bd6866027a6555db17905b9fc073d38d https://git.kernel.org/stable/c/9dd5481f960e337b81d7dfe429529495c1c481c0 https://git.kernel.org/stable/c/b778b6d095421619c331fd2d7751143cd5387103 https://git.kernel.org/stable/c/f9c52a6ba9780bd27e0bf4c044fd91c13c778b6e

Subscriptions

Linux Linux Kernel

MITRE

Status: PUBLISHED

Assigner: Linux

Published: 2026-05-27T12:59:04.628Z

Updated: 2026-05-27T12:59:04.628Z

Reserved: 2026-05-13T15:03:33.097Z

Link: CVE-2026-46099

Vulnrichment

No data.

NVD

Status : Awaiting Analysis

Published: 2026-05-27T14:17:31.557

Modified: 2026-05-27T14:48:03.013

Link: CVE-2026-46099

Redhat

No data.

OpenCVE Enrichment

Updated: 2026-05-27T18:00:15Z

Weaknesses

CWE-665

Impact

Affected Systems

Risk and Exploitability

Tracking

Subscriptions

JSON object

JSON object

JSON object

JSON object

JSON object