CVE-2026-98101 ipv6: mcast: use copy-on-write RCU updates in ip6_mc_source()
A race condition in Linux kernel IPv6 multicast handling allows concurrent readers to observe torn or inconsistent source filter data, potentially leading to information disclosure or system instability.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-98101?
CVE-2026-98101 is a vulnerability in Linux, published on 25 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
ipv6: mcast: use copy-on-write RCU updates in ip6_mc_source()
pmc->sflist is read locklessly under rcu_read_lock() by inet6_mc_check() during packet reception in the UDP and RAW multicast receive paths.
ip6_mc_source() mutated psl->sl_addr and psl->sl_count in-place when adding or removing a source filter. Additionally, when expanding the filter buffer, newpsl was published via rcu_assign_pointer() before writing the new source into the array.
Because 16-byte struct in6_addr writes are not atomic and array shifting is not synchronized with RCU readers, concurrent readers in inet6_mc_check() could read torn IPv6 addresses or observe duplicated/missed source entries.
Fix this by switching ip6_mc_source() to copy-on-write RCU updates: allocate and fully populate newpsl before publishing it via rcu_assign_pointer(), and reclaim the old filter via kfree_rcu(), matching ip6_mc_msfilter().
Also remove the now unused IP6_SFBLOCK macro.
Which versions are affected by CVE-2026-98101?
How do I fix CVE-2026-98101?
Apply the vendor's patch.
Is CVE-2026-98101 being exploited?
There is no known exploitation of CVE-2026-98101 in the wild and no public exploit code.
Sources
Also known as AZL-104636, BELL-CVE-2026-98101, DEBIAN-CVE-2026-98101, GHSA-wv36-qwc6-r8c9, UBUNTU-CVE-2026-98101.